Showing 1–16 of 16 results for author: Wexler, J

First Full Dalitz Plot Measurement in Neutron $β$-Decay using the Nab Spectrometer and Implications for New Physics

Authors: Francisco M. Gonzalez, Jin Ha Choi, Himal Acharya, Skylar Clymer, Andrew Hagemeier, David G. Mathews, August Mendelsohn, Austin Nelsen, Hitesh Rahangdale, Love Richburg, Ricardo Alarcon, Ariella Atencio, Stefan Baeßler, Thomas Bailey, Noah Birge, Dennis Borissenko, Michael Bowler, Leah J. Broussard, Albert T. Bryant, Jimmy Caylor, Tim Chupp, Christopher Crawford, R. Alston Croley, Micah Cruz, George Dodson , et al. (67 additional authors not shown)

Abstract: Precision measurements of observables in neutron $β$-decay are used to test the Standard Model description of the weak interaction and search for evidence of new physics. The Nab experiment at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source was constructed to measure correlations in neutron decay by utilizing an asymmetric spectrometer and novel detection system to accura… ▽ More Precision measurements of observables in neutron $β$-decay are used to test the Standard Model description of the weak interaction and search for evidence of new physics. The Nab experiment at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source was constructed to measure correlations in neutron decay by utilizing an asymmetric spectrometer and novel detection system to accurately reconstruct the proton momentum and electron energy for each $β$-decay. This work describes the detection of neutron $β$-decay products in the Nab spectrometer and presents the first full Dalitz plot representation of the phase space of neutron $β$-decay for all electrons >100 keV. In addition, new constraints are placed on a possible excited neutron state, hypothesized to explain the disagreement between the appearance and disappearance neutron lifetime techniques. △ Less

Submitted 21 August, 2025; originally announced August 2025.

Improved limits on Fierz Interference using asymmetry measurements from the UCNA experiment

Authors: Xuan Sun, E. Adamek, B. Allgeier, Y. Bagdasarova, D. B. Berguno, M. Blatnik, T. J. Bowles, L. J. Broussard, M. A. -P. Brown, R. Carr, S. Clayton, C. Cude-Woods, S. Currie, E. B. Dees, X. Ding, B. W. Filippone, A. García, P. Geltenbort, S. Hasan, K. P. Hickerson, J. Hoagland, R. Hong, A. T. Holley, T. M. Ito, A. Knecht , et al. (34 additional authors not shown)

Abstract: The Ultracold Neutron Asymmetry (UCNA) experiment was designed to measure the $β$-decay asymmetry parameter, $A_0$, for free neutron decay. In the experiment, polarized ultracold neutrons are transported into a decay trap, and their $β$-decay electrons are detected with $\approx 4π$ acceptance into two detector packages which provide position and energy reconstruction. The experiment also has sens… ▽ More The Ultracold Neutron Asymmetry (UCNA) experiment was designed to measure the $β$-decay asymmetry parameter, $A_0$, for free neutron decay. In the experiment, polarized ultracold neutrons are transported into a decay trap, and their $β$-decay electrons are detected with $\approx 4π$ acceptance into two detector packages which provide position and energy reconstruction. The experiment also has sensitivity to $b_{n}$, the Fierz interference term in the neutron $β$-decay rate. In this work, we determine $b_{n}$ from the energy dependence of $A_0$ using the data taken during the UCNA 2011-2013 run. In addition, we present the same type of analysis using the earlier 2010 $A$ dataset. Motivated by improved statistics and comparable systematic errors compared to the 2010 data-taking run, we present a new $b_{n}$ measurement using the weighted average of our asymmetry dataset fits, to obtain $b_{n} = 0.066 \pm 0.041_{\text{stat}} \pm 0.024_{\text{syst}}$ which corresponds to a limit of $-0.012 < b_{n} < 0.144$ at the 90% confidence level. △ Less

Submitted 13 November, 2019; originally announced November 2019.

Comments: 6 pages, 3 figures. Submission to PRC

Journal ref: Phys. Rev. C 101, 035503 (2020)

Final results for the neutron $β$-asymmetry parameter $A_0$ from the UCNA experiment

Authors: B. Plaster, E. Adamek, B. Allgeier, J. Anaya, H. O. Back, Y. Bagdasarova, D. B. Berguno, M. Blatnik, J. G. Boissevain, T. J. Bowles, L. J. Broussard, M. A. -P. Brown, R. Carr, D. J. Clark, S. Clayton, C. Cude-Woods, S. Currie, E. B. Dees, X. Ding, S. Du, B. W. Filippone, A. Garcia, P. Geltenbort, S. Hasan, A. Hawari , et al. (69 additional authors not shown)

Abstract: The UCNA experiment was designed to measure the neutron $β$-asymmetry parameter $A_0$ using polarized ultracold neutrons (UCN). UCN produced via downscattering in solid deuterium were polarized via transport through a 7 T magnetic field, and then directed to a 1 T solenoidal electron spectrometer, where the decay electrons were detected in electron detector packages located on the two ends of the… ▽ More The UCNA experiment was designed to measure the neutron $β$-asymmetry parameter $A_0$ using polarized ultracold neutrons (UCN). UCN produced via downscattering in solid deuterium were polarized via transport through a 7 T magnetic field, and then directed to a 1 T solenoidal electron spectrometer, where the decay electrons were detected in electron detector packages located on the two ends of the spectrometer. A value for $A_0$ was then extracted from the asymmetry in the numbers of counts in the two detector packages. We summarize all of the results from the UCNA experiment, obtained during run periods in 2007, 2008--2009, 2010, and 2011--2013, which ultimately culminated in a 0.67\% precision result for $A_0$. △ Less

Submitted 10 April, 2019; originally announced April 2019.

Comments: 6 pages, 7 figures, to appear in proceedings of the International Workshop on Particle Physics at Neutron Sources 2018

A boron-coated CCD camera for direct detection of Ultracold Neutrons (UCN)

Authors: K. Kuk, C. Cude-Woods, C. R. Chavez, J. H. Choi, J. Estrada, M. Hoffbauer, M. Makela, P. Merkel, C. L. Morris, E. Ramberg, Z. Wang, T. Bailey, M. Blatnik, E. R. Adamek, L. J. Broussard, M. A. -P. Brown, N. B. Callahan, S. M. Clayton, S. A. Currie, X. Ding, D. Dinger, B. Filippone, E. M. Fries, P. Geltenbort, E. George , et al. (26 additional authors not shown)

Abstract: A new boron-coated CCD camera is described for direct detection of ultracold neutrons (UCN) through the capture reactions $^{10}$B (n,$α$0$γ$)$^7$Li (6%) and $^{10}$B(n,$α$1$γ$)$^7$Li (94%). The experiments, which extend earlier works using a boron-coated ZnS:Ag scintillator, are based on direct detections of the neutron-capture byproducts in silicon. The high position resolution, energy resolutio… ▽ More A new boron-coated CCD camera is described for direct detection of ultracold neutrons (UCN) through the capture reactions $^{10}$B (n,$α$0$γ$)$^7$Li (6%) and $^{10}$B(n,$α$1$γ$)$^7$Li (94%). The experiments, which extend earlier works using a boron-coated ZnS:Ag scintillator, are based on direct detections of the neutron-capture byproducts in silicon. The high position resolution, energy resolution and particle ID performance of a scientific CCD allows for observation and identification of all the byproducts $α$, $^7$Li and $γ$ (electron recoils). A signal-to-noise improvement on the order of 10$^4$ over the indirect method has been achieved. Sub-pixel position resolution of a few microns is demonstrated. The technology can also be used to build UCN detectors with an area on the order of 1 m$^2$. The combination of micrometer scale spatial resolution, few electrons ionization thresholds and large area paves the way to new research avenues including quantum physics of UCN and high-resolution neutron imaging and spectroscopy. △ Less

Submitted 28 February, 2019; originally announced March 2019.

Comments: 10 pages, 8 figures

Report number: Los Alamos National Laboratory Report number LA-UR-19-21706

Journal ref: Nuclear Instruments and Methods in Physics Research Section A, Volume 1003, 1 July 2021, 165306

Using Nab to determine correlations in unpolarized neutron decay

Authors: L. J. Broussard, S. Baeßler, T. L. Bailey, N. Birge, J. D. Bowman, C. B. Crawford, C. Cude-Woods, D. E. Fellers, N. Fomin, E. Frlež, M. T. W. Gericke, L. Hayen, A. P. Jezghani, H. Li, N. Macsai, M. F. Makela, R. R. Mammei, D. Mathews, P. L. McGaughey, P. E. Mueller, D. Počanić, C. A. Royse, A. Salas-Bacci, S. K. L. Sjue, J. C. Ramsey , et al. (6 additional authors not shown)

Abstract: The Nab experiment will measure the ratio of the weak axial-vector and vector coupling constants $λ=g_A/g_V$ with precision $δλ/λ\sim3\times10^{-4}$ and search for a Fierz term $b_F$ at a level $Δb_F<10^{-3}$. The Nab detection system uses thick, large area, segmented silicon detectors to very precisely determine the decay proton's time of flight and the decay electron's energy in coincidence and… ▽ More The Nab experiment will measure the ratio of the weak axial-vector and vector coupling constants $λ=g_A/g_V$ with precision $δλ/λ\sim3\times10^{-4}$ and search for a Fierz term $b_F$ at a level $Δb_F<10^{-3}$. The Nab detection system uses thick, large area, segmented silicon detectors to very precisely determine the decay proton's time of flight and the decay electron's energy in coincidence and reconstruct the correlation between the antineutrino and electron momenta. Excellent understanding of systematic effects affecting timing and energy reconstruction using this detection system are required. To explore these effects, a series of ex situ studies have been undertaken, including a search for a Fierz term at a less sensitive level of $Δb_F<10^{-2}$ in the beta decay of $^{45}$Ca using the UCNA spectrometer. △ Less

Submitted 19 December, 2018; originally announced December 2018.

Comments: Proceedings of the 7th International Syposium on Symmetries in Subatomic Physics SSP2018, Aachen (Germany), 10 - 15 Jun 2018. This is a pre-print of an article published in Hyperfine Interactions. The final authenticated version is available online at: https://doi.org/10.1007/s10751-018-1538-7

Journal ref: Hyperfine Interact. 240 (2019) no.1, 1

The Nab Experiment: A Precision Measurement of Unpolarized Neutron Beta Decay

Authors: J. Fry, R. Alarcon, S. Baessler, S. Balascuta, L. Barron-Palos, T. Bailey, K. Bass, N. Birge, A. Blose, D. Borissenko, J. D. Bowman, L. J. Broussard, A. T. Bryant, J. Byrne, J. R. Calarco, J. Caylor, K. Chang, T. Chupp, T. V. Cianciolo, C. Crawford, X. Ding, M. Doyle, W. Fan, W. Farrar, N. Fomin , et al. (47 additional authors not shown)

Abstract: Neutron beta decay is one of the most fundamental processes in nuclear physics and provides sensitive means to uncover the details of the weak interaction. Neutron beta decay can evaluate the ratio of axial-vector to vector coupling constants in the standard model, $λ= g_A / g_V$, through multiple decay correlations. The Nab experiment will carry out measurements of the electron-neutrino correlati… ▽ More Neutron beta decay is one of the most fundamental processes in nuclear physics and provides sensitive means to uncover the details of the weak interaction. Neutron beta decay can evaluate the ratio of axial-vector to vector coupling constants in the standard model, $λ= g_A / g_V$, through multiple decay correlations. The Nab experiment will carry out measurements of the electron-neutrino correlation parameter $a$ with a precision of $δa / a = 10^{-3}$ and the Fierz interference term $b$ to $δb = 3\times10^{-3}$ in unpolarized free neutron beta decay. These results, along with a more precise measurement of the neutron lifetime, aim to deliver an independent determination of the ratio $λ$ with a precision of $δλ/ λ= 0.03\%$ that will allow an evaluation of $V_{ud}$ and sensitively test CKM unitarity, independent of nuclear models. Nab utilizes a novel, long asymmetric spectrometer that guides the decay electron and proton to two large area silicon detectors in order to precisely determine the electron energy and an estimation of the proton momentum from the proton time of flight. The Nab spectrometer is being commissioned at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source at Oak Ridge National Lab. We present an overview of the Nab experiment and recent updates on the spectrometer, analysis, and systematic effects. △ Less

Submitted 7 January, 2020; v1 submitted 25 November, 2018; originally announced November 2018.

Comments: Presented at PPNS2018

Monte Carlo Simulations of Trapped Ultracold Neutrons in the UCNτ Experiment

Authors: Nathan Callahan, Chen-Yu Liu, Francisco Gonzalez, Evan Adamek, James David Bowman, Leah Broussard, S. M. Clayton, S. Currie, C. Cude-Woods, E. B. Dees, X. Ding, E. M. Egnel, D. Fellers, W. Fox, P. Geltenbort, K. P. Hickerson, M. A. Hoffbauer, A. T. Holley, A. Komives, S. W. T. MacDonald, M. Makela, C. L. Morris, J. D. Ortiz, R. W. Pattie Jr, J. Ramsey , et al. (15 additional authors not shown)

Abstract: In the UCNτ experiment, ultracold neutrons (UCN) are confined by magnetic fields and the Earth's gravitational field. Field-trapping mitigates the problem of UCN loss on material surfaces, which caused the largest correction in prior neutron experiments using material bottles. However, the neutron dynamics in field traps differ qualitatively from those in material bottles. In the latter case, neut… ▽ More In the UCNτ experiment, ultracold neutrons (UCN) are confined by magnetic fields and the Earth's gravitational field. Field-trapping mitigates the problem of UCN loss on material surfaces, which caused the largest correction in prior neutron experiments using material bottles. However, the neutron dynamics in field traps differ qualitatively from those in material bottles. In the latter case, neutrons bounce off material surfaces with significant diffusivity and the population quickly reaches a static spatial distribution with a density gradient induced by the gravitational potential. In contrast, the field-confined UCN -- whose dynamics can be described by Hamiltonian mechanics -- do not exhibit the stochastic behaviors typical of an ideal gas model as observed in material bottles. In this report, we will describe our efforts to simulate UCN trapping in the UCNτ magneto-gravitational trap. We compare the simulation output to the experimental results to determine the parameters of the neutron detector and the input neutron distribution. The tuned model is then used to understand the phase space evolution of neutrons observed in the UCNτ experiment. We will discuss the implications of chaotic dynamics on controlling the systematic effects, such as spectral cleaning and microphonic heating, for a successful UCN lifetime experiment to reach a 0.01% level of precision. △ Less

Submitted 16 October, 2018; originally announced October 2018.

Comments: 18 pages, 19 figures

Journal ref: Phys. Rev. C 100, 015501 (2019)

Search for dark matter decay of the free neutron from the UCNA experiment: n $\rightarrow χ+ e^+e^-$

Authors: X. Sun, E. Adamek, B. Allgeier, M. Blatnik, T. J. Bowles, L. J. Broussard, M. A. -P. Brown, R. Carr, S. Clayton, C. Cude-Woods, S. Currie, E. B. Dees, X. Ding, B. W. Filippone, A. García, P. Geltenbort, S. Hasan, K. P. Hickerson, J. Hoagland, R. Hong, G. E. Hogan, A. T. Holley, T. M. Ito, A. Knecht, C. -Y. Liu , et al. (35 additional authors not shown)

Abstract: It has been proposed recently that a previously unobserved neutron decay branch to a dark matter particle ($χ$) could account for the discrepancy in the neutron lifetime observed in experiments that use two different measurement techniques. One of the possible final states discussed includes a single $χ$ along with an $e^{+}e^{-}$ pair. We use data from the UCNA (Ultracold Neutron Asymmetry) exper… ▽ More It has been proposed recently that a previously unobserved neutron decay branch to a dark matter particle ($χ$) could account for the discrepancy in the neutron lifetime observed in experiments that use two different measurement techniques. One of the possible final states discussed includes a single $χ$ along with an $e^{+}e^{-}$ pair. We use data from the UCNA (Ultracold Neutron Asymmetry) experiment to set limits on this decay channel. Coincident electron-like events are detected with $\sim 4π$ acceptance using a pair of detectors that observe a volume of stored Ultracold Neutrons (UCNs). The summed kinetic energy ($E_{e^{+}e^{-}}$) from such events is used to set limits, as a function of the $χ$ mass, on the branching fraction for this decay channel. For $χ$ masses consistent with resolving the neutron lifetime discrepancy, we exclude this as the dominant dark matter decay channel at $\gg~5σ$ level for $100~\text{keV} < E_{e^{+}e^{-}} < 644~\text{keV}$. If the $χ+e^{+}e^{-}$ final state is not the only one, we set limits on its branching fraction of $< 10^{-4}$ for the above $E_{e^{+}e^{-}}$ range at $> 90\%$ confidence level. △ Less

Submitted 28 March, 2018; originally announced March 2018.

Comments: 5 pages, 5 figures

Journal ref: Phys. Rev. C 97, 052501 (2018)

New result for the neutron $β$-asymmetry parameter $A_0$ from UCNA

Authors: M. A. -P. Brown, E. B. Dees, E. Adamek, B. Allgeier, M. Blatnik, T. J. Bowles, L. J. Broussard, R. Carr, S. Clayton, C. Cude-Woods, S. Currie, X. Ding, B. W. Filippone, A. Garcia, P. Geltenbort, S. Hasan, K. P. Hickerson, J. Hoagland, R. Hong, G. E. Hogan, A. T. Holley, T. M. Ito, A. Knecht, C. -Y. Liu, J. Liu , et al. (34 additional authors not shown)

Abstract: The neutron $β$-decay asymmetry parameter $A_0$ defines the correlation between the spin of the neutron and the momentum of the emitted electron, which determines $λ=\frac{g_{A}}{g_{V}}$, the ratio of the axial-vector to vector weak coupling constants. The UCNA Experiment, located at the Ultracold Neutron facility at the Los Alamos Neutron Science Center, is the first to measure such a correlation… ▽ More The neutron $β$-decay asymmetry parameter $A_0$ defines the correlation between the spin of the neutron and the momentum of the emitted electron, which determines $λ=\frac{g_{A}}{g_{V}}$, the ratio of the axial-vector to vector weak coupling constants. The UCNA Experiment, located at the Ultracold Neutron facility at the Los Alamos Neutron Science Center, is the first to measure such a correlation coefficient using ultracold neutrons (UCN). Following improvements to the systematic uncertainties and increased statistics, we report the new result $A_0 = -0.12054(44)_{\mathrm{stat}}(68)_{\mathrm{syst}}$ which yields $λ\equiv \frac{g_{A}}{g_{V}}=-1.2783(22)$. Combination with the previous UCNA result and accounting for correlated systematic uncertainties produces $A_0=-0.12015(34)_{\mathrm{stat}}(63)_{\mathrm{syst}}$ and $λ\equiv \frac{g_{A}}{g_{V}}=-1.2772(20)$. △ Less

Submitted 14 August, 2018; v1 submitted 3 December, 2017; originally announced December 2017.

Comments: 9 pages, 7 figures, updated to as-published version

Journal ref: Phys. Rev. C 97, 035505 (2018)

Measurement of the neutron lifetime using an asymmetric magneto- gravitational trap and in situ detection

Authors: R. W. Pattie Jr., N. B. Callahan, C. Cude-Woods, E. R. Adamek, L. J. Broussard, S. M. Clayton, S. A. Currie, E. B. Dees, X. Ding, E. M. Engel, D. E. Fellers, W. Fox, K. P. Hickerson, M. A. Hoffbauer, A. T. Holley, A. Komives, C. -Y. Liu, S. W. T. MacDonald, M. Makela, C. L. Morris, J. D. Ortiz, J. Ramsey, D. J. Salvat, A. Saunders, S. J. Seestrom , et al. (13 additional authors not shown)

Abstract: The precise value of the mean neutron lifetime, $τ_n$, plays an important role in nuclear and particle physics and cosmology. It is a key input for predicting the ratio of protons to helium atoms in the primordial universe and is used to search for new physics beyond the Standard Model of particle physics. There is a 3.9 standard deviation discrepancy between $τ_n$ measured by counting the decay r… ▽ More The precise value of the mean neutron lifetime, $τ_n$, plays an important role in nuclear and particle physics and cosmology. It is a key input for predicting the ratio of protons to helium atoms in the primordial universe and is used to search for new physics beyond the Standard Model of particle physics. There is a 3.9 standard deviation discrepancy between $τ_n$ measured by counting the decay rate of free neutrons in a beam (887.7 $\pm$ 2.2 s) and by counting surviving ultracold neutrons stored for different storage times in a material trap (878.5$\pm$0.8 s). The experiment described here eliminates loss mechanisms present in previous trap experiments by levitating polarized ultracold neutrons above the surface of an asymmetric storage trap using a repulsive magnetic field gradient so that the stored neutrons do not interact with material trap walls and neutrons in quasi-stable orbits rapidly exit the trap. As a result of this approach and the use of a new in situ neutron detector, the lifetime reported here (877.7 $\pm$ 0.7 (stat) +0.4/-0.2 (sys) s) is the first modern measurement of $τ_n$ that does not require corrections larger than the quoted uncertainties. △ Less

Submitted 7 February, 2018; v1 submitted 6 July, 2017; originally announced July 2017.

Comments: 9 pages, 3 figures, 2 tables

A new method for measuring the neutron lifetime using an in situ neutron detector

Authors: C. L. Morris, E. R. Adamek, L. J. Broussard, N. B. Callahan, S. M. Clayton, C. Cude-Woods, S. A. Currie, X. Ding, W. Fox, K. P. Hickerson, A. T. Holley, A. Komives, C. -Y. Liu, M. Makela, R. W. Pattie Jr., J. Ramsey, D. J. Salvat, A. Saunders, S. J. Seestrom, E. I. Sharapov, S. K. Sjue, Z. Tang, J. Vanderwerp, B. Vogelaar, P. L. Walstrom , et al. (6 additional authors not shown)

Abstract: The neutron lifetime is important in understanding the production of light nuclei in the first minutes after the big bang and it provides basic information on the charged weak current of the standard model of particle physics. Two different methods have been used to measure the neutron lifetime: disappearance measurements using bottled ultracold neutrons and decay rate measurements using neutron b… ▽ More The neutron lifetime is important in understanding the production of light nuclei in the first minutes after the big bang and it provides basic information on the charged weak current of the standard model of particle physics. Two different methods have been used to measure the neutron lifetime: disappearance measurements using bottled ultracold neutrons and decay rate measurements using neutron beams. The best measurements using these two techniques give results that differ by nearly 4 standard deviations. In this paper we describe a new method for measuring surviving neutrons in neutron lifetime measurements using bottled ultracold neutrons that provides better characterization of systematic uncertainties and enables higher precision than previous measurement techniques. We present results obtained using our method. △ Less

Submitted 14 October, 2016; originally announced October 2016.

Report number: LA-UR-16-27352

Detection System for Neutron $β$ Decay Correlations in the UCNB and Nab experiments

Authors: L. J. Broussard, B. A. Zeck, E. R. Adamek, S. Baeßler, N. Birge, M. Blatnik, J. D. Bowman, A. E. Brandt, M. Brown, J. Burkhart, N. B. Callahan, S. M. Clayton, C. Crawford, C. Cude-Woods, S. Currie, E. B. Dees, X. Ding, N. Fomin, E. Frlez, J. Fry, F. E. Gray, S. Hasan, K. P. Hickerson, J. Hoagland, A. T. Holley , et al. (29 additional authors not shown)

Abstract: We describe a detection system designed for precise measurements of angular correlations in neutron $β$ decay. The system is based on thick, large area, highly segmented silicon detectors developed in collaboration with Micron Semiconductor, Ltd. The prototype system meets specifications for $β$ electron detection with energy thresholds below 10 keV, energy resolution of $\sim$3 keV FWHM, and rise… ▽ More We describe a detection system designed for precise measurements of angular correlations in neutron $β$ decay. The system is based on thick, large area, highly segmented silicon detectors developed in collaboration with Micron Semiconductor, Ltd. The prototype system meets specifications for $β$ electron detection with energy thresholds below 10 keV, energy resolution of $\sim$3 keV FWHM, and rise time of $\sim$50 ns with 19 of the 127 detector pixels instrumented. Using ultracold neutrons at the Los Alamos Neutron Science Center, we have demonstrated the coincident detection of $β$ particles and recoil protons from neutron $β$ decay. The fully instrumented detection system will be implemented in the UCNB and Nab experiments, to determine the neutron $β$ decay parameters $B$, $a$, and $b$. △ Less

Submitted 7 January, 2017; v1 submitted 9 July, 2016; originally announced July 2016.

Comments: Copyright 2016. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

New Measurements of the Transverse Beam Asymmetry for Elastic Electron Scattering from Selected Nuclei

Authors: The HAPPEX, PREX Collaborations, :, S. Abrahamyan, A. Acha, A. Afanasev, Z. Ahmed, H. Albataineh, K. Aniol, D. S. Armstrong, W. Armstrong, J. Arrington, T. Averett, B. Babineau, S. L. Bailey, J. Barber, A. Barbieri, A. Beck, V. Bellini, R. Beminiwattha, H. Benaoum, J. Benesch, F. Benmokhtar, P. Bertin, T. Bielarski , et al. (173 additional authors not shown)

Abstract: We have measured the beam-normal single-spin asymmetry $A_n$ in the elastic scattering of 1-3 GeV transversely polarized electrons from $^1$H and for the first time from $^4$He, $^{12}$C, and $^{208}$Pb. For $^1$H, $^4$He and $^{12}$C, the measurements are in agreement with calculations that relate $A_n$ to the imaginary part of the two-photon exchange amplitude including inelastic intermediate st… ▽ More We have measured the beam-normal single-spin asymmetry $A_n$ in the elastic scattering of 1-3 GeV transversely polarized electrons from $^1$H and for the first time from $^4$He, $^{12}$C, and $^{208}$Pb. For $^1$H, $^4$He and $^{12}$C, the measurements are in agreement with calculations that relate $A_n$ to the imaginary part of the two-photon exchange amplitude including inelastic intermediate states. Surprisingly, the $^{208}$Pb result is significantly smaller than the corresponding prediction using the same formalism. These results suggest that a systematic set of new $A_n$ measurements might emerge as a new and sensitive probe of the structure of heavy nuclei. △ Less

Submitted 12 October, 2012; v1 submitted 30 August, 2012; originally announced August 2012.

Comments: 5 pages, 3 figures, accepted by PRL. v3: fixed one author name and affiliation, otherwise no change

Journal ref: Phys. Rev. Lett. 109, (2012) 192501

Weak charge form factor and radius of 208Pb through parity violation in electron scattering

Authors: C. J. Horowitz, Z. Ahmed, C. -M. Jen, A. Rakhman, P. A. Souder, M. M. Dalton, N. Liyanage, K. D. Paschke, K. Saenboonruang, R. Silwal, G. B. Franklin, M. Friend, B. Quinn, K. S. Kumar, J. M. Mammei, D. McNulty, L. Mercado, S. Riordan, J. Wexler, R. W. Michaels, G. M. Urciuoli

Abstract: We use distorted wave electron scattering calculations to extract the weak charge form factor F_W(q), the weak charge radius R_W, and the point neutron radius R_n, of 208Pb from the PREX parity violating asymmetry measurement. The form factor is the Fourier transform of the weak charge density at the average momentum transfer q=0.475 fm$^{-1}$. We find F_W(q) =0.204 \pm 0.028 (exp) \pm 0.001 (mode… ▽ More We use distorted wave electron scattering calculations to extract the weak charge form factor F_W(q), the weak charge radius R_W, and the point neutron radius R_n, of 208Pb from the PREX parity violating asymmetry measurement. The form factor is the Fourier transform of the weak charge density at the average momentum transfer q=0.475 fm$^{-1}$. We find F_W(q) =0.204 \pm 0.028 (exp) \pm 0.001 (model). We use the Helm model to infer the weak radius from F_W(q). We find R_W= 5.826 \pm 0.181 (exp) \pm 0.027 (model) fm. Here the exp error includes PREX statistical and systematic errors, while the model error describes the uncertainty in R_W from uncertainties in the surface thickness σof the weak charge density. The weak radius is larger than the charge radius, implying a "weak charge skin" where the surface region is relatively enriched in weak charges compared to (electromagnetic) charges. We extract the point neutron radius R_n=5.751 \pm 0.175 (exp) \pm 0.026 (model) \pm 0.005 (strange) fm$, from R_W. Here there is only a very small error (strange) from possible strange quark contributions. We find R_n to be slightly smaller than R_W because of the nucleon's size. Finally, we find a neutron skin thickness of R_n-R_p=0.302\pm 0.175 (exp) \pm 0.026 (model) \pm 0.005 (strange) fm, where R_p is the point proton radius. △ Less

Submitted 13 February, 2014; v1 submitted 7 February, 2012; originally announced February 2012.

Comments: 5 pages, 1 figure, published in Phys Rev. C. Only one change in this version: we have added one author, also to metadata

Journal ref: Phys. Rev. C 85, 032501(R) (2012)

Measurement of the Neutron Radius of 208Pb Through Parity-Violation in Electron Scattering

Authors: S. Abrahamyan, Z. Ahmed, H. Albataineh, K. Aniol, D. S. Armstrong, W. Armstrong, T. Averett, B. Babineau, A. Barbieri, V. Bellini, R. Beminiwattha, J. Benesch, F. Benmokhtar, T. Bielarski, W. Boeglin, A. Camsonne, M. Canan, P. Carter, G. D. Cates, C. Chen, J. -P. Chen, O. Hen, F. Cusanno, M. M. Dalton, R. De Leo , et al. (110 additional authors not shown)

Abstract: We report the first measurement of the parity-violating asymmetry A_PV in the elastic scattering of polarized electrons from 208Pb. A_PV is sensitive to the radius of the neutron distribution (Rn). The result A_PV = 0.656 \pm 0.060 (stat) \pm 0.014 (syst) ppm corresponds to a difference between the radii of the neutron and proton distributions Rn - Rp = 0.33 +0.16 -0.18 fm and provides the first e… ▽ More We report the first measurement of the parity-violating asymmetry A_PV in the elastic scattering of polarized electrons from 208Pb. A_PV is sensitive to the radius of the neutron distribution (Rn). The result A_PV = 0.656 \pm 0.060 (stat) \pm 0.014 (syst) ppm corresponds to a difference between the radii of the neutron and proton distributions Rn - Rp = 0.33 +0.16 -0.18 fm and provides the first electroweak observation of the neutron skin which is expected in a heavy, neutron-rich nucleus. △ Less

Submitted 13 January, 2012; v1 submitted 12 January, 2012; originally announced January 2012.

Comments: 6 pages, 1 figure

Report number: JLAB-PHY-12-1480

Journal ref: Phys. Rev. Lett. 108, 112502 (2012)

New Precision Limit on the Strange Vector Form Factors of the Proton

Authors: HAPPEX collaboration, Z. Ahmed, K. Allada, K. A. Aniol, D. S. Armstrong, J. Arrington, P. Baturin, V. Bellini, J. Benesch, R. Beminiwattha, F. Benmokhtar, M. Canan, A. Camsonne, G. D. Cates, J. -P. Chen, E. Chudakov, E. Cisbani, M. M. Dalton, C. W. de Jager, R. De Leo, W. Deconinck, P. Decowski, X. Deng, A. Deur, C. Dutta , et al. (80 additional authors not shown)

Abstract: The parity-violating cross-section asymmetry in the elastic scattering of polarized electrons from unpolarized protons has been measured at a four-momentum transfer squared Q2 = 0.624 GeV and beam energy E =3.48 GeV to be A_PV = -23.80 +/- 0.78 (stat) +/- 0.36 (syst) parts per million. This result is consistent with zero contribution of strange quarks to the combination of electric and magnetic fo… ▽ More The parity-violating cross-section asymmetry in the elastic scattering of polarized electrons from unpolarized protons has been measured at a four-momentum transfer squared Q2 = 0.624 GeV and beam energy E =3.48 GeV to be A_PV = -23.80 +/- 0.78 (stat) +/- 0.36 (syst) parts per million. This result is consistent with zero contribution of strange quarks to the combination of electric and magnetic form factors G_E^s + 0.517 G_M^s = 0.003 +/- 0.010 (stat) +/- 0.004 (syst) +/- 0.009 (ff), where the third error is due to the limits of precision on the electromagnetic form factors and radiative corrections. With this measurement, the world data on strange contributions to nucleon form factors are seen to be consistent with zero and not more than a few percent of the proton form factors. △ Less

Submitted 5 July, 2011; originally announced July 2011.

Comments: 5 pages, 3 figures