Showing 1–50 of 149 results for author: Beck, R

Measurement of the Gerasimov-Drell-Hearn integrand for proton and deuteron from 200 to 1400 MeV

Authors: P. Pedroni, F. Afzal, S. Abt, P. Achenbach, J. R. M. Annand, H. J. Arends, S. D. Bass, M. Biroth, R. Beck, N. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, A. S. Dolzhikov, E. Downie, S. Fegan, A. Fix, D. Ghosal, I. Gorodnov, W. Gradl, G. Gurevich, L. Heijkenskjöld, D. Hornidge, G. M. Huber , et al. (37 additional authors not shown)

Abstract: New data for the total inclusive helicity-dependent cross section for the proton and deuteron were obtained in the photon energy interval 200-1400 MeV. The experiment was performed at the A2 tagged-photon facility of the Mainz Microtron (MAMI) using a circularly polarized photon beam and longitudinally polarized proton and deuteron targets. The reaction products were detected using the large-accep… ▽ More New data for the total inclusive helicity-dependent cross section for the proton and deuteron were obtained in the photon energy interval 200-1400 MeV. The experiment was performed at the A2 tagged-photon facility of the Mainz Microtron (MAMI) using a circularly polarized photon beam and longitudinally polarized proton and deuteron targets. The reaction products were detected using the large-acceptance Crystal Ball/TAPS calorimeter, which covers 97% of the full solid angle. These new results, obtained with fine energy binning, significantly expand both the quantity and the quality of the available data for these observables and enable a detailed comparison with state-of-the-art theoretical calculations. From the combination of the results for the deuteron and the proton, important information could also be extracted for the free neutron. Based on these data, and using existing models to evaluate the missing contributions from unmeasured photon energy regions, the validity of the Gerasimov-Drell-Hearn (GDH) sum rule has been verified for the proton, the neutron, and the deuteron. These new data provide a precise experimental benchmark for theoretical models used to study nucleons, both in their free state and when embedded in the nuclear medium. △ Less

Submitted 15 April, 2026; originally announced April 2026.

Comments: 16 pages, 9 figures

First measurement using elliptically polarized photons of the double-polarization observable $E$ for $γp \to p π^0$ and $γp \to n π^+$

Authors: A2 Collaboration, F. Afzal, K. Spieker, P. Hurck, S. Abt, P. Achenbach, P. Adlarson, Z. Ahmed, C. S. Akondi, J. R. M. Annand, H. J. Arends, M. Bashkanov, R. Beck, M. Biroth, N. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, S. Costanza, A. Denig, M. Dieterle, E. J. Downie, P. Drexler, S. Fegan , et al. (52 additional authors not shown)

Abstract: We report the measurement of the helicity asymmetry $E$ for the $pπ^0$ and $nπ^+$ final states using, for the first time, an elliptically polarized photon beam in combination with a longitudinally polarized target at the Crystal Ball experiment at MAMI. The results agree very well with data that were taken with a circularly polarized photon beam, showing that it is possible to simultaneously measu… ▽ More We report the measurement of the helicity asymmetry $E$ for the $pπ^0$ and $nπ^+$ final states using, for the first time, an elliptically polarized photon beam in combination with a longitudinally polarized target at the Crystal Ball experiment at MAMI. The results agree very well with data that were taken with a circularly polarized photon beam, showing that it is possible to simultaneously measure polarization observables that require linearly (e.g.~$G$) and circularly polarized photons (e.g.~$E$) and a longitudinally polarized target. The new data cover a photon energy range 270 - 1400 MeV for the $pπ^0$ final state (230 - 842 MeV for the $nπ^+$ final state) and the full range of pion polar angles, $θ$, providing the most precise measurement of the observable $E$. A moment analysis gives a clear observation of the $pη$ cusp in the $pπ^0$ final state. △ Less

Submitted 8 February, 2024; originally announced February 2024.

Evaluation of the E2/M1 ratio in the $N\to Δ(1232)$ transition from the $ \vecγ \vec{p} \to p π^0 $ reaction

Authors: E. Mornacchi, P. Pedroni, F. Afzal, Y. Wunderlich, S. Abt, P. Achenbach, J. R. M. Annand, H. J. Arends, M. Bashkanov, M. Biroth, R. Beck, N. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, A. Denig, A. S. Dolzhikov, E. Downie, S. Fegan, A. Fix, D. Ghosal, I. Gorodnov, W. Gradl, D. Gurevich , et al. (37 additional authors not shown)

Abstract: A new data set for the helicity-dependent differential cross section of the single-meson photoproduction reaction $γp \to p π^{0}$ was obtained for the photon energy interval 150-400 MeV. The experiment was performed at the A2 tagged photon facility of the Mainz Microtron MAMI using a circularly polarized photon beam and a longitudinally polarized proton target. The reaction products were detected… ▽ More A new data set for the helicity-dependent differential cross section of the single-meson photoproduction reaction $γp \to p π^{0}$ was obtained for the photon energy interval 150-400 MeV. The experiment was performed at the A2 tagged photon facility of the Mainz Microtron MAMI using a circularly polarized photon beam and a longitudinally polarized proton target. The reaction products were detected with the large acceptance Crystal Ball/TAPS calorimeter covering 97\% of the full solid angle. These new results, obtained with a fine energy and polar angle binning, greatly increase both the existing quantity and quality of the data available for this observable. A moment analysis, based on a finite expansion in Legendre polynomials, was applied to these data by using a bootstrap-based fitting method to correctly account for their systematic uncertainties. From the resulting decomposition of the differential cross sections, the $E2/M1$ ratio for the $N\to Δ(1232)$ transition was determined to be $[-2.38 \pm 0.16{\hbox{ (stat.+sys.)}} \pm 0.10 {\hbox{ (model)}}] \%$. Combining this value with previous results also allowed us to evaluate the most precise available estimate of the $E2/M1$ ratio to be used for all further reference and model comparisons. △ Less

Submitted 7 February, 2024; v1 submitted 13 December, 2023; originally announced December 2023.

Comments: 16 pages, 14 figures

Helicity dependent cross sections for the photoproduction of $π^0π^{\pm}$ pairs from quasi-free nucleons

Authors: A2 Collaboration, D. Ghosal, V. Sokhoyan, A. Fix, S. Lutterer, S. Abt, P. Achenbach, F. Afzal, Z. Ahmed, J. R. M. Annand, M. Bashkanov, R. Beck, M. Biroth, N. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicot, S. Costanza, A. Denig, M. Dieterle, A. S. Dolzhikov, E. J. Downie, P. Drexler, S. Fegan , et al. (49 additional authors not shown)

Abstract: Photoproduction of $π^0π^{\pm}$-pairs from quasifree nucleons bound in the deuteron has been investigated to study the helicity dependence of this reaction. Measurements with a liquid deuterium target were used to extract the unpolarized cross sections for reactions on protons and neutrons. A deuterated, longitudinally polarized solid-butanol target, together with a circularly polarized photon bea… ▽ More Photoproduction of $π^0π^{\pm}$-pairs from quasifree nucleons bound in the deuteron has been investigated to study the helicity dependence of this reaction. Measurements with a liquid deuterium target were used to extract the unpolarized cross sections for reactions on protons and neutrons. A deuterated, longitudinally polarized solid-butanol target, together with a circularly polarized photon beam, determined the double polarization observable $E$. From these results the spin-dependent cross sections $σ_{1/2}$ and $σ_{3/2}$, corresponding to the anti-parallel and parallel spin configurations of the beam photon and target nucleon, have been derived. The measurements were performed at the Mainz MAMI accelerator with tagged, circularly-polarized photon beams produced via bremsstrahlung from longitudinally polarized electron beams. The reaction products were detected with an almost $4π$ solid-angle covering calorimeter composed of the Crystal Ball and TAPS detectors, supplemented by plastic scintillation detectors for charged particle identification. The results are sensitive to sequential decays of nucleon resonances via intermediate states and also to the decay of nucleon resonances by emission of charged $ρ$ mesons, and are compared to recent model results. △ Less

Submitted 28 October, 2023; v1 submitted 29 August, 2023; originally announced August 2023.

Comments: 9 pages, 6 figures

Journal ref: Physics Letters B, Volume 847, 10 December 2023, 138273

The Two-Photon Exchange Experiment at DESY

Authors: R. Alarcon, R. Beck, J. C. Bernauer, M. Broering, A. Christopher, E. W. Cline, S. Dhital, B. Dongwi, I. Fernando, M. Finger, M. Finger Jr., I. Friščić, T. Gautam, G. N. Grauvogel, D. K. Hasell, O. Hen, T. Horn, E. Ihloff, R. Johnston, J. Kelsey, M. Kohl, T. Kutz, I. Lavrukhin, S. Lee, W. Lorenzon , et al. (15 additional authors not shown)

Abstract: We propose a new measurement of the ratio of positron-proton to electron-proton elastic scattering at DESY. The purpose is to determine the contributions beyond single-photon exchange, which are essential for the Quantum Electrodynamic (QED) description of the most fundamental process in hadronic physics. By utilizing a 20 cm long liquid hydrogen target in conjunction with the extracted beam from… ▽ More We propose a new measurement of the ratio of positron-proton to electron-proton elastic scattering at DESY. The purpose is to determine the contributions beyond single-photon exchange, which are essential for the Quantum Electrodynamic (QED) description of the most fundamental process in hadronic physics. By utilizing a 20 cm long liquid hydrogen target in conjunction with the extracted beam from the DESY synchrotron, we can achieve an average luminosity of $2.12\times10^{35}$ cm$^{-2}\cdot$s$^{-1}\cdot$sr$^{-1}$ ($\approx200$ times the luminosity achieved by OLYMPUS). The proposed TPEX experiment entails a commissioning run at 2 GeV, followed by measurements at 3 GeV, thereby providing new data up to $Q^2=4.6$ (GeV/$c$)$^2$ (twice the range of current measurements). We present and discuss the proposed experimental setup, run plan, and expectations. △ Less

Submitted 25 July, 2023; originally announced July 2023.

Comments: 10 pages, 14 figures. arXiv admin note: substantial text overlap with arXiv:2301.04708

Two-Photon EXchange -- TPEX

Authors: R. Alarcon, R. Beck, J. C. Bernauer, M. Broering, E. Cline, B. Dongwi, I. Fernando, M. Finger, M. Finger Jr., I. Friščić, T. Gautam, D. K. Hasell, O. Hen, J. Holmes, T. Horn, E. Ihloff, R. Johnston, J. Kelsey, M. Kohl, T. Kutz, I. Lavrukhin, S. Lee, W. Lorenzon, F. Maas, H. Merkel , et al. (12 additional authors not shown)

Abstract: We propose a new measurement of the ratio of positron-proton to electron-proton, elastic scattering at DESY to determine the contributions beyond single-photon exchange, which are essential to the QED description of the most fundamental process in hadronic physics. A 20~cm long liquid hydrogen target together with the extracted beam from the DESY synchrotron would yield an average luminosity of… ▽ More We propose a new measurement of the ratio of positron-proton to electron-proton, elastic scattering at DESY to determine the contributions beyond single-photon exchange, which are essential to the QED description of the most fundamental process in hadronic physics. A 20~cm long liquid hydrogen target together with the extracted beam from the DESY synchrotron would yield an average luminosity of $2.12\times10^{35}$~cm$^{-2}\cdot$s$^{-1}\cdot$sr$^{-1}$ ($\sim200$ times the luminosity achieved by OLYMPUS). A commissioning run at 2 GeV followed by measurements at 3 GeV would provide new data up to $Q^2=4.6$~(GeV/$c$)$^2$ (twice the range of current measurements). Lead tungstate calorimeters would be used to detect the scattered leptons at polar angles of $30^\circ$, $50^\circ$, $70^\circ$, $90^\circ$, and $110^\circ$. The measurements could be scheduled to not interfere with the operation of PETRA. We present rate estimates and simulations for the planned measurements including background considerations. Initial measurements at the DESY test beam facility using prototype lead tungstate calorimeters in 2019, 2021, and 2022 were made to check the Monte Carlo simulations and the performance of the calorimeters. These tests also investigated different readout schemes (triggered and streaming). Various upgrades are possible to shorten the running time and to make higher beam energies and thus greater $Q^2$ ranges accessible. △ Less

Submitted 11 January, 2023; originally announced January 2023.

Comments: 45 pages, 43 figures, submitted to the DESY PRC

The new APD-Based Readout of the Crystal Barrel Calorimeter -- An Overview

Authors: CBELSA/TAPS Collaboration, :, C. Honisch, P. Klassen, J. Müllers, M. Urban, F. Afzal, J. Bieling, S. Ciupka, J. Hartmann, P. Hoffmeister, M. Lang, D. Schaab, C. Schmidt, M. Steinacher, D. Walther, R. Beck, K. -T. Brinkmann, V. Crede, H. Dutz, D. Elsner, W. Erni, E. Fix, F. Frommberger, M. Grüner , et al. (26 additional authors not shown)

Abstract: The Crystal Barrel is an electromagnetic calorimeter consisting of 1380 CsI(Tl) scintillators, and is currently installed at the CBELSA/TAPS experiment where it is used to detect decay products from photoproduction of mesons. The readout of the Crystal Barrel has been upgraded in order to integrate the detector into the first level of the trigger and to increase its sensitivity for neutral final s… ▽ More The Crystal Barrel is an electromagnetic calorimeter consisting of 1380 CsI(Tl) scintillators, and is currently installed at the CBELSA/TAPS experiment where it is used to detect decay products from photoproduction of mesons. The readout of the Crystal Barrel has been upgraded in order to integrate the detector into the first level of the trigger and to increase its sensitivity for neutral final states. The new readout uses avalanche photodiodes in the front-end and a dual back-end with branches optimized for energy and time measurement, respectively. An FPGA-based cluster finder processes the whole hit pattern within less than 100 ns. The important downside of APDs -- the temperature dependence of their gain -- is handled with a temperature stabilization and a compensating bias voltage supply. Additionally, a light pulser system allows the APDs' gains to be measured during beamtimes. △ Less

Submitted 16 January, 2023; v1 submitted 23 December, 2022; originally announced December 2022.

Neutron polarisation transfer, $C_{x'}^n$, in $π^+$ photoproduction off the proton

Authors: M. Bashkanov, D. P. Watts, S. J. D. Kay, S. Abt, P. Achenbach, P. Adlarson, F. Afzal, Z. Ahmed, C. S. Akondi, J. R. M. Annand, R. Beck, M. Biroth, N. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, S. Costanza, A. Denig, E. J. Downie, P. Drexler, S. Fegan, A. Fix, S. Gardner, D. Ghosal , et al. (41 additional authors not shown)

Abstract: We report a first measurement of the double-polarisation observable, $C_{x'}$, in $π^+$ photoproduction off the proton. The $C_{x'}$ double-polarisation observable represents the transfer of polarisation from a circularly polarised photon beam to the recoiling neutron. The MAMI circularly polarised photon beam impinged on a liquid deuterium target cell, with reaction products detected in the Cryst… ▽ More We report a first measurement of the double-polarisation observable, $C_{x'}$, in $π^+$ photoproduction off the proton. The $C_{x'}$ double-polarisation observable represents the transfer of polarisation from a circularly polarised photon beam to the recoiling neutron. The MAMI circularly polarised photon beam impinged on a liquid deuterium target cell, with reaction products detected in the Crystal Ball calorimeter. Ancillary apparatus surrounding the target provided tracking, particle identification and determination of recoil nucleon polarisation. The $C_{x'}$ observable is determined for photon energies 800-1400 MeV, providing new constraints on models aiming to elucidate the spectrum and properties of nucleon resonances. This is the first determination of any polarisation observable from the beam-recoil group of observables for this reaction, providing a valuable constraint and systematic check of the current solutions of partial wave analysis based theoretical models. △ Less

Submitted 17 November, 2022; originally announced November 2022.

Polarization observables in double neutral pion photoproduction

Authors: CBELSA/TAPS Collaboration, :, T. Seifen, J. Hartmann, F. Afzal, A. V. Anisovich, R. Beck, M. Becker, A. Berlin, M. Bichow, K. -Th. Brinkmann, V. Crede, M. Dieterle, H. Dutz, H. Eberhardt, D. Elsner, K. Fornet-Ponse, St. Friedrich, F. Frommberger, Ch. Funke, M. Gottschall, M. Grüner, St. Görtz, E. Gutz, Ch. Hammann , et al. (52 additional authors not shown)

Abstract: Measurements of target asymmetries and double-polarization observables for the reaction $γp\to pπ^0π^0$ are reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility (Bonn University) using the Bonn frozen-spin butanol (C$_4$H$_9$OH) target, which provided transversely polarized protons. Linearly polarized photons were produced via bremsstrahlung off a diamond crystal. The… ▽ More Measurements of target asymmetries and double-polarization observables for the reaction $γp\to pπ^0π^0$ are reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility (Bonn University) using the Bonn frozen-spin butanol (C$_4$H$_9$OH) target, which provided transversely polarized protons. Linearly polarized photons were produced via bremsstrahlung off a diamond crystal. The data cover the photon energy range from $E_γ$=650 MeV to $E_γ$=2600 MeV and nearly the complete angular range. The results have been included in the BnGa partial wave analysis. Experimental results and the fit agree very well. Observed systematic differences in the branching ratios for decays of $N^*$ and $Δ^*$ resonances are attributed to the internal structure of these excited nucleon states. Resonances which can be assigned to SU(6)$\times$O(3) two-oscillator configurations show larger branching ratios to intermediate states with non-zero intrinsic orbital angular momenta than resonances assigned to one-oscillator configurations. △ Less

Submitted 3 June, 2025; v1 submitted 5 July, 2022; originally announced July 2022.

Comments: 22 pages, 27 figures; extended the introduction and clarified some sentences; added further references

Journal ref: Eur. Phys. J. A 61, 173 (2025)

First measurement of polarisation transfer $C^n_{x'}$ in deuteron photodisintegration

Authors: M. Bashkanov, D. P. Watts, S. J. D. Kay, S. Abt, P. Achenbach, P. Adlarson, F. Afzal, Z. Ahmed, C. S. Akondi, J. R. M. Annand, H. J. Arends, R. Beck, M. Biroth, N. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, S. Costanza, A. Denig, E. J. Downie, P. Drexler, S. Fegan, A. Fix, S. Gardner , et al. (44 additional authors not shown)

Abstract: A first measurement of the polarisation transfer from a circularly-polarised photon to the final state neutron ($C^n_{x'}$) in deuterium photodisintegration has been carried out. This quantity is determined over the photon energy range 370~--~700~MeV and for neutron centre-of-mass breakup angles $\sim45-120^{\circ}$. The polarisation of the final state neutrons was determined by an ancillary large… ▽ More A first measurement of the polarisation transfer from a circularly-polarised photon to the final state neutron ($C^n_{x'}$) in deuterium photodisintegration has been carried out. This quantity is determined over the photon energy range 370~--~700~MeV and for neutron centre-of-mass breakup angles $\sim45-120^{\circ}$. The polarisation of the final state neutrons was determined by an ancillary large-acceptance nucleon polarimeter, surrounding a cryogenic liquid deuterium target within the Crystal Ball detector at MAMI. The polarimeter characterised $(n,p)$ charge exchange of the ejected neutrons to determine their polarisation. The new $C^n_{x'}$ data are also compared to a theoretical model based on nucleonic and nucleon resonance degrees of freedom constrained by the current world-database of deuterium photodisintegration measurements. Structures in $C^n_{x'}$ observed in the region of the $d^*(2380)$ could not be explained by conventional models of deuteron photodisintegration. △ Less

Submitted 3 July, 2023; v1 submitted 24 June, 2022; originally announced June 2022.

Measurement of the helicity dependence for single $π^{0}$ photoproduction from the deuteron

Authors: The A2 collaboration, F. Cividini, M. Dieterle, S. Abt, P. Achenbach, P. Adlarson, F. Afzal, Z. Ahmed, J. R. M. Annand, H. J. Arends, M. Bashkanov, R. Beck, M. Biroth, N. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, S. Costanza, A. Denig, A. S. Dolzhikov, E. J. Downie, P. Drexler, S. Fegan, A. Fix , et al. (51 additional authors not shown)

Abstract: The helicity-dependent single $π^{0}$ photoproduction cross section on the deuteron and the angular dependence of the double polarisation observable $E$ for the quasi-free single $π^0$ production off the proton and the neutron have been measured for the first time from the threshold region up to the photon energy 1.4 GeV. The experiment was performed at the tagged photon facility of the MAMI accel… ▽ More The helicity-dependent single $π^{0}$ photoproduction cross section on the deuteron and the angular dependence of the double polarisation observable $E$ for the quasi-free single $π^0$ production off the proton and the neutron have been measured for the first time from the threshold region up to the photon energy 1.4 GeV. The experiment was performed at the tagged photon facility of the MAMI accelerator and used a circularly polarised photon beam and longitudinally polarised deuteron target. The reaction products were detected using the large acceptance Crystal Ball/TAPS calorimeter, which covered 97% of the full solid angle. Comparing the cross section from the deuteron with the sum of free nucleon cross sections provides a quantitative estimate of the effects of the nuclear medium on pion production. In contrast, comparison of $E$ helicity asymmetry data from quasi-free protons off deuterium with data from a free proton target indicates that nuclear effects do not significantly affect this observable. As a consequence, it is deduced that the helicity asymmetry $E$ on a free neutron can be reliably extracted from measurements on a deuteron in quasi-free kinematics. △ Less

Submitted 3 June, 2022; v1 submitted 1 March, 2022; originally announced March 2022.

Comments: 22 pages, 22 figures; version accepted for publication on Eur. Phys. J. A

Evidence of a dibaryon spectrum in coherent $π^0π^0 d$ photoproduction at forward deuteron angles

Authors: T. C. Jude, S. Alef, R. Beck, A. Braghieri, P. L. Cole, D. Elsner, R. Di Salvo, A. Fantini, O. Freyermuth, F. Frommberger, F. Ghio, A. Gridnev, K. Kohl, N. Kozlenko, A. Lapik, P. Levi Sandri, V. Lisin, G. Mandaglio, D. Moricciani, V. Nedorezov, D. Novinsky, P. Pedroni, A. Polonskiy, B. -E. Reitz, M. Romaniuk , et al. (5 additional authors not shown)

Abstract: The coherent reaction, $γd \rightarrow π^0π^0 d$ was studied with the BGOOD experiment at ELSA from threshold to a centre-of-mass energy of 2850\,MeV. A full kinematic reconstruction was made, with final state deuterons identified in the forward spectrometer and $π^0$ decays in the central BGO Rugby Ball. The strength of the differential cross section exceeds what can be described by models of coh… ▽ More The coherent reaction, $γd \rightarrow π^0π^0 d$ was studied with the BGOOD experiment at ELSA from threshold to a centre-of-mass energy of 2850\,MeV. A full kinematic reconstruction was made, with final state deuterons identified in the forward spectrometer and $π^0$ decays in the central BGO Rugby Ball. The strength of the differential cross section exceeds what can be described by models of coherent photoproduction and instead supports the three isoscalar dibaryon candidates reported by the ELPH collaboration at 2.38, 2.47 and 2.63\,GeV/c$^2$. A low mass enhancement in the $π^0π^0$ invariant mass is also observed at the $d^*(2380)$ centre-of-mass energy which is consistent with the ABC effect. At higher centre-of-mass energies, a narrow peak in the $π^0 d$ invariant mass at 2114\,MeV/c$^2$ with a width of 20\,MeV/c$^2$ supports a sequential two-dibaryon decay mechanism. △ Less

Submitted 29 October, 2024; v1 submitted 17 February, 2022; originally announced February 2022.

Comments: 8 pages, 10 figures. Additional references (recommended by colleagues) added since the first revision

Measurement of Compton scattering at MAMI for the extraction of the electric and magnetic polarizabilities of the proton

Authors: A2 Collaboration, E. Mornacchi, P. P. Martel, S. Abt, P. Achenbach, P. Adlarson, F. Afzal, Z. Ahmed, J. R. M. Annand, H. J. Arends, M. Bashkanov, R. Beck, M. Biroth, N. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, S. Costanza, A. Denig, A. S. Dolzhikov, E. J. Downie, P. Drexler, S. Fegan, S. Gardner , et al. (43 additional authors not shown)

Abstract: A precise measurement of the differential cross-sections $dσ/dΩ$ and the linearly polarized photon beam asymmetry $Σ_3$ for Compton scattering on the proton below pion threshold has been performed with a tagged photon beam and almost $4π$ detector at the Mainz Microtron. The incident photons were produced by the recently upgraded Glasgow-Mainz photon tagging facility and impinged on a cryogenic li… ▽ More A precise measurement of the differential cross-sections $dσ/dΩ$ and the linearly polarized photon beam asymmetry $Σ_3$ for Compton scattering on the proton below pion threshold has been performed with a tagged photon beam and almost $4π$ detector at the Mainz Microtron. The incident photons were produced by the recently upgraded Glasgow-Mainz photon tagging facility and impinged on a cryogenic liquid hydrogen target, with the scattered photons detected in the Crystal Ball/TAPS set-up. Using the highest statistics Compton scattering data ever measured on the proton along with two effective field theories (both covariant baryon and heavy-baryon) and one fixed-$t$ dispersion relation model, constraining the fits with the Baldin sum rule, we have obtained the proton electric and magnetic polarizabilities with unprecedented precision: \begin{align*} &{}α_{E1} = 10.99 \pm 0.16 \pm 0.47 \pm 0.17 \pm 0.34 &{}β_{M1} = 3.14 \pm 0.21 \pm 0.24 \pm 0.20 \pm 0.35 \end{align*} in units of $10^{-4}$\,fm$^3$ where the errors are statistical, systematic, spin polarizability dependent and model dependent. △ Less

Submitted 3 March, 2022; v1 submitted 29 October, 2021; originally announced October 2021.

Observation of a structure in the M$_{pη}$ invariant mass distribution near 1700 MeV/$c^2$ in the $\mathbf{γp \rightarrow p π^0 η} $ reaction

Authors: V. Metag, M. Nanova, J. Hartmann, P. Mahlberg, F. Afzal, C. Bartels, D. Bayadilov, R. Beck, M. Becker, E. Blanke, K. -T. Brinkmann, S. Ciupka, V. Crede, M. Dieterle, H. Dutz, D. Elsner, F. Frommberger, A. Gridnev, M. Gottschall, M. Grüner, Ch. Hammann, J. Hannappel, W. Hillert, J. Hoff, Ph. Hoffmeister , et al. (52 additional authors not shown)

Abstract: The reaction $γp \rightarrow p π^0 η$ has been studied with the CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for incident photon energies from threshold up to 3.1 GeV. This paper has been motivated by the recently claimed observation of a narrow structure in the M$_{Nη}$ invariant mass distribution at a mass of 1678 MeV/$c^2$. The existence of this structure cannot be co… ▽ More The reaction $γp \rightarrow p π^0 η$ has been studied with the CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for incident photon energies from threshold up to 3.1 GeV. This paper has been motivated by the recently claimed observation of a narrow structure in the M$_{Nη}$ invariant mass distribution at a mass of 1678 MeV/$c^2$. The existence of this structure cannot be confirmed in the present work. Instead, for E$_γ$ = 1400 - 1500 MeV and the cut M$_{pπ^0} \le 1190 $ MeV/$c^2$ a statistically significant structure in the M$_{pη}$ invariant mass distribution near 1700 MeV/$c^2$ is observed with a width of $Γ\approx 35$ MeV/$c^2$ while the mass resolution is $σ_{res}$ = 5 MeV/$c^2$. Increasing the incident photon energy from 1420 to 1540 MeV this structure shifts in mass from $\approx$ 1700MeV/c$^2$ to $\approx$ 1725 MeV/$c^2$; the width increases to about 50 MeV/$c^2$ and decreases thereafter. The cross section associated with this structure reaches a maximum of $\approx$ 100 nb around E$_γ \approx$ 1490 MeV (W $\approx $ 1920 MeV), which coincides with the $p a_0$ threshold. Three scenarios are discussed which might be the origin of this structure in the M$_{pη}$ invariant mass distribution. The most likely interpretation is that it is due to a triangular singularity in the $γp \rightarrow p a_0 \rightarrow p π^0 η$ reaction △ Less

Submitted 18 November, 2021; v1 submitted 11 October, 2021; originally announced October 2021.

Comments: 16 pages, 18 figure

Measurement of the $γn\rightarrow K^0Σ^0$ differential cross section over the $K^*$ threshold

Authors: K. Kohl, T. C. Jude, S. Alef, R. Beck, A. Braghieri, P. Cole, R. D. Salvo, A. Fantini, O. Freyermuth, F. Frommberger, F. Ghio, A. Gridnev, D. Hammann, J. Hannappel, N. Kozlenko, A. Lapik, P. Levi Sandri, V. Lisin, G. Mandaglio, R. Messi, D. Moricciani, V. Nedorezov, V. A. Nikonov, D. Novinsky, P. Pedroni , et al. (8 additional authors not shown)

Abstract: The differential cross section for the quasi-free photoproduction reaction $γn\rightarrow K^0Σ^0$ was measured at BGOOD at ELSA from threshold to a centre-of-mass energy of 2400 MeV. Close to threshold the results are consistent with existing data and are in agreement with partial wave analysis solutions over the full measured energy range, with a large coupling to the $Δ$(1900)1/2- evident. This… ▽ More The differential cross section for the quasi-free photoproduction reaction $γn\rightarrow K^0Σ^0$ was measured at BGOOD at ELSA from threshold to a centre-of-mass energy of 2400 MeV. Close to threshold the results are consistent with existing data and are in agreement with partial wave analysis solutions over the full measured energy range, with a large coupling to the $Δ$(1900)1/2- evident. This is the first dataset covering the K* threshold region, where there are model predictions of dynamically generated vector meson-baryon resonance contributions. △ Less

Submitted 22 September, 2023; v1 submitted 30 August, 2021; originally announced August 2021.

Comments: 6 pages, 8 figures

Photoproduction of $K^+Λ(1405) \rightarrow K^+π^0Σ^0$ extending to forward angles and low momentum transfer

Authors: G. Scheluchin, T. C. Jude, S. Alef, P. Bauer, D. Bayadilov, R. Beck, A. Braghieri, P. L. Cole, R. Di Salvo, D. Elsner, A. Fantini, O. Freyermuth, F. Fromberger, F. Ghio, A. Gridnev, D. Hammann, J. Hannappel, K. Kohl, N. Kozlenko, A. Lapik, P. Levi Sandri, V. Lisin, G. Mandaglio, R. Messi, D. Moricciani , et al. (10 additional authors not shown)

Abstract: $K^+Λ(1405)$ photoproduction has been studied at the BGOOD experiment via the all neutral decay, $Λ(1405)\rightarrowΣ^0π^0$. The unique BGOOD experimental setup allows both the cross section and $Λ(1405)$ invariant mass distribution (line shape) to be measured over a broad $K^+$ polar angle range, extending to extreme forward $K^+… ▽ More $K^+Λ(1405)$ photoproduction has been studied at the BGOOD experiment via the all neutral decay, $Λ(1405)\rightarrowΣ^0π^0$. The unique BGOOD experimental setup allows both the cross section and $Λ(1405)$ invariant mass distribution (line shape) to be measured over a broad $K^+$ polar angle range, extending to extreme forward $K^+$ angles unattainable at previous experiments. Evidence is provided for the role of a triangle singularity driven by the $N^*(2030)$ resonance, which appears to contribute significantly to $K^+Λ(1405)$ photoproduction. This is observed in both the angular distributions and the integrated cross section which was determined with unprecedented energy resolution. The measured line shape is also in agreement with the previous results of CLAS and ANKE, and is consistent with two poles derived in $χ$PT based models. △ Less

Submitted 6 April, 2022; v1 submitted 27 August, 2021; originally announced August 2021.

Single $π^0$ Production Off Neutrons Bound in Deuteron with Linearly Polarized Photons

Authors: C. Mullen, S. Gardner, D. I. Glazier, S. J. D. Kay, K. Livingston, I. I. Strakovsky, R. L. Workman, S. Abt, P. Achenbach, F. Afzal, Z. Ahmed, C. S. Akondi, J. R. M. Annand, M. Bashkanov, R. Beck, M. Biroth, N. S. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, S. Costanza, A. Denig, M. Dieterle, E. J. Downie , et al. (57 additional authors not shown)

Abstract: The quasifree $\overrightarrowγ d\toπ^0n(p)$ photon beam asymmetry, $Σ$, has been measured at photon energies, $E_γ$, from 390 to 610 MeV, corresponding to center of mass energy from 1.271 to 1.424 GeV, for the first time. The data were collected in the A2 hall of the MAMI electron beam facility with the Crystal Ball and TAPS calorimeters covering pion center-of-mass angles from 49 to 148$^\circ$.… ▽ More The quasifree $\overrightarrowγ d\toπ^0n(p)$ photon beam asymmetry, $Σ$, has been measured at photon energies, $E_γ$, from 390 to 610 MeV, corresponding to center of mass energy from 1.271 to 1.424 GeV, for the first time. The data were collected in the A2 hall of the MAMI electron beam facility with the Crystal Ball and TAPS calorimeters covering pion center-of-mass angles from 49 to 148$^\circ$. In this kinematic region, polarization observables are sensitive to contributions from the $Δ(1232)$ and $N(1440)$ resonances. The extracted values of $Σ$ have been compared to predictions based on partial-wave analyses (PWAs) of the existing pion photoproduction database. Our comparison includes the SAID, MAID, and Bonn-Gatchina analyses; while a revised SAID fit, including the new $Σ$ measurements, has also been performed. In addition, isospin symmetry is examined as a way to predict $π^0n$ photoproduction observables, based on fits to published data in the channels $π^0p$, $π^+n$, and $π^-p$. △ Less

Submitted 16 March, 2021; v1 submitted 15 March, 2021; originally announced March 2021.

Comments: 11 pages, 9 figures, 5 tables; fixed 2 glitches

PANDA Phase One

Authors: G. Barucca, F. Davì, G. Lancioni, P. Mengucci, L. Montalto, P. P. Natali, N. Paone, D. Rinaldi, L. Scalise, B. Krusche, M. Steinacher, Z. Liu, C. Liu, B. Liu, X. Shen, S. Sun, G. Zhao, J. Zhao, M. Albrecht, W. Alkakhi, S. Bökelmann, S. Coen, F. Feldbauer, M. Fink, J. Frech , et al. (399 additional authors not shown)

Abstract: The Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany, provides unique possibilities for a new generation of hadron-, nuclear- and atomic physics experiments. The future antiProton ANnihilations at DArmstadt (PANDA or $\overline{\rm P}$ANDA) experiment at FAIR will offer a broad physics programme, covering different aspects of the strong interaction. Understanding the latter in… ▽ More The Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany, provides unique possibilities for a new generation of hadron-, nuclear- and atomic physics experiments. The future antiProton ANnihilations at DArmstadt (PANDA or $\overline{\rm P}$ANDA) experiment at FAIR will offer a broad physics programme, covering different aspects of the strong interaction. Understanding the latter in the non-perturbative regime remains one of the greatest challenges in contemporary physics. The antiproton-nucleon interaction studied with PANDA provides crucial tests in this area. Furthermore, the high-intensity, low-energy domain of PANDA allows for searches for physics beyond the Standard Model, e.g. through high precision symmetry tests. This paper takes into account a staged approach for the detector setup and for the delivered luminosity from the accelerator. The available detector setup at the time of the delivery of the first antiproton beams in the HESR storage ring is referred to as the \textit{Phase One} setup. The physics programme that is achievable during Phase One is outlined in this paper. △ Less

Submitted 9 June, 2021; v1 submitted 28 January, 2021; originally announced January 2021.

Comments: 35 pages, 15 figures

Journal ref: European Physics Journal A57, 44 (2021)

Measurement of the Charge-Averaged Elastic Lepton-Proton Scattering Cross Section by the OLYMPUS Experiment

Authors: J. C. Bernauer, A. Schmidt, B. S. Henderson, L. D. Ice, D. Khaneft, C. O'Connor, R. Russell, N. Akopov, R. Alarcon, O. Ates, A. Avetisyan, R. Beck, S. Belostotski, J. Bessuille, F. Brinker, J. R. Calarco, V. Carassiti, E. Cisbani, G. Ciullo, M. Contalbrigo, R. De Leo, J. Diefenbach, T. W. Donnelly, K. Dow, G. Elbakian , et al. (45 additional authors not shown)

Abstract: We report the first measurement of the average of the electron-proton and positron-proton elastic scattering cross sections. This lepton charge-averaged cross section is insensitive to the leading effects of hard two-photon exchange, giving more robust access to the proton's electromagnetic form factors. The cross section was extracted from data taken by the OLYMPUS experiment at DESY, in which al… ▽ More We report the first measurement of the average of the electron-proton and positron-proton elastic scattering cross sections. This lepton charge-averaged cross section is insensitive to the leading effects of hard two-photon exchange, giving more robust access to the proton's electromagnetic form factors. The cross section was extracted from data taken by the OLYMPUS experiment at DESY, in which alternating stored electron and positron beams were scattered from a windowless gaseous hydrogen target. Elastic scattering events were identified from the coincident detection of the scattered lepton and recoil proton in a large-acceptance toroidal spectrometer. The luminosity was determined from the rates of Møller, Bhabha and elastic scattering in forward electromagnetic calorimeters. The data provide some selectivity between existing form factor global fits and will provide valuable constraints to future fits. △ Less

Submitted 28 September, 2023; v1 submitted 12 August, 2020; originally announced August 2020.

Comments: 6 pages, 3 figures, submitted to PRL

Report number: DESY 20-132

Journal ref: Phys. Rev. Lett. 126, 162501 (2021)

Strangeness photoproduction at the BGO-OD experiment

Authors: Georg Scheluchin, Stefan Alef, Patrick Bauer, Reinhard Beck, Alessandro Braghieri, Philip Cole, Rachele Di Salvo, Daniel Elsner, Alessia Fantini, Oliver Freyermuth, Francesco Ghio, Anatoly Gridnev, Daniel Hammann, Jürgen Hannappel, Thomas Jude, Katrin Kohl, Nikolay Kozlenko, Alexander Lapik, Paolo Levi Sandri, Valery Lisin, Giuseppe Mandaglio, Roberto Messi, Dario Moricciani, Vladimir Nedorezov, Dmitry Novinsky , et al. (8 additional authors not shown)

Abstract: The BGO-OD experiment at the University of Bonn's ELSA accelerator facility in Germany is ideally suited to investigate photoproduction at extreme forward angles. It combines a highly segmented BGO electromagnetic calorimeter at central angles and an open dipole magnetic spectrometer in the forward direction. This allows the detection of forward going kaons, and complex final states of mixed charg… ▽ More The BGO-OD experiment at the University of Bonn's ELSA accelerator facility in Germany is ideally suited to investigate photoproduction at extreme forward angles. It combines a highly segmented BGO electromagnetic calorimeter at central angles and an open dipole magnetic spectrometer in the forward direction. This allows the detection of forward going kaons, and complex final states of mixed charge from hyperon decays. Current projects at the BGO-OD experiment include strangeness production of $γp \rightarrow K^+ Λ/Σ^0$ at forward angles, $K^0Σ^0$ with a deuteron target and $K^+Λ(1405)$ line shape and cross section measurements. △ Less

Submitted 6 August, 2020; v1 submitted 5 August, 2020; originally announced August 2020.

Comments: Contribution to the proceedings of "HADRON2019: 18th International Conference on Hadron Spectroscopy and Structure", limited to 5 pages

$K^+Λ$(1405) photoproduction at the BGO-OD experiment

Authors: Georg Scheluchin, Stefan Alef, Patrick Bauer, Reinhard Beck, Alessandro Braghieri, Philip Cole, Rachele Di Salvo, Daniel Elsner, Alessia Fantini, Oliver Freyermuth, Francesco Ghio, Anatoly Gridnev, Daniel Hammann, Jürgen Hannappel, Thomas Jude, Katrin Kohl, Nikolay Kozlenko, Alexander Lapik, Paolo Levi Sandri, Valery Lisin, Giuseppe Mandaglio, Roberto Messi, Dario Moricciani, Vladimir Nedorezov, Dmitry Novinsky , et al. (8 additional authors not shown)

Abstract: Since the discovery of the $Λ(1405)$, it remains poorly described by conventional constituent quark models, and it is a candidate for having an "exotic" meson-baryon or "penta-quark" structure, similar to states recently reported in the hidden charm sector. The $Λ(1405)$ can be produced in the reaction $γp \rightarrow K^+Λ(1405)$. The pure I=0 decay mode into $Σ^0π^0$ is prohibited for the mass-… ▽ More Since the discovery of the $Λ(1405)$, it remains poorly described by conventional constituent quark models, and it is a candidate for having an "exotic" meson-baryon or "penta-quark" structure, similar to states recently reported in the hidden charm sector. The $Λ(1405)$ can be produced in the reaction $γp \rightarrow K^+Λ(1405)$. The pure I=0 decay mode into $Σ^0π^0$ is prohibited for the mass-overlapping $Σ(1385)$. Combining a large aperture forward magnetic spectrometer and a central BGO crystal calorimeter, the BGO-OD experiment is ideally suited to measure this decay with the $K^+$ in the forward direction. Preliminary results are presented. *Supported by DFG (PN 388979758, 405882627). △ Less

Submitted 19 August, 2020; v1 submitted 17 July, 2020; originally announced July 2020.

Comments: Contribution to the proceedings of "NSTAR2019: 12th International Workshop on the Physics of Excited Nucleons"

Strangeness Photoproduction at the BGO-OD experiment

Authors: T. C. Jude, S. Alef, P. Bauer, R. Beck, A. Braghieri, P. L. Cole, R. Di Salvo, D. Elsner, A. Fantini, O. Freyermuth, F. Ghio, A. Gridnev, D. Hammann, J. Hannappel, K. Kohl, N. Kozlenko, A. Lapik, P. Levi Sandri, V. Lisin, G. Mandaglio, R. Messi, D. Moricciani, V. Nedorezov, D. Novinsky, P. Pedroni , et al. (8 additional authors not shown)

Abstract: The BGO-OD experiment at the ELSA accelerator facility uses an energy tagged bremsstrahlung photon beam to investigate the excitation structure of the nucleon. The setup consists of a highly segmented BGO calorimeter surrounding the target, with a particle tracking magnetic spectrometer at forward angles. BGO-OD is ideal for investigating low momentum transfer processes due to the acceptance and… ▽ More The BGO-OD experiment at the ELSA accelerator facility uses an energy tagged bremsstrahlung photon beam to investigate the excitation structure of the nucleon. The setup consists of a highly segmented BGO calorimeter surrounding the target, with a particle tracking magnetic spectrometer at forward angles. BGO-OD is ideal for investigating low momentum transfer processes due to the acceptance and high momentum resolution at forward angles. In particular, this enables the investigation of strangeness photoproduction where t-channel exchange mechanisms play an important role. This also allows access to low momentum exchange kinematics where extended, molecular structure may manifest in reaction mechanisms. First key results at low $t$ indicate a cusp-like structure in $K^+Σ^0$ photoproduction at $W = 1900$\,MeV, line shapes and differential cross sections for $K^+Λ$(1405)$\rightarrow K^+Σ^0π^0$, and a peak structure in $K^0_SΣ^0$ photoproduction. The peak in the $K^0_SΣ^0$ channel appears consistent with meson-baryon generated states, where equivalent models have been used to describe the $P_C$ pentaquark candidates in the heavy charmed quark sector. △ Less

Submitted 17 July, 2020; originally announced July 2020.

Comments: Proceedings of The 12th International Workshop on the Physics of Excited Nucleons (NSTAR 2019)

Helicity-dependent cross sections for the photoproduction of $π^0$ pairs from nucleons

Authors: M. Dieterle, L. Witthauer, A. Fix, S. Abt, P. Achenbach, P. Adlarson, F. Afzal, P. Aguar Bartolome, Z. Ahmed, J. R. M. Annand, H. J. Arends, M. Bashkanov, R. Beck, M. Biroth, N. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, S. Costanza, A. Denig, A. S. Dolzhikov, E. J. Downie, P. Drexler, S. Gardner , et al. (66 additional authors not shown)

Abstract: The double-polarization observable $E$ and helicity-dependent cross sections $σ_{1/2}$, $σ_{3/2}$ have been measured for the photoproduction of $π^0$ pairs off quasi-free protons and neutrons at the Mainz MAMI accelerator with the Crystal Ball/TAPS setup. A circularly polarized photon beam was produced by bremsstrahlung from longitudinally polarized electrons and impinged on a longitudinally polar… ▽ More The double-polarization observable $E$ and helicity-dependent cross sections $σ_{1/2}$, $σ_{3/2}$ have been measured for the photoproduction of $π^0$ pairs off quasi-free protons and neutrons at the Mainz MAMI accelerator with the Crystal Ball/TAPS setup. A circularly polarized photon beam was produced by bremsstrahlung from longitudinally polarized electrons and impinged on a longitudinally polarized deuterated butanol target. The reaction products were detected with an almost $4π$ covering calorimeter. The results reveal for the first time the helicity- and isospin-dependent structure of the $γN\rightarrow Nπ^0π^0$ reaction. They are compared to predictions from reaction models in view of nucleon resonance contributions and also to a refit of one model that predicted results for the proton and for the neutron target. The comparison of the prediction and the refit demonstrate the large impact of the new data. △ Less

Submitted 12 July, 2020; originally announced July 2020.

Comments: Submitted to Phys. Rev. Lett

Journal ref: Phys. Rev. Lett. 125, 062001 (2020)

Observation of a cusp-like structure in the $γp \rightarrow K^+Σ^0$ cross section at forward angles and low momentum transfer

Authors: T. C. Jude, S. Alef, P. Bauer, D. Bayadilov, R. Beck, A. Bella, J. Bieling, A. Braghieri, P. L. Cole, D. Elsner, R. Di Salvo, A. Fantini, O. Freyermuth, F. Frommberger, F. Ghio, S. Goertz, A. Gridnev, D. Hammann, J. Hannappel, K. Kohl, N. Kozlenko, A. Lapik P. Levi Sandri, V. Lisin, G. Mandaglio, F. Messi , et al. (17 additional authors not shown)

Abstract: The $γp \rightarrow K^+Σ^0$ differential cross section at extremely forward angles was measured at the BGOOD experiment. A three-quarter drop in strength over a narrow range in energy and a strong dependence on the polar angle of the $K^+$ in the centre-of-mass of the reaction is observed at a centre-of-mass energy of 1900\,MeV. Residing close to multiple open and hidden strangeness thresholds, th… ▽ More The $γp \rightarrow K^+Σ^0$ differential cross section at extremely forward angles was measured at the BGOOD experiment. A three-quarter drop in strength over a narrow range in energy and a strong dependence on the polar angle of the $K^+$ in the centre-of-mass of the reaction is observed at a centre-of-mass energy of 1900\,MeV. Residing close to multiple open and hidden strangeness thresholds, the structure appears consistent with meson-baryon threshold effects which may contribute to the reaction mechanism. △ Less

Submitted 4 August, 2021; v1 submitted 22 June, 2020; originally announced June 2020.

Comments: 9 pages (including a 2 page appendix of tabulated data), 10 figures

$K^+Λ$ photoproduction at forward angles and low momentum transfer

Authors: S. Alef, P. Bauer, D. Bayadilov, R. Beck, A. Bella, J. Bieling, A. Braghieri, P. L. Cole, D. Elsner, R. Di Salvo, A. Fantini, O. Freyermuth, F. Frommberger, F. Ghio, S. Goertz, A. Gridnev, D. Hammann, J. Hannappel, T. C. Jude, K. Kohl, N. Kozlenko, A. Lapik P. Levi Sandri, V. Lisin, G. Mandaglio, F. Messi , et al. (17 additional authors not shown)

Abstract: $γp \rightarrow K^+ Λ$ differential cross sections and recoil polarisation data from threshold for extremely forward angles are presented. The measurements were performed at the BGOOD experiment at ELSA, utilising the high angular and momentum resolution forward spectrometer for charged particle identification. The data discriminates between conflicting results in the world data set and enable ext… ▽ More $γp \rightarrow K^+ Λ$ differential cross sections and recoil polarisation data from threshold for extremely forward angles are presented. The measurements were performed at the BGOOD experiment at ELSA, utilising the high angular and momentum resolution forward spectrometer for charged particle identification. The data discriminates between conflicting results in the world data set and enable extraction of the cross section as the minimum momentum transfer to the recoiling hyperon is approached. △ Less

Submitted 1 March, 2021; v1 submitted 22 June, 2020; originally announced June 2020.

Comments: 12 pages (including a 2 page appendix), 14 figures

Journal ref: Eur. Phys. J. A 57, 80 (2021)

Antiproton over proton and K$^-$ over K$^+$ multiplicity ratios at high $z$ in DIS

Authors: M. G. Alexeev, G. D. Alexeev, A. Amoroso, V. Andrieux, V. Anosov, A. Antoshkin, K. Augsten, W. Augustyniak, C. D. R. Azevedo, B. Badelek, F. Balestra, M. Ball, J. Barth, R. Beck, Y. Bedfer, J. Berenguer Antequera, J. Bernhard, M. Bodlak, F. Bradamante, A. Bressan, M. Buechele, V. E. Burtsev, W. -C. Chang, C. Chatterjee, M. Chiosso , et al. (174 additional authors not shown)

Abstract: The $\bar{\rm p} $ over p multiplicity ratio is measured in deep-inelastic scattering for the first time using (anti-) protons carrying a large fraction of the virtual-photon energy, $z>0.5$. The data were obtained by the COMPASS Collaboration using a 160 GeV muon beam impinging on an isoscalar $^6$LiD target. The regime of deep-inelastic scattering is ensured by requiring $Q^2$ > 1 (GeV/$c$)$^2$… ▽ More The $\bar{\rm p} $ over p multiplicity ratio is measured in deep-inelastic scattering for the first time using (anti-) protons carrying a large fraction of the virtual-photon energy, $z>0.5$. The data were obtained by the COMPASS Collaboration using a 160 GeV muon beam impinging on an isoscalar $^6$LiD target. The regime of deep-inelastic scattering is ensured by requiring $Q^2$ > 1 (GeV/$c$)$^2$ for the photon virtuality and $W > 5$ GeV/$c^2$ for the invariant mass of the produced hadronic system. The range in Bjorken-$x$ is restricted to $0.01 < x < 0.40$. Protons and antiprotons are identified in the momentum range $20 ÷60$ GeV/$c$. In the whole studied $z$-region, the $\bar{\rm p}$ over p multiplicity ratio is found to be below the lower limit expected from calculations based on leading-order perturbative Quantum Chromodynamics (pQCD). Extending our earlier analysis of the K$^-$ over K$^+$ multiplicity ratio by including now events with larger virtual-photon energies, this ratio becomes closer to the expectation of next-to-leading order pQCD. The results of both analyses strengthen our earlier conclusion that the phase space available for hadronisation should be taken into account in the pQCD formalism. △ Less

Submitted 26 March, 2020; originally announced March 2020.

Comments: 19 pages, 7 figures, 3 tables

Report number: CERN-EP-2020-041

Signatures of the $d^*(2380)$ hexaquark in d($γ$,$p\vec{n}$)

Authors: M. Bashkanov, D. P. Watts, S. J. D. Kay, S. Abt, P. Achenbach, P. Adlarson, F. Afzal, P. Aguar Bartolome, Z. Ahmed, C. S. Akondi, J. R. M. Annand, H. J. Arends, R. Beck, M. Biroth, N. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, S. Costanza, A. Denig, M. Dieterle, E. J. Downie, P. Drexler, S. Garni , et al. (52 additional authors not shown)

Abstract: We report a measurement of the spin polarisation of the recoiling neutron in deuterium photodisintegration, utilising a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured photon energy range of 300~--~700~MeV provides the first measurement of recoil neutron polarisation at photon energies where the quark substructure of the deuteron plays a role, thereby providing impor… ▽ More We report a measurement of the spin polarisation of the recoiling neutron in deuterium photodisintegration, utilising a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured photon energy range of 300~--~700~MeV provides the first measurement of recoil neutron polarisation at photon energies where the quark substructure of the deuteron plays a role, thereby providing important new constraints on photodisintegration mechanisms. A very high neutron polarisation in a narrow structure centred around $E_γ\sim$~570~MeV is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition suggests this behaviour could be related to the excitation of the $d^*(2380)$ hexaquark. △ Less

Submitted 19 November, 2019; originally announced November 2019.

Journal ref: Phys. Rev. Lett. 124, 132001 (2020)

The BGOOD experimental setup at ELSA

Authors: S. Alef, P. Bauer, D. Bayadilov, R. Beck, M. Becker, A. Bella, J. Bieling, S. Boese, A. Braghieri, K. -Th. Brinkmann, P. Cole, R. Di Salvo, D. Elsner, A. Fantini, O. Freyermuth, F. Frommberger, G. Gervino, F. Ghio, S. Goertz, A. Gridnev, E. Gutz, D. Hammann, J. Hannappel, W. Hillert, O. Jahn , et al. (36 additional authors not shown)

Abstract: The BGOOD experiment at the ELSA facility in Bonn has been commissioned within the framework of an international collaboration. The experiment pursues a systematic investigation of non-strange and strange meson photoproduction, in particular $t$-channel processes at low momentum transfer. The setup uniquely combines a central almost $4π$ acceptance BGO crystal calorimeter with a large aperture for… ▽ More The BGOOD experiment at the ELSA facility in Bonn has been commissioned within the framework of an international collaboration. The experiment pursues a systematic investigation of non-strange and strange meson photoproduction, in particular $t$-channel processes at low momentum transfer. The setup uniquely combines a central almost $4π$ acceptance BGO crystal calorimeter with a large aperture forward magnetic spectrometer providing excellent detection of both neutral and charged particles, complementary to other setups such as Crystal Barrel, Crystal Ball, LEPS and CLAS. △ Less

Submitted 18 February, 2020; v1 submitted 24 October, 2019; originally announced October 2019.

Comments: 26 pages, 37 figures

Journal ref: Eur. Phys. J. A (2020) 56:104

New data on $\vecγ \vec{p}\rightarrow ηp$ with polarized photons and protons and their implications for $N^* \to Nη$ decays

Authors: J. Müller, J. Hartmann, M. Grüner, F. Afzal, A. V. Anisovich, B. Bantes, D. Bayadilov, R. Beck, M. Becker, Y. Beloglazov, M. Berlin, M. Bichow, S. Böse, K. -T. Brinkmann, T. Challand, V. Crede, F. Dietz, M. Dieterle, P. Drexler, H. Dutz, H. Eberhardt, D. Elsner, R. Ewald, K. Fornet-Ponse, S. Friedrich , et al. (64 additional authors not shown)

Abstract: The polarization observables $T, E, P, H$, and $G$ in photoproduction of $η$ mesons off protons are measured for photon energies from threshold to $W=2400\,$MeV ($T$), 2280 MeV ($E$), 1620 MeV ($P, H$), or 1820 MeV ($G$), covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, JüBo, and BnGa partial-wave analyses. A refit within the BnGa approach including f… ▽ More The polarization observables $T, E, P, H$, and $G$ in photoproduction of $η$ mesons off protons are measured for photon energies from threshold to $W=2400\,$MeV ($T$), 2280 MeV ($E$), 1620 MeV ($P, H$), or 1820 MeV ($G$), covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, JüBo, and BnGa partial-wave analyses. A refit within the BnGa approach including further data yields precise branching ratios for the $Nη$ decay of nucleon resonances. A $Nη$-branching ratio of $0.33\pm 0.04$ for $N(1650)1/2^-$ is found, which reduces the large and controversially discussed $Nη$-branching ratio difference of the two lowest mass $J^P=1/2^-$-resonances significantly. △ Less

Submitted 18 September, 2019; originally announced September 2019.

Comments: 10 pages, 11 figures

Spin polarizabilities of the proton by measurement of Compton double-polarization observables

Authors: D. Paudyal, P. P. Martel, G. M. Huber, D. Hornidge, S. Abt, P. Achenbach, P. Adlarson, F. Afzal, Z. Ahmed, C. S. Akondi, J. R. M. Annand, H. J. Arends, M. Bashkanov, R. Beck, M. Biroth, N. S. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, S. Costanza, C. Collicott, A. Denig, M. Dieterle, E. J. Downie, P. Drexler , et al. (68 additional authors not shown)

Abstract: The Compton double-polarization observable $Σ_{2z}$ has been measured for the first time in the $Δ(1232)$ resonance region using a circularly polarized photon beam incident on a longitudinally polarized target at the Mainz Microtron. This paper reports these results, together with the model-dependent extraction of four proton spin polarizabilities from fits to additional asymmetry data using dispe… ▽ More The Compton double-polarization observable $Σ_{2z}$ has been measured for the first time in the $Δ(1232)$ resonance region using a circularly polarized photon beam incident on a longitudinally polarized target at the Mainz Microtron. This paper reports these results, together with the model-dependent extraction of four proton spin polarizabilities from fits to additional asymmetry data using dispersion relation and chiral perturbation theory calculations, with the former resulting in: $γ_{E1E1} = -3.18 \pm 0.52$, $γ_{M1M1} = 2.98 \pm 0.43$, $γ_{E1M2} = -0.44 \pm 0.67$ and $γ_{M1E2} = 1.58 \pm 0.43$, in units of $10^{-4}~\mathrm{fm}^{4}$. △ Less

Submitted 26 August, 2020; v1 submitted 4 September, 2019; originally announced September 2019.

Comments: 7 pages, 5 figures, 2 tables, accepted for publication in PRC

Journal ref: Phys. Rev. C 102, 035205 (2020)

Cross Section for $γn \to π^0 n$ measured at Mainz/A2

Authors: W. J. Briscoe, M. Hadzimehmedovi, A. E. Kudryavtsev, V. V. Kulikov, M. A. Martemianov, I. I. Strakovsky, A. Svarc, V. E. Tarasov, R. L. Workman, S. Abt, P. Achenbach, C. S. Akondi, F. Afzal, P. Aguar-Bartolome, Z. Ahmed, J. R. M. Annand, H. J. Arends, K. Bantawa, M. Bashkanov, R. Beck, M. Biroth, N. Borisov, A. Braghieri, S. A. Bulychjov, F. Cividini , et al. (67 additional authors not shown)

Abstract: The $γn \to π^0 n$ differential cross section evaluated for 27 energy bins span the photon-energy range 290-813 MeV (W = 1.195-1.553 GeV) and the pion c.m. polar production angles, ranging from 18 deg to 162 deg, making use of model-dependent nuclear corrections to extract pi0 production data on the neutron from measurements on the deuteron target. Additionally, the total photoabsorption cross sec… ▽ More The $γn \to π^0 n$ differential cross section evaluated for 27 energy bins span the photon-energy range 290-813 MeV (W = 1.195-1.553 GeV) and the pion c.m. polar production angles, ranging from 18 deg to 162 deg, making use of model-dependent nuclear corrections to extract pi0 production data on the neutron from measurements on the deuteron target. Additionally, the total photoabsorption cross section was measured. The tagged photon beam produced by the 883-MeV electron beam of the Mainz Microtron MAMI was used for the 0-meson production. Our accumulation of 3.6 x 10^6 $γn \to π^0 n$ events allowed a detailed study of the reaction dynamics. Our data are in reasonable agreement with previous A2 measurements and extend them to lower energies. The data are compared to predictions of previous SAID, MAID, and BnGa partial-wave analyses and to the latest SAID fit MA19 that included our data. Selected photon decay amplitudes $N^* \to γn$ at the resonance poles are determined for the first time. △ Less

Submitted 7 August, 2019; originally announced August 2019.

Comments: 16 pages, 12 figures, 3 tables

Journal ref: Phys. Rev. C 100, 065205 (2019)

Measurement of the beam-helicity asymmetry in photoproduction of $π^{0}η$ pairs on carbon, aluminum, and lead

Authors: V. Sokhoyan, S. Prakhov, A. Fix, S. Abt, P. Achenbach, P. Adlarson, F. Afzal, P. Aguar-Bartolomé, Z. Ahmed, K. Altangerel, J. R. M. Annand, H. J. Arends, K. Bantawa, M. Bashkanov, R. Beck, M. Biroth, N. S. Borisov, A. Braghieri, W. J. Briscoe, S. Cherepnya, F. Cividini, C. Collicott, S. Costanza, A. Denig, M. Dieterle , et al. (66 additional authors not shown)

Abstract: The beam-helicity asymmetry was measured, for the first time, in photoproduction of $π^{0}η$ pairs on carbon, aluminum, and lead, with the A2 experimental setup at MAMI. The results are compared to an earlier measurement on a free proton and to the corresponding theoretical calculations. The Mainz model is used to predict the beam-helicity asymmetry for the nuclear targets. The present results ind… ▽ More The beam-helicity asymmetry was measured, for the first time, in photoproduction of $π^{0}η$ pairs on carbon, aluminum, and lead, with the A2 experimental setup at MAMI. The results are compared to an earlier measurement on a free proton and to the corresponding theoretical calculations. The Mainz model is used to predict the beam-helicity asymmetry for the nuclear targets. The present results indicate that the photoproduction mechanism for $π^{0}η$ pairs on nuclei is similar to photoproduction on a free nucleon. This process is dominated by the $D_{33}$ partial wave with the $ηΔ(1232)$ intermediate state. △ Less

Submitted 23 January, 2020; v1 submitted 29 June, 2019; originally announced July 2019.

Comments: Accepted for publication in Physics Letters B (2020)

Measurement of the Helicity Asymmetry $E$ for the reaction $ γp\to π^0 p$

Authors: CBELSA/TAPS collaboration, :, M. Gottschall, F. Afzal, A. V. Anisovich, D. Bayadilov, R. Beck, M. Bichow, K. -Th. Brinkmann, V. Crede, M. Dieterle, F. Dietz, H. Dutz, H. Eberhardt, D. Elsner, R. Ewald, K. Fornet-Ponse, St. Friedrich, F. Frommberger, A. Gridnev, M. Grüner, E. Gutz, Ch. Hammann, J. Hannappel, J. Hartmann , et al. (45 additional authors not shown)

Abstract: A measurement of the double-polarization observable $E$ for the reaction $γp\to π^0 p$ is reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility in Bonn using the Bonn frozen-spin butanol (C$_4$H$_9$OH) target, which provided longitudinally-polarized protons. Circularly-polarized photons were produced via bremsstrahlung of longitudinally-polarized electrons. The data co… ▽ More A measurement of the double-polarization observable $E$ for the reaction $γp\to π^0 p$ is reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility in Bonn using the Bonn frozen-spin butanol (C$_4$H$_9$OH) target, which provided longitudinally-polarized protons. Circularly-polarized photons were produced via bremsstrahlung of longitudinally-polarized electrons. The data cover the photon energy range from $E_γ=600$~MeV to $E_γ=2310$~MeV and nearly the complete angular range. The results are compared to and have been included in recent partial wave analyses. △ Less

Submitted 29 April, 2019; originally announced April 2019.

Comments: 21 pages

Experimental study of the $γp\rightarrow K^0Σ^+$, $γn\rightarrow K^0Λ$, and $γn\rightarrow K^0 Σ^0$ reactions at the Mainz Microtron

Authors: C. S. Akondi, K. Bantawa, D. M. Manley, S. Abt, P. Achenbach, F. Afzal, P. Aguar-Bartolomé, Z. Ahmed, J. R. M. Annand, H. J. Arends, M. Bashkanov, R. Beck, M. Biroth, N. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, S. Costanza, A. Denig, M. Dieterle, E. J. Downie, P. Drexler, M. I. Ferretti Bondy, S. Gardner , et al. (56 additional authors not shown)

Abstract: This work measured $dσ/dΩ$ for neutral kaon photoproduction reactions from threshold up to a c.m.\ energy of 1855MeV, focussing specifically on the $γp\rightarrow K^0Σ^+$, $γn\rightarrow K^0Λ$, and $γn\rightarrow K^0 Σ^0$ reactions. Our results for $γn\rightarrow K^0 Σ^0$ are the first-ever measurements for that reaction. These data will provide insight into the properties of $N^*$ resonances and,… ▽ More This work measured $dσ/dΩ$ for neutral kaon photoproduction reactions from threshold up to a c.m.\ energy of 1855MeV, focussing specifically on the $γp\rightarrow K^0Σ^+$, $γn\rightarrow K^0Λ$, and $γn\rightarrow K^0 Σ^0$ reactions. Our results for $γn\rightarrow K^0 Σ^0$ are the first-ever measurements for that reaction. These data will provide insight into the properties of $N^*$ resonances and, in particular, will lead to an improved knowledge about those states that couple only weakly to the $πN$ channel. Integrated cross sections were extracted by fitting the differential cross sections for each reaction as a series of Legendre polynomials and our results are compared with prior experimental results and theoretical predictions. △ Less

Submitted 13 November, 2018; originally announced November 2018.

Comments: 16 pages, 13 figures, 7 tables

First measurement of helicity-dependent cross sections in pi0-eta photoproduction from quasi-free nucleons

Authors: A. Käser, M. Dieterle, L. Witthauer, S. Abt, P. Achenbach, P. Adlarson, F. Afzal, Z. Ahmed, J. Ahrens, J. R. M. Annand, H. J. Arends, M. Bashkanov, R. Beck, M. Biroth, N. S. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, S. Costanza, A. Denig, E. J. Downie, P. Drexler, A. Fix, S. Garni , et al. (58 additional authors not shown)

Abstract: The helicity-dependent cross sections for the photoproduction of $π^0η$ pairs have been measured for the first time. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball - TAPS calorimeter. The experiment used a polarized deuterated butanol target and a circularly polarized photon beam. This arrangement allowed the $σ_{1/2}$ (photo… ▽ More The helicity-dependent cross sections for the photoproduction of $π^0η$ pairs have been measured for the first time. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball - TAPS calorimeter. The experiment used a polarized deuterated butanol target and a circularly polarized photon beam. This arrangement allowed the $σ_{1/2}$ (photon and target spin antiparallel) and $σ_{3/2}$ (parallel spins) components to be measured for quasi-free production of $π^0η$ pairs off protons and neutrons. The main finding is that the two helicity components contribute identically, within uncertainties, for both participant protons and neutrons. The absolute couplings for protons and neutrons are also identical. This means that nucleon resonances contributing to this reaction in the investigated energy range have almost equal electromagnetic helicity couplings, $A_{1/2}^{n,p}$ and $A_{3/2}^{n,p}$. Identical couplings for protons and neutrons are typical for $Δ$ resonances and identical $A_{1/2}$ and $A_{3/2}$ components are only possible for $J\geq 3/2$ states, which constrains possible contributions of nucleon resonances. △ Less

Submitted 5 October, 2018; originally announced October 2018.

Comments: accepted for publication in Phys. Lett. B

The η'-carbon potential at low meson momenta

Authors: M. Nanova, S. Friedrich, V. Metag, E. Ya. Paryev, F. N. Afzal, D. Bayadilov, B. Bantes, R. Beck, M. Becker, S. Böse, K. -T. Brinkmann, V. Crede, P. Drexler, H. Eberhardt, D. Elsner, F. Frommberger, Ch. Funke, M. Gottschall, M. Grüner, E. Gutz, Ch. Hammann, J. Hannappel, J. Hartmann, W. Hillert, Ph. Hoffmeister , et al. (36 additional authors not shown)

Abstract: The production of $η^\prime$ mesons in coincidence with forward-going protons has been studied in photon-induced reactions on $^{12}$C and on a liquid hydrogen (LH$_2$) target for incoming photon energies of 1.3-2.6 GeV at the electron accelerator ELSA. The $η^\prime$ mesons have been identified via the $η^\prime\rightarrow π^0 π^0η\rightarrow 6 γ$ decay registered with the CBELSA/TAPS detector sy… ▽ More The production of $η^\prime$ mesons in coincidence with forward-going protons has been studied in photon-induced reactions on $^{12}$C and on a liquid hydrogen (LH$_2$) target for incoming photon energies of 1.3-2.6 GeV at the electron accelerator ELSA. The $η^\prime$ mesons have been identified via the $η^\prime\rightarrow π^0 π^0η\rightarrow 6 γ$ decay registered with the CBELSA/TAPS detector system. Coincident protons have been identified in the MiniTAPS BaF$_2$ array at polar angles of $2^{\circ} \le θ_{p} \le 11^{\circ}$. Under these kinematic constraints the $η^\prime$ mesons are produced with relatively low kinetic energy ($\approx$ 150 MeV) since the coincident protons take over most of the momentum of the incident-photon beam. For the C-target this allows the determination of the real part of the $η^\prime$-carbon potential at low meson momenta by comparing with collision model calculations of the $η^\prime$ kinetic energy distribution and excitation function. Fitting the latter data for $η^\prime$ mesons going backwards in the center-of-mass system yields a potential depth of V = $-$(44 $\pm$ 16(stat)$\pm$15(syst)) MeV, consistent with earlier determinations of the potential depth in inclusive measurements for average $η^\prime$ momenta of $\approx$ 1.1 GeV/$c$. Within the experimental uncertainties, there is no indication of a momentum dependence of the $η^\prime$-carbon potential. The LH$_2$ data, taken as a reference to check the data analysis and the model calculations, provide differential and integral cross sections in good agreement with previous results for $η^\prime$ photoproduction off the free proton. △ Less

Submitted 1 October, 2018; originally announced October 2018.

Comments: 9 pages, 13 figures. arXiv admin note: text overlap with arXiv:1608.06074

Journal ref: Eur. Phys. J. A, 54 (2018) 182

Deuteron photodisintegration by polarized photons in the region of the $d^*(2380)$

Authors: M. Bashkanov, S. Kay, D. P. Watts, C. Mullen, S. Abt, P. Achenbach, P. Adlarson, F. Afzal, Z. Ahmed, C. S. Akondi, J. R. M. Annand, H. J. Arends, R. Beck, M. Biroth, N. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, S. Costanza, A. Denig, E. J. Downie, P. Drexler, S. Garni, D. I. Glazier , et al. (48 additional authors not shown)

Abstract: We report the first large-acceptance measurement of the beam-spin asymmetry for deuteron photodisintegration ($\vecγ d\to pn$) in the photon energy range $400<E_γ<630$~MeV. The measurement provides important new constraints on the mechanisms of photodisintegration above the delta resonance and on the photocoupling of the recently discovered $d^*(2380)$ hexaquark. We report the first large-acceptance measurement of the beam-spin asymmetry for deuteron photodisintegration ($\vecγ d\to pn$) in the photon energy range $400<E_γ<630$~MeV. The measurement provides important new constraints on the mechanisms of photodisintegration above the delta resonance and on the photocoupling of the recently discovered $d^*(2380)$ hexaquark. △ Less

Submitted 14 December, 2018; v1 submitted 25 September, 2018; originally announced September 2018.

Photoproduction of {\boldmath{$π^{0}$}} Mesons off Protons and Neutrons in the Second and Third Nucleon Resonance Region

Authors: M. Dieterle, D. Werthmüller, S. Abt, F. Afzal, P. Aguar Bartolome, Z. Ahmed, J. Ahrens, J. R. M. Annand, H. J. Arends, M. Bashkanov, R. Beck, M. Biroth, N. Borisov, A. Braghieri, W. J. Briscoe, S. Cherepnya, F. Cividini, C. Collicott, S. Costanza, A. Denig, E. J. Downie, P. Drexler, L. V. Fil'kov, S. Garni, D. I. Glazier , et al. (59 additional authors not shown)

Abstract: Photoproduction of mesons off quasi-free nucleons bound in the deuteron allows to study the electromagnetic excitation spectrum of the neutron and the isospin structure of the excitation of nucleon resonances. The database for such reactions is much more sparse than for free proton targets. Single $π^0$ photoproduction off quasi-free nucleons from the deuteron was experimentally studied. Nuclear e… ▽ More Photoproduction of mesons off quasi-free nucleons bound in the deuteron allows to study the electromagnetic excitation spectrum of the neutron and the isospin structure of the excitation of nucleon resonances. The database for such reactions is much more sparse than for free proton targets. Single $π^0$ photoproduction off quasi-free nucleons from the deuteron was experimentally studied. Nuclear effects were investigated by a comparison of the results for free protons and quasi-free protons and used as a correction for the quasi-free neutron data. The experiment was performed at the tagged photon beam of the Mainz MAMI accelerator for photon energies between 0.45~GeV and 1.4~GeV, using an almost $4π$ electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. A complete kinematic reconstruction of the final state removed the effects of Fermi motion. Reaction model predictions and PWA for $γn\rightarrow nπ^{0}$, based on fits to data for the other isospin channels, disagreed between themselves and no model provided a good description of the new data. The results demonstrate clearly the importance of a measurement of the fully neutral final state for the isospin decomposition of the cross section. Model refits, for example from the Bonn-Gatchina analysis, show that the new and the previous data for the other three isospin channels can be simultaneously described when the contributions of several partial waves are modified. The results are also relevant for the suppression of the higher resonance bumps in total photoabsorption on nuclei, which are not well understood. △ Less

Submitted 26 May, 2018; originally announced May 2018.

Comments: excepted for publication in Phys. Rev. C

High-statistics measurement of the eta->3pi^0 decay at the Mainz Microtron

Authors: S. Prakhov, S. Abt, P. Achenbach, P. Adlarson, F. Afzal, P. Aguar-Bartolomé, Z. Ahmed, J. Ahrens, J. R. M. Annand, H. J. Arends, K. Bantawa, M. Bashkanov, R. Beck, M. Biroth, N. S. Borisov, A. Braghieri, W. J. Briscoe, S. Cherepnya, F. Cividini, C. Collicott, S. Costanza, A. Denig, M. Dieterle, E. J. Downie, P. Drexler , et al. (63 additional authors not shown)

Abstract: The largest, at the moment, statistics of 7x10^6 eta->3pi^0 decays, based on 6.2x10^7 eta mesons produced in the gamma p -> eta p reaction, has been accumulated by the A2 Collaboration at the Mainz Microtron, MAMI. It allowed a detailed study of the eta->3pi^0 dynamics beyond its conventional parametrization with just the quadratic slope parameter alpha and enabled, for the first time, a measureme… ▽ More The largest, at the moment, statistics of 7x10^6 eta->3pi^0 decays, based on 6.2x10^7 eta mesons produced in the gamma p -> eta p reaction, has been accumulated by the A2 Collaboration at the Mainz Microtron, MAMI. It allowed a detailed study of the eta->3pi^0 dynamics beyond its conventional parametrization with just the quadratic slope parameter alpha and enabled, for the first time, a measurement of the second-order term and a better understanding of the cusp structure in the neutral decay. The present data are also compared to recent theoretical calculations that predict a nonlinear dependence along the quadratic distance from the Dalitz-plot center. △ Less

Submitted 11 June, 2018; v1 submitted 6 March, 2018; originally announced March 2018.

Comments: 11 pages, 6 figures, 1 table

Journal ref: Phys. Rev. C 97, 065203 (2018)

Study of the $γp\to π^0ηp$ reaction with the A2 setup at MAMI

Authors: V. Sokhoyan, S. Prakhov, A. Fix, S. Abt, P. Achenbach, P. Adlarson, F. Afzal, P. Aguar-Bartolomé, Z. Ahmed, J. Ahrens, J. R. M. Annand, H. J. Arends, K. Bantawa, M. Bashkanov, R. Beck, M. Biroth, N. S. Borisov, A. Braghieri, W. J. Briscoe, S. Cherepnya, F. Cividini, C. Collicott, S. Costanza, A. Denig, M. Dieterle , et al. (77 additional authors not shown)

Abstract: The data available from the A2 Collaboration at MAMI were analyzed to select the $γp\to π^0ηp$ reaction on an event-by-event basis, which allows for partial-wave analyses of three-body final states to obtain more reliable results, compared to fits to measured distributions. These data provide the world's best statistical accuracy in the energy range from threshold to $E_γ=1.45$ GeV, allowing a fin… ▽ More The data available from the A2 Collaboration at MAMI were analyzed to select the $γp\to π^0ηp$ reaction on an event-by-event basis, which allows for partial-wave analyses of three-body final states to obtain more reliable results, compared to fits to measured distributions. These data provide the world's best statistical accuracy in the energy range from threshold to $E_γ=1.45$ GeV, allowing a finer energy binning in the measurement of all observables needed for understanding the reaction dynamics. The results obtained for the measured observables are compared to existing models, and the impact from the new data is checked by the fit with the revised Mainz model. △ Less

Submitted 2 June, 2018; v1 submitted 2 March, 2018; originally announced March 2018.

Comments: 15 pages, 18 figures

Journal ref: Phys. Rev. C 97, 055212 (2018)

Transverse Extension of Partons in the Proton probed by Deeply Virtual Compton Scattering

Authors: R. Akhunzyanov, M. G. Alexeev, G. D. Alexeev, A. Amoroso, V. Andrieux, N. V. Anfimov, V. Anosov, A. Antoshkin, K. Augsten, W. Augustyniak, A. Austregesilo, C. D. R. Azevedo, B. Badelek, F. Balestra, M. Ball, J. Barth, R. Beck, Y. Bedfer, J. Bernhard, K. Bicker, E. R. Bielert, R. Birsa, M. Bodlak, P. Bordalo, F. Bradamante , et al. (202 additional authors not shown)

Abstract: We report on the first measurement of exclusive single-photon muoproduction on the proton by COMPASS using 160 GeV/$c$ polarized $μ^+$ and $μ^-$ beams of the CERN SPS impinging on a liquid hydrogen target. We determine the dependence of the average of the measured $μ^+$ and $μ^-$ cross sections for deeply virtual Compton scattering on the squared four-momentum transfer $t$ from the initial to the… ▽ More We report on the first measurement of exclusive single-photon muoproduction on the proton by COMPASS using 160 GeV/$c$ polarized $μ^+$ and $μ^-$ beams of the CERN SPS impinging on a liquid hydrogen target. We determine the dependence of the average of the measured $μ^+$ and $μ^-$ cross sections for deeply virtual Compton scattering on the squared four-momentum transfer $t$ from the initial to the final final proton. The slope $B$ of the $t$-dependence is fitted with a single exponential function, which yields $B=(4.3 \ \pm \ 0.6_{\text{stat}}\_{- \ 0.3}^{+ \ 0.1}\big\rvert_{\text{sys}}) (\text{GeV}/c)^{-2}$. This result can be converted into an average transverse extension of partons in the proton, $\sqrt{\langle r_{\perp}^2 \rangle} = (0.58 \ \pm \ 0.04_{\text{stat}}\_{- \ 0.02}^{+ \ 0.01}\big\rvert_{\text{sys}})\text{fm}$. For this measurement, the average virtuality of the photon mediating the interaction is $\langle Q^2 \rangle = 1.8\,(\text{GeV/}c)^2$ and the average value of the Bjorken variable is $\langle x_{\text{Bj}} \rangle = 0.056$. △ Less

Submitted 11 December, 2019; v1 submitted 8 February, 2018; originally announced February 2018.

Comments: 13 pages, 5 figures

Report number: CERN-EP-2018-016

Transverse-momentum-dependent Multiplicities of Charged Hadrons in Muon-Deuteron Deep Inelastic Scattering

Authors: M. Aghasyan, R. Akhunzyanov, M. G. Alexeev, G. D. Alexeev, A. Amoroso, V. Andrieux, N. V. Anfimov, V. Anosov, A. Antoshkin, K. Augsten, W. Augustyniak, A. Austregesilo, C. D. R. Azevedo, B. Badelek, F. Balestra, M. Ball, J. Barth, R. Beck, Y. Bedfer, J. Bernhard, K. Bicker, E. R. Bielert, R. Birsa, M. Bodlak, P. Bordalo , et al. (206 additional authors not shown)

Abstract: A semi-inclusive measurement of charged hadron multiplicities in deep inelastic muon scattering off an isoscalar target was performed using data collected by the COMPASS Collaboration at CERN. The following kinematic domain is covered by the data: photon virtuality $Q^{2}>1$ (GeV/$c$)$^2$, invariant mass of the hadronic system $W > 5$ GeV/$c^2$, Bjorken scaling variable in the range… ▽ More A semi-inclusive measurement of charged hadron multiplicities in deep inelastic muon scattering off an isoscalar target was performed using data collected by the COMPASS Collaboration at CERN. The following kinematic domain is covered by the data: photon virtuality $Q^{2}>1$ (GeV/$c$)$^2$, invariant mass of the hadronic system $W > 5$ GeV/$c^2$, Bjorken scaling variable in the range $0.003 < x < 0.4$, fraction of the virtual photon energy carried by the hadron in the range $0.2 < z < 0.8$, square of the hadron transverse momentum with respect to the virtual photon direction in the range 0.02 (GeV/$c)^2 < P_{\rm{hT}}^{2} < 3$ (GeV/$c$)$^2$. The multiplicities are presented as a function of $P_{\rm{hT}}^{2}$ in three-dimensional bins of $x$, $Q^2$, $z$ and compared to previous semi-inclusive measurements. We explore the small-$P_{\rm{hT}}^{2}$ region, i.e. $P_{\rm{hT}}^{2} < 1$ (GeV/$c$)$^2$, where hadron transverse momenta are expected to arise from non-perturbative effects, and also the domain of larger $P_{\rm{hT}}^{2}$, where contributions from higher-order perturbative QCD are expected to dominate. The multiplicities are fitted using a single-exponential function at small $P_{\rm{hT}}^{2}$ to study the dependence of the average transverse momentum $\langle P_{\rm{hT}}^{2}\rangle$ on $x$, $Q^2$ and $z$. The power-law behaviour of the multiplicities at large $P_{\rm{hT}}^{2}$ is investigated using various functional forms. The fits describe the data reasonably well over the full measured range. △ Less

Submitted 21 September, 2017; originally announced September 2017.

Comments: 28 pages, 20 figures

Report number: CERN-EP-2017-253

Journal ref: Phys. Rev. D 97, 032006 (2018)

First measurement of the polarization observable $E$ and helicity-dependent cross sections in single $π^{0}$ photoproduction from quasi-free nucleons

Authors: M. Dieterle, L. Witthauer, F. Cividini, S. Abt, P. Achenbach, P. Adlarson, F. Afzal, Z. Ahmed, C. S. Akondi, J. R. M. Annand, H. J. Arends, M. Bashkanov, R. Beck, M. Biroth, N. S. Borisov, A. Braghieri, W. J. Briscoe, S. Costanza, C. Collicott, A. Denig, E. J. Downie, P. Drexler, M. I. Ferretti-Bondy, S. Gardner, S. Garni , et al. (62 additional authors not shown)

Abstract: The double-polarization observable $E$ and the helicity-dependent cross sections $σ_{1/2}$ and $σ_{3/2}$ have been measured for the first time for single $π^{0}$ photoproduction from protons and neutrons bound in the deuteron at the electron accelerator facility MAMI in Mainz, Germany. The experiment used a circularly polarized photon beam and a longitudinally polarized deuterated butanol target.… ▽ More The double-polarization observable $E$ and the helicity-dependent cross sections $σ_{1/2}$ and $σ_{3/2}$ have been measured for the first time for single $π^{0}$ photoproduction from protons and neutrons bound in the deuteron at the electron accelerator facility MAMI in Mainz, Germany. The experiment used a circularly polarized photon beam and a longitudinally polarized deuterated butanol target. The reaction products, recoil nucleons and decay photons from the $π^0$ meson were detected with the Crystal Ball and TAPS electromagnetic calorimeters. Effects from nuclear Fermi motion were removed by a kinematic reconstruction of the $π^{0}N$ final state. A comparison to data measured with a free proton target showed that the absolute scale of the cross sections is significantly modified by nuclear final-state interaction (FSI) effects. However, there is no significant effect on the asymmetry $E$ since the $σ_{1/2}$ and $σ_{3/2}$ components appear to be influenced in a similar way. Thus, the best approximation of the two helicity-dependent cross sections for the free neutron is obtained by combining the asymmetry $E$ measured with quasi-free neutrons and the unpolarized cross section corrected for FSI effects under the assumption that the FSI effects are similar for neutrons and protons. △ Less

Submitted 20 May, 2017; originally announced May 2017.

Comments: accepted for publication in Phys. Lett. B

Journal ref: Phys. Lett. B 770 (2017) 523

Helicity-dependent cross sections and double-polarization observable E in eta photoproduction from quasi-free protons and neutrons

Authors: L. Witthauer, M. Dieterle, S. Abt, P. Achenbach, F. Afzal, Z. Ahmed, C. S. Akondi, J. R. M. Annand, H. J. Arends, M. Bashkanov, R. Beck, M. Biroth, N. S. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, S. Costanza, C. Collicott, A. Denig, E. J. Downie, P. Drexler, M. I. Ferretti-Bondy, S. Gardner, S. Garni, D. I. Glazier , et al. (62 additional authors not shown)

Abstract: Precise helicity-dependent cross sections and the double-polarization observable $E$ were measured for $η$ photoproduction from quasi-free protons and neutrons bound in the deuteron. The $η\rightarrow 2γ$ and $η\rightarrow 3π^0\rightarrow 6γ$ decay modes were used to optimize the statistical quality of the data and to estimate systematic uncertainties. The measurement used the A2 detector setup at… ▽ More Precise helicity-dependent cross sections and the double-polarization observable $E$ were measured for $η$ photoproduction from quasi-free protons and neutrons bound in the deuteron. The $η\rightarrow 2γ$ and $η\rightarrow 3π^0\rightarrow 6γ$ decay modes were used to optimize the statistical quality of the data and to estimate systematic uncertainties. The measurement used the A2 detector setup at the tagged photon beam of the electron accelerator MAMI in Mainz. A longitudinally polarized deuterated butanol target was used in combination with a circularly polarized photon beam from bremsstrahlung of a longitudinally polarized electron beam. The reaction products were detected with the electromagnetic calorimeters Crystal Ball and TAPS, which covered 98\% of the full solid angle. The results show that the narrow structure observed earlier in the unpolarized excitation function of $η$ photoproduction off the neutron appears only in reactions with antiparallel photon and nucleon spin ($σ_{1/2}$). It is absent for reactions with parallel spin orientation ($σ_{3/2}$) and thus very probably related to partial waves with total spin 1/2. The behavior of the angular distributions of the helicity-dependent cross sections was analyzed by fitting them with Legendre polynomials. The results are in good agreement with a model from the Bonn-Gatchina group, which uses an interference of $P_{11}$ and $S_{11}$ partial waves to explain the narrow structure. △ Less

Submitted 3 April, 2017; originally announced April 2017.

Comments: accepted for publication in Phys. Rev. C

Journal ref: Phys. Rev. C 95 (2017) 055201

Photoproduction of eta mesons from the neutron: cross sections and double polarization observable E

Authors: L. Witthauer, M. Dieterle, F. Afzal, A. V. Anisovich, B. Bantes, D. Bayadilov, R. Beck, M. Bichow, K. -T. Brinkmann, S. Böse, Th. Challand, V. Crede, H. Dutz, H. Eberhardt, D. Elsner, R. Ewald, K. Fornet-Ponse, St. Friedrich, F. Frommberger, Ch. Funke, St. Goertz, M. Gottschall, A. Gridnev, M. Grüner, E. Gutzınst , et al. (48 additional authors not shown)

Abstract: Photoproduction of $η$ mesons from neutrons} \abstract{Results from measurements of the photoproduction of $η$ mesons from quasifree protons and neutrons are summarized. The experiments were performed with the CBELSA/TAPS detector at the electron accelerator ELSA in Bonn using the $η\to3π^{0}\to6γ$ decay. A liquid deuterium target was used for the measurement of total cross sections and angular di… ▽ More Photoproduction of $η$ mesons from neutrons} \abstract{Results from measurements of the photoproduction of $η$ mesons from quasifree protons and neutrons are summarized. The experiments were performed with the CBELSA/TAPS detector at the electron accelerator ELSA in Bonn using the $η\to3π^{0}\to6γ$ decay. A liquid deuterium target was used for the measurement of total cross sections and angular distributions. The results confirm earlier measurements from Bonn and the MAMI facility in Mainz about the existence of a narrow structure in the excitation function of $γn\rightarrow nη$. The current angular distributions show a forward-backward asymmetry, which was previously not seen, but was predicted by model calculations including an additional narrow $P_{11}$ state. Furthermore, data obtained with a longitudinally polarized, deuterated butanol target and a circularly polarized photon beam were analyzed to determine the double polarization observable $E$. Both data sets together were also used to extract the helicity dependent cross sections $σ_{1/2}$ and $σ_{3/2}$. The narrow structure in the excitation function of $γn\rightarrow nη$ appears associated with the helicity-1/2 component of the reaction. △ Less

Submitted 3 April, 2017; originally announced April 2017.

Journal ref: Eur, Phys. J. A 53 (2017) 58

Insight into the narrow structure in {\boldmath{$η$}}-photoproduction on the neutron from helicity dependent cross sections

Authors: L. Witthauer, M. Dieterle, S. Abt, P. Achenbach, F. Afzal, Z. Ahmed, J. R. M. Annand, H. J. Arends, M. Bashkanov, R. Beck, M. Biroth, N. S. Borisov, A. Braghieri, W. J. Briscoe, F. Cividini, S. Costanza, C. Collicott, A. Denig, E. J. Downie, P. Drexler, M. I. Ferretti-Bondy, S. Gardner, S. Garni, D. I. Glazier, D. Glowa , et al. (61 additional authors not shown)

Abstract: The double polarization observable $E$ and the helicity dependent cross sections $σ_{1/2}$ and $σ_{3/2}$ were measured for $η$ photoproduction from quasi-free protons and neutrons. The circularly polarized tagged photon beam of the A2 experiment at the Mainz MAMI accelerator was used in combination with a longitudinally polarized deuterated butanol target. The almost $4π$ detector setup of the Cry… ▽ More The double polarization observable $E$ and the helicity dependent cross sections $σ_{1/2}$ and $σ_{3/2}$ were measured for $η$ photoproduction from quasi-free protons and neutrons. The circularly polarized tagged photon beam of the A2 experiment at the Mainz MAMI accelerator was used in combination with a longitudinally polarized deuterated butanol target. The almost $4π$ detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from $η\rightarrow 2γ$ and $η\rightarrow 3π^0$. The results show that the narrow structure previously observed in $η$ photoproduction from the neutron is only apparent in $σ_{1/2}$ and hence, most likely related to a spin-1/2 amplitude. Nucleon resonances that contribute to this partial wave in $η$ production are only $N1/2^-$ ($S_{11}$) and $N1/2^+$ ($P_{11}$). Furthermore, the extracted Legendre coefficients of the angular distributions for $σ_{1/2}$ are in good agreement with recent reaction model predictions assuming a narrow resonance in the $P_{11}$ wave as the origin of this structure. △ Less

Submitted 5 February, 2017; originally announced February 2017.

Journal ref: Phys. Rev. Lett. 117 (2016) 132502

Study of $η$ and $η'$ photoproduction at MAMI

Authors: V. L. Kashevarov, P. Ott, S. Prakhov, P. Adlarson, F. Afzal, Z. Ahmed, C. S. Akondi, J. R. M. Annand, H. J. Arends, R. Beck, A. Braghieri, W. J. Briscoe, F. Cividini, R. Codling, C. Collicott, S. Costanza, A. Denig, E. J. Downie, M. Dieterle, M. I. Ferretti Bondy, L. V. Filkov, A. Fix, S. Gardner, S. Garni, D. I. Glazier , et al. (65 additional authors not shown)

Abstract: The reactions $γp\to ηp$ and $γp\to η' p$ have been measured from their thresholds up to the center-of-mass energy $W=1.96$GeV with the tagged-photon facilities at the Mainz Microtron, MAMI. Differential cross sections were obtained with unprecedented accuracy, providing fine energy binning and full production-angle coverage. A strong cusp is observed in the total cross section and excitation func… ▽ More The reactions $γp\to ηp$ and $γp\to η' p$ have been measured from their thresholds up to the center-of-mass energy $W=1.96$GeV with the tagged-photon facilities at the Mainz Microtron, MAMI. Differential cross sections were obtained with unprecedented accuracy, providing fine energy binning and full production-angle coverage. A strong cusp is observed in the total cross section and excitation functions for $η$ photoproduction at the energies in vicinity of the $η'$ threshold, $W=1896$MeV ($E_γ=1447$MeV). This behavior is explained in a revised $η$MAID isobar model by a significant branching of the $N(1895)1/2^-$ nucleon resonance to both, $ηp$ and $η' p$, confirming the existence and constraining the properties of this poorly known state. △ Less

Submitted 2 January, 2017; originally announced January 2017.

Journal ref: Phys. Rev. Lett. 118, 212001 (2017)

Measurement of the pi^0 -> e^+e^-gamma Dalitz decay at the Mainz Microtron

Authors: P. Adlarson, F. Afzal, P. Aguar-Bartolomé, Z. Ahmed, C. S. Akondi, J. R. M. Annand, H. J. Arends, K. Bantawa, R. Beck, H. Berghäuser, M. Biroth, N. S. Borisov, A. Braghieri, W. J. Briscoe, S. Cherepnya, F. Cividini, C. Collicott, S. Costanza, A. Denig, M. Dieterle, E. J. Downie, P. Drexler, M. I. Ferretti Bondy, L. V. Fil'kov, S. Gardner , et al. (72 additional authors not shown)

Abstract: The Dalitz decay pi^0 -> e^+e^-gamma has been measured in the gamma p -> pi^0 p reaction with the A2 tagged-photon facility at the Mainz Microtron, MAMI. The value obtained for the slope parameter of the pi^0 electromagnetic transition form factor, a_pi = 0.030+/-0.010_tot, is in agreement with existing measurements of this decay and with recent theoretical calculations. The uncertainty obtained i… ▽ More The Dalitz decay pi^0 -> e^+e^-gamma has been measured in the gamma p -> pi^0 p reaction with the A2 tagged-photon facility at the Mainz Microtron, MAMI. The value obtained for the slope parameter of the pi^0 electromagnetic transition form factor, a_pi = 0.030+/-0.010_tot, is in agreement with existing measurements of this decay and with recent theoretical calculations. The uncertainty obtained in the value of a_pi is lower than in previous results based on the pi^0 -> e^+e^-gamma decay. △ Less

Submitted 18 February, 2017; v1 submitted 15 November, 2016; originally announced November 2016.

Comments: 11 pages, 7 figures, 1 table

Journal ref: Phys. Rev. C 95, 025202 (2017)

Hard Two-Photon Contribution to Elastic Lepton-Proton Scattering: Determined by the OLYMPUS Experiment

Authors: B. S. Henderson, L. D. Ice, D. Khaneft, C. O'Connor, R. Russell, A. Schmidt, J. C. Bernauer, M. Kohl, N. Akopov, R. Alarcon, O. Ates, A. Avetisyan, R. Beck, S. Belostotski, J. Bessuille, F. Brinker, J. R. Calarco, V. Carassiti, E. Cisbani, G. Ciullo, M. Contalbrigo, R. De Leo, J. Diefenbach, T. W. Donnelly, K. Dow , et al. (45 additional authors not shown)

Abstract: The OLYMPUS collaboration reports on a precision measurement of the positron-proton to electron-proton elastic cross section ratio, $R_{2γ}$, a direct measure of the contribution of hard two-photon exchange to the elastic cross section. In the OLYMPUS measurement, 2.01~GeV electron and positron beams were directed through a hydrogen gas target internal to the DORIS storage ring at DESY. A toroidal… ▽ More The OLYMPUS collaboration reports on a precision measurement of the positron-proton to electron-proton elastic cross section ratio, $R_{2γ}$, a direct measure of the contribution of hard two-photon exchange to the elastic cross section. In the OLYMPUS measurement, 2.01~GeV electron and positron beams were directed through a hydrogen gas target internal to the DORIS storage ring at DESY. A toroidal magnetic spectrometer instrumented with drift chambers and time-of-flight scintillators detected elastically scattered leptons in coincidence with recoiling protons over a scattering angle range of $\approx 20\degree$ to $80\degree$. The relative luminosity between the two beam species was monitored using tracking telescopes of interleaved GEM and MWPC detectors at $12\degree$, as well as symmetric Møller/Bhabha calorimeters at $1.29\degree$. A total integrated luminosity of 4.5~fb$^{-1}$ was collected. In the extraction of $R_{2γ}$, radiative effects were taken into account using a Monte Carlo generator to simulate the convolutions of internal bremsstrahlung with experiment-specific conditions such as detector acceptance and reconstruction efficiency. The resulting values of $R_{2γ}$, presented here for a wide range of virtual photon polarization $0.456<ε<0.978$, are smaller than some hadronic two-photon exchange calculations predict, but are in reasonable agreement with a subtracted dispersion model and a phenomenological fit to the form factor data. △ Less

Submitted 19 December, 2016; v1 submitted 14 November, 2016; originally announced November 2016.

Comments: 5 pages, 3 figures, 2 tables

Journal ref: Phys. Rev. Lett. 118, 092501 (2017)

Determination of the scalar polarizabilities of the proton using beam asymmetry $Σ_{3}$ in Compton scattering

Authors: V. Sokhoyan, E. J. Downie, E. Mornacchi, J. A. McGovern, N. Krupina, F. Afzal, J. Ahrens, C. S. Akondi, J. R. M. Annand, H. J. Arends, R. Beck, A. Braghieri, W. J. Briscoe, F. Cividini, C. Collicott, S. Costanza, A. Denig, M. Dieterle, M. Ferretti, S. Gardner, S. Garni, D. I. Glazier, D. Glowa, W. Gradl, G. Gurevich , et al. (51 additional authors not shown)

Abstract: The scalar dipole polarizabilities, $α_{E1}$ and $β_{M1}$, are fundamental properties related to the internal dynamics of the nucleon. The currently accepted values of the proton polarizabilities were determined by fitting to unpolarized proton Compton scattering cross section data. The measurement of the beam asymmetry $Σ_{3}$ in a certain kinematical range provides an alternative approach to the… ▽ More The scalar dipole polarizabilities, $α_{E1}$ and $β_{M1}$, are fundamental properties related to the internal dynamics of the nucleon. The currently accepted values of the proton polarizabilities were determined by fitting to unpolarized proton Compton scattering cross section data. The measurement of the beam asymmetry $Σ_{3}$ in a certain kinematical range provides an alternative approach to the extraction of the scalar polarizabilities. At the Mainz Microtron (MAMI) the beam asymmetry was measured for Compton scattering below pion photoproduction threshold for the first time. The results are compared with model calculations and the influence of the experimental data on the extraction of the scalar polarizabilities is determined. △ Less

Submitted 10 January, 2017; v1 submitted 11 November, 2016; originally announced November 2016.

Comments: 6 pages, 5 figures

Journal ref: Eur. Phys. J. A (2017) 53: 14