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Accurate Charge Radius Measurement of $^{14}$C Confronts \textit{Ab Initio} Theory
Authors:
Kristian König,
Patrick Müller,
Tobias Gesser,
Emily Burbach,
Stefano Gandolfi,
Matthias Heinz,
Phillip Imgram,
Alessandro Lovato,
Pieter Maris,
Takayuki Miyagi,
Wilfried Nörtershäuser,
Robert Roth,
Julien Spahn,
Achim Schwenk
Abstract:
Located at the neutron shell closure $N = 8$, the long-lived radioactive isotope \(^{14}\mathrm{C} \) plays a critical role in geochronology and nuclear structure studies. Despite its widespread use, the nuclear charge radius of $^{14}$C has remained less precisely known compared to its stable counterpart $^{12}$C. Here, we report a high-precision determination of the $^{14}$C charge radius using…
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Located at the neutron shell closure $N = 8$, the long-lived radioactive isotope \(^{14}\mathrm{C} \) plays a critical role in geochronology and nuclear structure studies. Despite its widespread use, the nuclear charge radius of $^{14}$C has remained less precisely known compared to its stable counterpart $^{12}$C. Here, we report a high-precision determination of the $^{14}$C charge radius using collinear laser spectroscopy at the COALA setup at TU Darmstadt, improving upon the precision of previous muonic measurements by a factor $5$ and revealing a $1.9σ$ discrepancy of combined uncertainty, indicating a likely underestimated uncertainty in the muonic determination. This measurement challenges state-of-the-art \textit{ab initio} nuclear theory calculations, including auxiliary field diffusion Monte Carlo, the valence-space in-medium similarity renormalization group, and the no-core shell model, augmented by neural-network techniques. With $^{12}$C and $^{14}$C now forming one of the most precisely characterized even-even isotope pairs, these results also enable improved QED tests.
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Submitted 17 August, 2026;
originally announced August 2026.
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The dipole strength distribution of $^8$He and decay characteristics
Authors:
C. Lehr,
T. Aumann,
M. Duer,
A. T. Saito,
T. Nakamura,
N. L. Achouri,
D. Ahn,
H. Baba,
S. Bacca,
C. A. Bertulani,
M. Böhmer,
F. Bonaiti,
K. Boretzky,
C. Caesar,
N. Chiga,
D. Cortina-Gil,
C. A. Douma,
F. Dufter,
Z. Elekes,
J. Feng,
B. Fernández-Domínguez,
U. Forsberg,
N. Fukuda,
I. Gasparic,
Z. Ge
, et al. (69 additional authors not shown)
Abstract:
The weak binding and spatially extended neutron densities characteristic of drip-line nuclei give rise to a distinctive low-energy dipole response. The drip-line nucleus $^8$He is the most neutron-rich bound nucleus with a mass-to-charge ratio of $A/Z=4$. We measure the dipole response of $^8$He, including for the first time the four-neutron decay channel. A total dipole strength of…
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The weak binding and spatially extended neutron densities characteristic of drip-line nuclei give rise to a distinctive low-energy dipole response. The drip-line nucleus $^8$He is the most neutron-rich bound nucleus with a mass-to-charge ratio of $A/Z=4$. We measure the dipole response of $^8$He, including for the first time the four-neutron decay channel. A total dipole strength of $\sum B(E1)(E^*<15$~MeV$)=0.95(16)~e^2$fm$^2$ and a dipole polarizability of $α_D = 0.61(1)$~fm$^3$ are extracted from the differential Coulomb-excitation cross section and compared to state-of-the-art theoretical calculations employing coupled cluster and three-body approaches. We find that the dipole continuum is dominated, even at high excitation energies well above the $4n$ decay threshold, by two-neutron emission, pointing to a $^6$He$+2n$ structure of the excited dipole mode. No indication was found for a $4n$ final-state correlation, while pronounced $nn$ and $^6$He-$n$ final-state correlations are apparent.
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Submitted 3 April, 2026; v1 submitted 25 March, 2026;
originally announced March 2026.
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Anisotropic flow in fixed-target $^{208}$Pb+$^{20}$Ne collisions as a probe of quark-gluon plasma
Authors:
Giuliano Giacalone,
Wenbin Zhao,
Benjamin Bally,
Shihang Shen,
Thomas Duguet,
Jean-Paul Ebran,
Serdar Elhatisari,
Mikael Frosini,
Timo A. Lähde,
Dean Lee,
Bing-Nan Lu,
Yuan-Zhuo Ma,
Ulf-G. Meißner,
Govert Nijs,
Jacquelyn Noronha-Hostler,
Christopher Plumberg,
Tomás R. Rodríguez,
Robert Roth,
Wilke van der Schee,
Björn Schenke,
Chun Shen,
Vittorio Somà
Abstract:
The System for Measuring Overlap with Gas (SMOG2) at the LHCb detector enables the study of fixed-target ion-ion collisions at relativistic energies ($\sqrt{s_{\rm NN}}\sim100$ GeV in the centre-of-mass). With input from \textit{ab initio} calculations of the structure of $^{16}$O and $^{20}$Ne, we compute 3+1D hydrodynamic predictions for the anisotropic flow of Pb+Ne and Pb+O collisions, to be t…
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The System for Measuring Overlap with Gas (SMOG2) at the LHCb detector enables the study of fixed-target ion-ion collisions at relativistic energies ($\sqrt{s_{\rm NN}}\sim100$ GeV in the centre-of-mass). With input from \textit{ab initio} calculations of the structure of $^{16}$O and $^{20}$Ne, we compute 3+1D hydrodynamic predictions for the anisotropic flow of Pb+Ne and Pb+O collisions, to be tested with upcoming LHCb data. This will allow the detailed study of quark-gluon plasma (QGP) formation as well as experimental tests of the predicted nuclear shapes. Elliptic flow ($v_2$) in Pb+Ne collisions is greatly enhanced compared to the Pb+O baseline due to the shape of $^{20}$Ne, which is deformed in a bowling-pin geometry. Owing to the large $^{208}$Pb radius, this effect is seen in a broad centrality range, a unique feature of this collision configuration. Larger elliptic flow further enhances the quadrangular flow ($v_4$) of Pb+Ne collisions via non-linear coupling, and impacts the sign of the kurtosis of the elliptic flow vector distribution ($c_2\{4\}$). Exploiting the shape of $^{20}$Ne proves thus an ideal method to investigate the formation of QGP in fixed-target experiments at LHCb, and demonstrates the power of SMOG2 as a tool to image nuclear ground states.
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Submitted 26 February, 2025; v1 submitted 30 May, 2024;
originally announced May 2024.
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Ab initio description of monopole resonances in light- and medium-mass nuclei: II. Ab initio PGCM calculations in $^{46}$Ti, $^{28}$Si and $^{24}$Mg
Authors:
Andrea Porro,
Thomas Duguet,
Jean-Paul Ebran,
Mikael Frosini,
Robert Roth,
Vittorio Somà
Abstract:
Giant resonances (GRs) are a striking manifestation of collective motions in atomic nuclei. The present paper is the second in a series of four dedicated to the use of the projected generator coordinate method (PGCM) for the ab initio determination of the isoscalar giant monopole resonance (GMR) in closed- and open-shell mid-mass nuclei.
While the first paper was dedicated to quantifying various…
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Giant resonances (GRs) are a striking manifestation of collective motions in atomic nuclei. The present paper is the second in a series of four dedicated to the use of the projected generator coordinate method (PGCM) for the ab initio determination of the isoscalar giant monopole resonance (GMR) in closed- and open-shell mid-mass nuclei.
While the first paper was dedicated to quantifying various uncertainty sources, the present paper focuses on the first applications to three doubly-open shell nuclei, namely $^{46}$Ti, $^{28}$Si and $^{24}$Mg. In particular, the goal is to investigate from an ab initio standpoint (i) the coupling of the GMR with the giant quadrupole resonance (GQR) in intrinsically-deformed nuclei, (ii) the possible impact of shape coexistence and shape mixing on the GMR, (iii) the GMR based on shape isomers and (iv) the impact of anharmonic effects on the monopole response. The latter is studied by comparing PGCM results to those obtained via the quasi-particle random phase approximation (QRPA), the traditional many-body approach to giant resonances, performed in a consistent setting.
Eventually, PGCM results for sd-shell nuclei are in excellent agreement with experimental data, which is attributed to the capacity of the PGCM to capture the important fragmentation of the monopole response in light, intrinsically-deformed systems. Still, the comparison to data in $^{28}$Si and $^{24}$Mg illustrates the challenge (and the potential benefit) of extracting unambiguous experimental information.
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Submitted 24 February, 2024;
originally announced February 2024.
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The unexpected uses of a bowling pin: exploiting $^{20}$Ne isotopes for precision characterizations of collectivity in small systems
Authors:
Giuliano Giacalone,
Benjamin Bally,
Govert Nijs,
Shihang Shen,
Thomas Duguet,
Jean-Paul Ebran,
Serdar Elhatisari,
Mikael Frosini,
Timo A. Lähde,
Dean Lee,
Bing-Nan Lu,
Yuan-Zhuo Ma,
Ulf-G. Meißner,
Jacquelyn Noronha-Hostler,
Christopher Plumberg,
Tomás R. Rodríguez,
Robert Roth,
Wilke van der Schee,
Vittorio Somà
Abstract:
Whether or not femto-scale droplets of quark-gluon plasma (QGP) are formed in so-called small systems at high-energy colliders is a pressing question in the phenomenology of the strong interaction. For proton-proton or proton-nucleus collisions the answer is inconclusive due to the large theoretical uncertainties plaguing the description of these processes. While upcoming data on collisions of…
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Whether or not femto-scale droplets of quark-gluon plasma (QGP) are formed in so-called small systems at high-energy colliders is a pressing question in the phenomenology of the strong interaction. For proton-proton or proton-nucleus collisions the answer is inconclusive due to the large theoretical uncertainties plaguing the description of these processes. While upcoming data on collisions of $^{16}$O nuclei may mitigate these uncertainties in the near future, here we demonstrate the unique possibilities offered by complementing $^{16}$O$^{16}$O data with collisions of $^{20}$Ne ions. We couple both NLEFT and PGCM ab initio descriptions of the structure of $^{20}$Ne and $^{16}$O to hydrodynamic simulations of $^{16}$O$^{16}$O and $^{20}$Ne$^{20}$Ne collisions at high energy. We isolate the imprints of the bowling-pin shape of $^{20}$Ne on the collective flow of hadrons, which can be used to perform quantitative tests of the hydrodynamic QGP paradigm. In particular, we predict that the elliptic flow of $^{20}$Ne$^{20}$Ne collisions is enhanced by as much as 1.170(8)$_{\rm stat.}$(30)$_{\rm syst.}$ for NLEFT and 1.139(6)$_{\rm stat.}$(39)$_{\rm syst.}$ for PGCM relative to $^{16}$O$^{16}$O collisions for the 1% most central events. At the same time, theoretical uncertainties largely cancel when studying relative variations of observables between two systems. This demonstrates a method based on experiments with two light-ion species for precision characterizations of the collective dynamics and its emergence in a small system.
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Submitted 8 February, 2024;
originally announced February 2024.
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Ab initio description of monopole resonances in light- and medium-mass nuclei: I. Technical aspects and uncertainties of ab initio PGCM calculations
Authors:
Andrea Porro,
Thomas Duguet,
Jean-Paul Ebran,
Mikael Frosini,
Robert Roth,
Vittorio Somá
Abstract:
Giant resonances (GRs) are a striking manifestation of collective motions in mesoscopic systems such as atomic nuclei. Until recently, theoretical investigations have essentially relied on the (quasiparticle) random phase approximation ((Q)RPA), and extensions of it, based on phenomenological energy density functionals (EDFs). As part of a current effort to describe GRs within an ab initio theoret…
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Giant resonances (GRs) are a striking manifestation of collective motions in mesoscopic systems such as atomic nuclei. Until recently, theoretical investigations have essentially relied on the (quasiparticle) random phase approximation ((Q)RPA), and extensions of it, based on phenomenological energy density functionals (EDFs). As part of a current effort to describe GRs within an ab initio theoretical scheme, the present work promotes the use of the projected generator coordinate method (PGCM). This method, which can handle anharmonic effects while satisfying symmetries of the nuclear Hamiltonian, displays a favorable (i.e. mean-field-like) scaling with system's size. Presently focusing on the isoscalar giant monopole resonance (GMR) of light- and medium-mass nuclei, PGCM's potential to deliver wide-range ab initio studies of GRs in closed- and open-shell nuclei encompassing pairing, deformation, and shape coexistence effects is demonstrated. The comparison with consistent QRPA calculations highlights PGCM's unique attributes and sheds light on the intricate interplay of nuclear collective excitations. The present paper is the first in a series of four and focuses on technical aspects and uncertainty quantification of ab initio PGCM calculations of GMR using the doubly open-shell $^{46}$Ti as an illustrative example. The second paper displays results for a set of nuclei of physical interest and proceeds to the comparison with consistent (deformed) ab initio QRPA calculations. While the third paper analyzes useful moments of the monopolar strength function and different ways to access them within PGCM calculations, the fourth paper focuses on the effect of the symmetry restoration on the monopole strength function.
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Submitted 3 February, 2024;
originally announced February 2024.
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Charge radii of $^{55,56}$Ni reveal a surprisingly similar behavior at $N=28$ in Ca and Ni isotopes
Authors:
F. Sommer,
K. König,
D. M. Rossi,
N. Everett,
D. Garand,
R. P. de Groote,
J. D. Holt,
P. Imgram,
A. Incorvati,
C. Kalman,
A. Klose,
J. Lantis,
Y. Liu,
A. J. Miller,
K. Minamisono,
T. Miyagi,
W. Nazarewicz,
W. Nörtershäuser,
S. V. Pineda,
R. Powel,
P. -G. Reinhard,
L. Renth,
E. Romero-Romero,
R. Roth,
A. Schwenk
, et al. (2 additional authors not shown)
Abstract:
Nuclear charge radii of $^{55,56}$Ni were measured by collinear laser spectroscopy. The obtained information completes the behavior of the charge radii at the shell closure of the doubly magic nucleus $^{56}$Ni. The trend of charge radii across the shell closures in calcium and nickel is surprisingly similar despite the fact that the $^{56}$Ni core is supposed to be much softer than the $^{48}$Ca…
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Nuclear charge radii of $^{55,56}$Ni were measured by collinear laser spectroscopy. The obtained information completes the behavior of the charge radii at the shell closure of the doubly magic nucleus $^{56}$Ni. The trend of charge radii across the shell closures in calcium and nickel is surprisingly similar despite the fact that the $^{56}$Ni core is supposed to be much softer than the $^{48}$Ca core. The very low magnetic moment $μ(^{55}\mathrm{Ni})=-1.108(20)\,μ_N$ indicates the impact of M1 excitations between spin-orbit partners across the $N,Z=28$ shell gaps. Our charge-radii results are compared to \textit{ab initio} and nuclear density functional theory calculations, showing good agreement within theoretical uncertainties.
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Submitted 4 October, 2022;
originally announced October 2022.
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Damping of the isovector giant dipole resonance in $^{40,48}$Ca
Authors:
J. Carter,
L. M. Donaldson,
H. Fujita,
Y. Fujita,
M. Jingo,
C. O. Kureba,
M. B. Latif,
E. Litvinova,
F. Nemulodi,
P. von Neumann-Cosel,
R. Neveling,
P. Papakonstantinou,
P. Papka,
L. Pellegri,
V. Yu. Ponomarev,
A. Richter,
R. Roth,
E. Sideras-Haddad,
F. D. Smit,
J. A. Swartz,
A. Tamii,
R. Trippel,
I. T. Usman,
H. Wibowo
Abstract:
The fine structure of the IsoVector Giant Dipole Resonance (IVGDR) in the doubly-magic nuclei $^{40,48}$Ca observed in inelastic proton scattering experiments under $0^\circ$ is used to investigate the role of different mechanisms contributing to the IVGDR decay width. Characteristic energy scales are extracted from the fine structure by means of wavelet analysis. The experimental scales are compa…
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The fine structure of the IsoVector Giant Dipole Resonance (IVGDR) in the doubly-magic nuclei $^{40,48}$Ca observed in inelastic proton scattering experiments under $0^\circ$ is used to investigate the role of different mechanisms contributing to the IVGDR decay width. Characteristic energy scales are extracted from the fine structure by means of wavelet analysis. The experimental scales are compared to different theoretical approaches allowing for the inclusion of complex configurations beyond the mean-field level. Calculations are performed in the framework of RPA and beyond-RPA in a relativistic approach based on an effective meson-exchange interaction, with the UCOM effective interaction and, for the first time, with realistic two- plus three-nucleon interactions from chiral effective field theory employing the in-medium similarity renormalization group. All models highlight the role of Landau fragmentation for the damping of the IVGDR, while the differences in the coupling strength between one particle-one hole (1p-1h) and two particle-two hole (2p-2h) correlated (relativistic) and non-correlated (non-relativistic) configurations lead to very different pictures of the importance of the spreading width resulting in wavelet scales being a sensitive measure of their interplay. The relativistic approach with particle-vibration coupling, in particular, shows impressive agreement with the number and absolute values of the scales extracted from the experimental data.
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Submitted 11 July, 2022; v1 submitted 26 April, 2022;
originally announced April 2022.
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Role of chiral two-body currents in $^6$Li magnetic properties in light of a new precision measurement with the relative self-absorption technique
Authors:
U. Friman-Gayer,
C. Romig,
T. Hüther,
K. Albe,
S. Bacca,
T. Beck,
M. Berger,
J. Birkhan,
K. Hebeler,
O. J. Hernandez,
J. Isaak,
S. König,
N. Pietralla,
P. C. Ries,
J. Rohrer,
R. Roth,
D. Savran,
M. Scheck,
A. Schwenk,
R. Seutin,
V. Werner
Abstract:
A direct measurement of the decay width of the excited $0^+_1$ state of $^6$Li using the relative self-absorption technique is reported. Our value of $Γ_{γ, 0^+_1 \to 1^+_1} = 8.17(14)_\mathrm{stat.}(11)_\mathrm{syst.} \mathrm{eV}$ provides sufficiently low experimental uncertainties to test modern theories of nuclear forces. The corresponding transition rate is compared to the results of ab initi…
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A direct measurement of the decay width of the excited $0^+_1$ state of $^6$Li using the relative self-absorption technique is reported. Our value of $Γ_{γ, 0^+_1 \to 1^+_1} = 8.17(14)_\mathrm{stat.}(11)_\mathrm{syst.} \mathrm{eV}$ provides sufficiently low experimental uncertainties to test modern theories of nuclear forces. The corresponding transition rate is compared to the results of ab initio calculations based on chiral effective field theory that take into account contributions to the magnetic dipole operator beyond leading order. This enables a precision test of the impact of two-body currents that enter at next-to-leading order.
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Submitted 20 January, 2021; v1 submitted 15 May, 2020;
originally announced May 2020.
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Precision measurement of the $E2$ transition strength to the 2$^+_1$ state of $^{12}$C
Authors:
A. D'Alessio,
T. Mongelli,
M. Arnold,
S. Bassauer,
J. Birkhan,
M. Hilcker,
T. Hüther,
J. Isaak,
L. Jürgensen,
T. Klaus,
P. von Neumann-Cosel,
N. Pietralla,
V. Yu. Ponomarev,
P. C. Ries,
R. Roth,
M. Singer,
G. Steinhilber,
K. Vobig,
V. Werner
Abstract:
The form factor of the electromagnetic excitation of $^{12}$C to its 2$^+_1$ state was measured at extremely low momentum transfers in an electron scattering experiment at the S-DALINAC. A combined analysis with the world form factor data results in a reduced transition strength $B(E2; 2^+_1\rightarrow 0^+_1) =7.63(19)$ e$^2$fm$^4$ with an accuracy improved to 2.5\%. In-Medium-No Core Shell Model…
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The form factor of the electromagnetic excitation of $^{12}$C to its 2$^+_1$ state was measured at extremely low momentum transfers in an electron scattering experiment at the S-DALINAC. A combined analysis with the world form factor data results in a reduced transition strength $B(E2; 2^+_1\rightarrow 0^+_1) =7.63(19)$ e$^2$fm$^4$ with an accuracy improved to 2.5\%. In-Medium-No Core Shell Model results with interactions derived from chiral effective field theory are capable to reproduce the result. A quadrupole moment $Q(2^+_1) = 5.97(30)$ efm$^2$ can be extracted from the strict correlation with the $B((E2)$ strength emerging in the calculations.
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Submitted 12 May, 2020; v1 submitted 8 May, 2020;
originally announced May 2020.
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Electromagnetic properties of $^{21}$O for benchmarking nuclear Hamiltonians
Authors:
S. Heil,
M. Petri,
K. Vobig,
D. Bazin,
J. Belarge,
P. Bender,
B. A. Brown,
R. Elder,
B. Elman,
A. Gade,
T. Haylett,
J. D. Holt,
T. Hüther,
A. Hufnagel,
H. Iwasaki,
N. Kobayashi,
C. Loelius,
B. Longfellow,
E. Lunderberg,
M. Mathy,
J. Menéndez,
S. Paschalis,
R. Roth,
A. Schwenk,
J. Simonis
, et al. (3 additional authors not shown)
Abstract:
The structure of exotic nuclei provides valuable tests for state-of-the-art nuclear theory. In particular electromagnetic transition rates are more sensitive to aspects of nuclear forces and many-body physics than excitation energies alone. We report the first lifetime measurement of excited states in $^{21}$O, finding $τ_{1/2^+}=420^{+35}_{-32}\text{(stat)}^{+34}_{-12}\text{(sys)}$\,ps. This resu…
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The structure of exotic nuclei provides valuable tests for state-of-the-art nuclear theory. In particular electromagnetic transition rates are more sensitive to aspects of nuclear forces and many-body physics than excitation energies alone. We report the first lifetime measurement of excited states in $^{21}$O, finding $τ_{1/2^+}=420^{+35}_{-32}\text{(stat)}^{+34}_{-12}\text{(sys)}$\,ps. This result together with the deduced level scheme and branching ratio of several $γ$-ray decays are compared to both phenomenological shell-model and ab initio calculations based on two- and three-nucleon forces derived from chiral effective field theory. We find that the electric quadrupole reduced transition probability of $\rm B(E2;1/2^+ \rightarrow 5/2^+_{g.s.}) = 0.71^{+0.07\ +0.02}_{-0.06\ -0.06}$~e$^2$fm$^4$, derived from the lifetime of the $1/2^+$ state, is smaller than the phenomenological result where standard effective charges are employed, suggesting the need for modifications of the latter in neutron-rich oxygen isotopes. We compare this result to both large-space and valence-space ab initio calculations, and by using multiple input interactions we explore the sensitivity of this observable to underlying details of nuclear forces.
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Submitted 5 December, 2019;
originally announced December 2019.
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Few- and many-nucleon systems with semilocal coordinate-space regularized chiral two- and three-body forces
Authors:
E. Epelbaum,
J. Golak,
K. Hebeler,
T. Hüther,
H. Kamada,
H. Krebs,
P. Maris,
U. -G. Meißner,
A. Nogga,
R. Roth,
R. Skibiński,
K. Topolnicki,
J. P. Vary,
K. Vobig,
H. Witała
Abstract:
We present a complete calculation of nucleon-deuteron scattering as well as ground and low-lying excited states of light nuclei in the mass range A=3-16 up through next-to-next-to-leading order in chiral effective field theory using semilocal coordinate-space regularized two- and three-nucleon forces. It is shown that both of the low-energy constants entering the three-nucleon force at this order…
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We present a complete calculation of nucleon-deuteron scattering as well as ground and low-lying excited states of light nuclei in the mass range A=3-16 up through next-to-next-to-leading order in chiral effective field theory using semilocal coordinate-space regularized two- and three-nucleon forces. It is shown that both of the low-energy constants entering the three-nucleon force at this order can be reliably determined from the triton binding energy and the differential cross section minimum in elastic nucleon-deuteron scattering. The inclusion of the three-nucleon force is found to improve the agreement with the data for most of the considered observables.
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Submitted 8 July, 2018;
originally announced July 2018.
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Light Neutron-Rich Hypernuclei from the Importance-Truncated No-Core Shell Model
Authors:
Roland Wirth,
Robert Roth
Abstract:
We explore the systematics of ground-state and excitation energies in singly-strange hypernuclei throughout the helium and lithium isotopic chains --- from $^5_Λ$He to $^{11}_Λ$He and from $^7_Λ$Li to $^{12}_Λ$Li --- in the ab initio no-core shell model with importance truncation. All calculations are based on two- and three-baryon interaction from chiral effective field theory and we employ a sim…
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We explore the systematics of ground-state and excitation energies in singly-strange hypernuclei throughout the helium and lithium isotopic chains --- from $^5_Λ$He to $^{11}_Λ$He and from $^7_Λ$Li to $^{12}_Λ$Li --- in the ab initio no-core shell model with importance truncation. All calculations are based on two- and three-baryon interaction from chiral effective field theory and we employ a similarity renormalization group transformation consistently up to the three-baryon level to improve the model-space convergence. While the absolute energies of hypernuclear states show a systematic variation with the regulator cutoff of the hyperon-nucleon interaction, the resulting neutron separation energies are very stable and in good agreement with available data for both nucleonic parents and their daughter hypernuclei. We provide predictions for the neutron separation energies and the spectra of neutron-rich hypernuclei that have not yet been observed experimentally. Furthermore, we find that the neutron drip lines in the helium and lithium isotopic chains are not changed by the addition of a hyperon.
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Submitted 28 February, 2018; v1 submitted 13 October, 2017;
originally announced October 2017.
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Electromagnetic Strength Distributions from the Ab Initio No-Core Shell Model
Authors:
Christina Stumpf,
Tobias Wolfgruber,
Robert Roth
Abstract:
We present an ab initio approach for the description of collective excitations and transition strength distributions of arbitrary nuclei up into the sd-shell that based on the No-Core Shell Model in combination with the Lanczos strength-function method. Starting from two- and three-nucleon interactions from chiral effective field theory, we investigate the electric monopole, dipole, and quadrupole…
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We present an ab initio approach for the description of collective excitations and transition strength distributions of arbitrary nuclei up into the sd-shell that based on the No-Core Shell Model in combination with the Lanczos strength-function method. Starting from two- and three-nucleon interactions from chiral effective field theory, we investigate the electric monopole, dipole, and quadrupole response of the even oxygen isotopes from 16-O to 24-O. The method describes the full energy range from low-lying excitations to the giant resonance region and beyond in a unified and consistent framework, including a complete description of fragmentation and fine-structure. This opens unique opportunities for understanding dynamic properties of nuclei from first principles and to further constrain nuclear interactions. We demonstrate the computational efficiency and the robust model-space convergence of our approach and compare to established approximate methods, such as the Random Phase Approximation, shedding new light on their deficiencies.
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Submitted 20 September, 2017;
originally announced September 2017.
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Structure of the lightest tin isotopes
Authors:
T. D. Morris,
J. Simonis,
S. R. Stroberg,
C. Stumpf,
G. Hagen,
J. D. Holt,
G. R. Jansen,
T. Papenbrock,
R. Roth,
A. Schwenk
Abstract:
We link the structure of nuclei around $^{100}$Sn, the heaviest doubly magic nucleus with equal neutron and proton numbers ($N=Z=50$), to nucleon-nucleon ($NN$) and three-nucleon ($NNN$) forces constrained by data of few-nucleon systems. Our results indicate that $^{100}$Sn is doubly magic, and we predict its quadrupole collectivity. We present precise computations of $^{101}$Sn based on three-par…
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We link the structure of nuclei around $^{100}$Sn, the heaviest doubly magic nucleus with equal neutron and proton numbers ($N=Z=50$), to nucleon-nucleon ($NN$) and three-nucleon ($NNN$) forces constrained by data of few-nucleon systems. Our results indicate that $^{100}$Sn is doubly magic, and we predict its quadrupole collectivity. We present precise computations of $^{101}$Sn based on three-particle--two-hole excitations of $^{100}$Sn, and reproduce the small splitting between the lowest $J^π=7/2^+$ and $5/2^+$ states. Our results are consistent with the sparse available data.
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Submitted 8 September, 2017;
originally announced September 2017.
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The nuclear force imprints revealed on the elastic scattering of protons with $^{10}$C
Authors:
A. Kumar,
R. Kanungo,
A. Calci,
P. Navratil,
A. Sanetullaev,
M. Alcorta,
V. Bildstein,
G. Christian,
B. Davids,
J. Dohet-Eraly,
J. Fallis,
A. T. Gallant,
G. Hackman,
B. Hadinia,
G. Hupin,
S. Ishimoto,
R. Krücken,
A. T. Laffoley,
J. Lighthall,
D. Miller,
S. Quaglioni,
J. S. Randhawa,
E. T. Rand,
A. Rojas,
R. Roth
, et al. (4 additional authors not shown)
Abstract:
How does nature hold together protons and neutrons to form the wide variety of complex nuclei in the universe? Describing many-nucleon systems from the fundamental theory of quantum chromodynamics has been the greatest challenge in answering this question. The chiral effective field theory description of the nuclear force now makes this possible but requires certain parameters that are not uniquel…
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How does nature hold together protons and neutrons to form the wide variety of complex nuclei in the universe? Describing many-nucleon systems from the fundamental theory of quantum chromodynamics has been the greatest challenge in answering this question. The chiral effective field theory description of the nuclear force now makes this possible but requires certain parameters that are not uniquely determined. Defining the nuclear force needs identification of observables sensitive to the different parametrizations. From a measurement of proton elastic scattering on $^{10}$C at TRIUMF and ab initio nuclear reaction calculations we show that the shape and magnitude of the measured differential cross section is strongly sensitive to the nuclear force prescription.
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Submitted 15 May, 2017;
originally announced May 2017.
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Can Ab Initio Theory Explain the Phenomenon of Parity Inversion in ${}^{11}$Be?
Authors:
Angelo Calci,
Petr Navrátil,
Robert Roth,
Jérémy Dohet-Eraly,
Sofia Quaglioni,
Guillaume Hupin
Abstract:
The weakly bound exotic ${}^{11}$Be nucleus, famous for its ground-state parity inversion and distinct n+ ${}^{10}$Be halo structure, is investigated from first principles using chiral two- and three-nucleon forces. An explicit treatment of continuum effects is found to be indispensable. We study the sensitivity of the ${}^{11}$Be spectrum to the details of the three-nucleon force and demonstrate…
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The weakly bound exotic ${}^{11}$Be nucleus, famous for its ground-state parity inversion and distinct n+ ${}^{10}$Be halo structure, is investigated from first principles using chiral two- and three-nucleon forces. An explicit treatment of continuum effects is found to be indispensable. We study the sensitivity of the ${}^{11}$Be spectrum to the details of the three-nucleon force and demonstrate that only certain chiral interactions are capable of reproducing the parity inversion. With such interactions, the extremely large E1 transition between the bound states is reproduced. We compare our photodisintegration calculations to conflicting experimental data and predict a distinct dip around the $3/2^-_1$ resonance energy. Finally, we predict low-lying $3/2^+$ and $9/2^+$ resonances that are not or not sufficiently measured in experiments.
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Submitted 30 November, 2016; v1 submitted 10 August, 2016;
originally announced August 2016.
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Prediction for a four-neutron resonance
Authors:
A. M. Shirokov,
G. Papadimitriou,
A. I. Mazur,
I. A. Mazur,
R. Roth,
J. P. Vary
Abstract:
We utilize various {\em ab initio} approaches to search for a low-lying resonance in the four-neutron ($4n$) system using the JISP16 realistic $NN$ interaction. Our most accurate prediction is obtained using a $J$-matrix extension of the No-Core Shell Model and suggests a $4n$ resonant state at an energy near $E_r = 0.8$ MeV with a width of approximately $Γ= 1.4$ MeV.
We utilize various {\em ab initio} approaches to search for a low-lying resonance in the four-neutron ($4n$) system using the JISP16 realistic $NN$ interaction. Our most accurate prediction is obtained using a $J$-matrix extension of the No-Core Shell Model and suggests a $4n$ resonant state at an energy near $E_r = 0.8$ MeV with a width of approximately $Γ= 1.4$ MeV.
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Submitted 26 July, 2016; v1 submitted 19 July, 2016;
originally announced July 2016.
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A nucleus-dependent valence-space approach to nuclear structure
Authors:
S. R. Stroberg,
A. Calci,
H. Hergert,
J. D. Holt,
S. K. Bogner,
R. Roth,
A. Schwenk
Abstract:
We present a nucleus-dependent valence-space approach for calculating ground and excited states of nuclei, which generalizes the shell-model in-medium similarity renormalization group to an ensemble reference with fractionally filled orbitals. Because the ensemble is used only as a reference, and not to represent physical states, no symmetry restoration is required. This allows us to capture 3N fo…
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We present a nucleus-dependent valence-space approach for calculating ground and excited states of nuclei, which generalizes the shell-model in-medium similarity renormalization group to an ensemble reference with fractionally filled orbitals. Because the ensemble is used only as a reference, and not to represent physical states, no symmetry restoration is required. This allows us to capture 3N forces among valence nucleons with a valence-space Hamiltonian specifically targeted to each nucleus of interest. Predicted ground-state energies from carbon through nickel agree with results of other large-space ab initio methods, generally to the 1\% level. In addition, we show that this new approach is required in order to obtain convergence for nuclei in the upper $p$ and $sd$ shells. Finally, we address the $1^+$/$3^+$ ground-state inversion problem in $^{22}\text{Na}$ and $^{46}\text{V}$. This approach extends the reach of ab initio nuclear structure calculations to essentially all light- and medium-mass nuclei.
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Submitted 12 July, 2016;
originally announced July 2016.
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Origin of fine structure of the giant dipole resonance in sd-shell nuclei
Authors:
R. W. Fearick,
B. Erler,
H. Matsubara,
P. von Neumann-Cosel,
A. Richter,
R. Roth,
A. Tamii
Abstract:
A set of high resolution zero-degree inelastic proton scattering data on 24Mg, 28Si, 32S, and 40Ca provides new insight into the long-standing puzzle of the origin of fragmentation of the Giant Dipole Resonance (GDR) in sd-shell nuclei. Understanding is provided by state-of-the-art theoretical Random Phase Approximation (RPA) calculatios for deformed nuclei using for the first time a realistic nuc…
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A set of high resolution zero-degree inelastic proton scattering data on 24Mg, 28Si, 32S, and 40Ca provides new insight into the long-standing puzzle of the origin of fragmentation of the Giant Dipole Resonance (GDR) in sd-shell nuclei. Understanding is provided by state-of-the-art theoretical Random Phase Approximation (RPA) calculatios for deformed nuclei using for the first time a realistic nucleon-nucleon interaction derived from the Argonne V18 potential with the unitary correlation operator method and supplemented by a phenomenological three-nucleon contact interaction. A wavelet analysis allows to extract significant scales both in the data and calculations characterizing the fine structure of the GDR. The fair agreement supports that the fine structure arises from ground-state deformation driven by alpha clustering.
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Submitted 6 April, 2018; v1 submitted 3 September, 2014;
originally announced September 2014.
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Ab Initio Multi-Reference In-Medium Similarity Renormalization Group Calculations of Even Calcium and Nickel Isotopes
Authors:
H. Hergert,
S. K. Bogner,
T. D. Morris,
S. Binder,
A. Calci,
J. Langhammer,
R. Roth
Abstract:
We use the newly developed Multi-Reference In-Medium Similarity Renormalization Group to study all even isotopes of the calcium and nickel isotopic chains, based on two- plus three-nucleon interactions derived from chiral effective field theory. We present results for ground-state and two-neutron separation energies and quantify their theoretical uncertainties. At shell closures, we find excellent…
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We use the newly developed Multi-Reference In-Medium Similarity Renormalization Group to study all even isotopes of the calcium and nickel isotopic chains, based on two- plus three-nucleon interactions derived from chiral effective field theory. We present results for ground-state and two-neutron separation energies and quantify their theoretical uncertainties. At shell closures, we find excellent agreement with Coupled Cluster results obtained with the same Hamiltonians. Our results highlight the importance of the chiral 3N interaction to obtain a correct reproduction of experimental energy trends, and their subtle impact on the location of the neutron drip lines in the Ca and Ni chains. At the same time, we uncover and discuss deficiencies of the input Hamiltonians which need to be addressed by the next generation of chiral interactions.
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Submitted 28 October, 2014; v1 submitted 27 August, 2014;
originally announced August 2014.
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Nonperturbative shell-model interactions from the in-medium similarity renormalization group
Authors:
S. K. Bogner,
H. Hergert,
J. D. Holt,
A. Schwenk,
S. Binder,
A. Calci,
J. Langhammer,
R. Roth
Abstract:
We present the first ab initio construction of valence-space Hamiltonians for medium-mass nuclei based on chiral two- and three-nucleon interactions using the in-medium similarity renormalization group. When applied to the oxygen isotopes, we find experimental ground-state energies are well reproduced, including the flat trend beyond the drip line at 24O. Similarly, natural-parity spectra in 21,22…
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We present the first ab initio construction of valence-space Hamiltonians for medium-mass nuclei based on chiral two- and three-nucleon interactions using the in-medium similarity renormalization group. When applied to the oxygen isotopes, we find experimental ground-state energies are well reproduced, including the flat trend beyond the drip line at 24O. Similarly, natural-parity spectra in 21,22,23,24O are in agreement with experiment, and we present predictions for excited states in 25,26O. The results exhibit a weak dependence on the harmonic-oscillator basis parameter and reproduce spectroscopy within the standard sd valence space.
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Submitted 3 October, 2014; v1 submitted 6 February, 2014;
originally announced February 2014.
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Isospin properties of electric dipole excitations in 48Ca
Authors:
V. Derya,
D. Savran,
J. Endres,
M. N. Harakeh,
H. Hergert,
J. H. Kelley,
P. Papakonstantinou,
N. Pietralla,
V. Yu. Ponomarev,
R. Roth,
G. Rusev,
A. P. Tonchev,
W. Tornow,
H. J. Wörtche,
A. Zilges
Abstract:
Two different experimental approaches were combined to study the electric dipole strength in the doubly-magic nucleus 48Ca below the neutron threshold. Real-photon scattering experiments using bremsstrahlung up to 9.9 MeV and nearly mono-energetic linearly polarized photons with energies between 6.6 and 9.51 MeV provided strength distribution and parities, and an (α,α'γ) experiment at E_α=136 MeV…
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Two different experimental approaches were combined to study the electric dipole strength in the doubly-magic nucleus 48Ca below the neutron threshold. Real-photon scattering experiments using bremsstrahlung up to 9.9 MeV and nearly mono-energetic linearly polarized photons with energies between 6.6 and 9.51 MeV provided strength distribution and parities, and an (α,α'γ) experiment at E_α=136 MeV gave cross sections for an isoscalar probe. The unexpected difference observed in the dipole response is compared to calculations using the first-order random-phase approximation and points to an energy-dependent isospin character. A strong isoscalar state at 7.6 MeV was identified for the first time supporting a recent theoretical prediction.
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Submitted 3 February, 2014;
originally announced February 2014.
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Low-energy electric dipole response of Sn isotopes
Authors:
P. Papakonstantinou,
H. Hergert,
V. Yu. Ponomarev,
R. Roth
Abstract:
We study the low-energy dipole (LED) strength distribution along the Sn isotopic chain in both the isoscalar (IS) and the isovector (IV, or E1) electric channels, to provide testable predictions and guidance for new experiments with stable targets and radioactive beams. We use the self-consistent Quasi-particle Random-Phase Approximation (QRPA) with finite-range interactions and mainly the Gogny D…
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We study the low-energy dipole (LED) strength distribution along the Sn isotopic chain in both the isoscalar (IS) and the isovector (IV, or E1) electric channels, to provide testable predictions and guidance for new experiments with stable targets and radioactive beams. We use the self-consistent Quasi-particle Random-Phase Approximation (QRPA) with finite-range interactions and mainly the Gogny D1S force. We analyze also the performance of a realistic two-body interaction supplemented by a phenomenological three-body contact term. We find that from N=50 and up to the N=82 shell closure (132Sn) the lowest-energy part of the IS-LED spectrum is dominated by a collective transition whose properties vary smoothly with neutron number and which cannot be interpreted as a neutron-skin oscillation. For the neutron-rich species this state contributes to the E1 strength below particle threshold, but much more E1 strength is carried by other, weak but numerous transitions around or above threshold. We find that strong structural changes in the spectrum take effect beyond N=82, namely increased LED strength and lower excitation energies. Our results with the Gogny interaction are compatible with existing data. On this basis we predict that a) the summed IS strength below particle threshold shall be of the same order of magnitude for N=50-82, b) the summed E1 strength up to approximately 12 MeV shall be similar for N=50-82 MeV, while c) the summed E1 strength below threshold shall be of the same order of magnitude for N ~ 64 - 82 and much weaker for the lighter, more-symmetric isotopes. We point out a general agreement of our results with other non-relativistic studies, the absence of a collective IS mode in some of those studies, and a possibly radical disagreement with relativistic models.
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Submitted 10 March, 2014; v1 submitted 9 October, 2013;
originally announced October 2013.
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Ab Initio Calculations of Even Oxygen Isotopes with Chiral Two- Plus Three-Nucleon Interactions
Authors:
H. Hergert,
S. Binder,
A. Calci,
J. Langhammer,
R. Roth
Abstract:
We formulate the In-Medium Similarity Renormalization Group (IM-SRG) for open-shell nuclei using a multi-reference formalism based on a generalized Wick theorem introduced in quantum chemistry. The resulting multi-reference IM-SRG (MR-IM-SRG) is used to perform the first ab initio study of even oxygen isotopes with chiral NN and 3N Hamiltonians, from the proton to the neutron drip lines. We obtain…
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We formulate the In-Medium Similarity Renormalization Group (IM-SRG) for open-shell nuclei using a multi-reference formalism based on a generalized Wick theorem introduced in quantum chemistry. The resulting multi-reference IM-SRG (MR-IM-SRG) is used to perform the first ab initio study of even oxygen isotopes with chiral NN and 3N Hamiltonians, from the proton to the neutron drip lines. We obtain an excellent reproduction of experimental ground-state energies with quantified uncertainties, which is validated by results from the Importance-Truncated No-Core Shell Model and the Coupled Cluster method. The agreement between conceptually different many-body approaches and experiment highlights the predictive power of current chiral two- and three-nucleon interactions, and establishes the MR-IM-SRG as a promising new tool for ab initio calculations of medium-mass nuclei far from shell closures.
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Submitted 12 June, 2013; v1 submitted 28 February, 2013;
originally announced February 2013.
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Low-energy dipole strength and the critical case of 48Ca
Authors:
P. Papakonstantinou,
H. Hergert,
V. Yu. Ponomarev,
R. Roth
Abstract:
Recent theoretical work has not led to a consensus regarding the nature of the low-energy E1 strength in the 40,44,48Ca isotopes, for which high-resolution (gamma,gamma') data exist. Here we revisit this problem using the first-order quasi-particle random-phase approximation (QRPA) and different interactions. First we examine all even Ca isotopes with N=14-40. All isotopes are predicted to undergo…
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Recent theoretical work has not led to a consensus regarding the nature of the low-energy E1 strength in the 40,44,48Ca isotopes, for which high-resolution (gamma,gamma') data exist. Here we revisit this problem using the first-order quasi-particle random-phase approximation (QRPA) and different interactions. First we examine all even Ca isotopes with N=14-40. All isotopes are predicted to undergo dipole transitions at low energy, of large and comparable isoscalar strength but of varying E1 strength. Provided a moderate and uniform energetic shift is introduced to the results, QRPA with the Gogny D1S interaction is able to account for the (gamma,gamma') data, because, up to N=28, it yields a rather pure isoscalar oscillation. A neutron-skin oscillation is anticipated for N larger or equal to 30. This contradicts existing predictions that 44,48Ca develop a neutron-skin mode. Which theoretical result is correct cannot be resolved conclusively using the available data. We propose that alpha-scattering, possibly followed by an electroexcitation experiment, could resolve the situation and thereby help to improve the different models aspiring to describe reliably the low-energy dipole strength of nuclei.
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Submitted 5 October, 2011;
originally announced October 2011.
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Systematics of 2+ states in C isotopes from the ab initio no-core shell model
Authors:
Christian Forssén,
Robert Roth,
Petr Navrátil
Abstract:
We study low-lying states of even carbon isotopes in the range A = 10 - 20 within the large- scale no-core shell model (NCSM). Using several accurate nucleon-nucleon (NN) as well as NN plus three-nucleon (NNN) interactions, we calculate excitation energies of the lowest 2+ state, the electromagnetic B(E2; 2+1 -> 0+1) transition rates, the 2+1 quadrupole moments as well as se- lected electromagneti…
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We study low-lying states of even carbon isotopes in the range A = 10 - 20 within the large- scale no-core shell model (NCSM). Using several accurate nucleon-nucleon (NN) as well as NN plus three-nucleon (NNN) interactions, we calculate excitation energies of the lowest 2+ state, the electromagnetic B(E2; 2+1 -> 0+1) transition rates, the 2+1 quadrupole moments as well as se- lected electromagnetic transitions among other states. Recent experimental campaigns to measure 2+-state lifetimes indicate an interesting evolution of nuclear structure that pose a challenge to reproduce theoretically from first principles. Our calculations do not include any effective charges or other fitting parameters. However, calculated results extrapolated to infinite model spaces are also presented. The model-dependence of those results is discussed. Overall, we find a good agree- ment with the experimentally observed trends, although our extrapolated B(E2; 2+1 -> 0+1) value for 16C is lower compared to the most recent measurements. Relative transition strengths from higher excited states are investigated and the influence of NNN forces is discussed. In particular for 16C we find a remarkable sensitivity of the transition rates from higher excited states to the details of the nuclear interactions.
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Submitted 19 March, 2013; v1 submitted 4 October, 2011;
originally announced October 2011.
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No-Core MCSM calculation for $^{10}$Be and $^{12}$Be low-lying spectra
Authors:
Lang Liu,
Takaharu Otsuka,
Noritaka Shimizu,
Yutaka Utsuno,
Robert Roth
Abstract:
The low-lying excited states of $^{10}$Be and $^{12}$Be are investigated within a no-core Monte Carlo Shell Model (MCSM) framework employing a realistic potential obtained via the Unitary Correlation Operator Method. The excitation energies of the 2$^+_1$ and 2$^+_2$ states and the B(E2;$\,2^+_{1,2}\rightarrow$ 0$^+_{g.s.}$) for $^{10}$Be in the MCSM with a standard treatment of spurious center-of…
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The low-lying excited states of $^{10}$Be and $^{12}$Be are investigated within a no-core Monte Carlo Shell Model (MCSM) framework employing a realistic potential obtained via the Unitary Correlation Operator Method. The excitation energies of the 2$^+_1$ and 2$^+_2$ states and the B(E2;$\,2^+_{1,2}\rightarrow$ 0$^+_{g.s.}$) for $^{10}$Be in the MCSM with a standard treatment of spurious center-of-mass motion show good agreement with experimental data. Some properties of low-lying states of $^{10}$Be are studied in terms of quadrupole moments, E2 transitions and single-particle occupation numbers. The E2 transition probability of $^{10}$C, the mirror nucleus of $^{10}$Be, is also presented with a good agreement to experiment. The triaxial deformation of $^{10}$Be and $^{10}$C is discussed in terms of the B(E2) values. The removal of the spurious center-of-mass motion affects differently on various states: for instance, negligible effects on the 2$^+_1$ and 2$^+_2$ levels of $^{10}$Be, while significant and favorable shift for the 1$^-_1$ level. It is suggested that the description of $^{12}$Be needs a larger model space as well as some other higher excited states of $^{10}$Be, as an indicator that these are dominated by intruder configurations.
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Submitted 15 June, 2012; v1 submitted 15 May, 2011;
originally announced May 2011.
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Fine structure of the isoscalar giant quadrupole resonance in 40Ca due to Landau damping?
Authors:
I. Usman,
Z. Buthelezi,
J. Carter,
G. R. J. Cooper,
R. W. Fearick,
S. V. Förtsch,
H. Fujita Y. Fujita,
Y. Kalmykov,
P. von Neumann-Cosel,
R. Neveling,
P. Papakonstantinou,
A. Richter,
R. Roth,
A. Shevchenko,
E. Sideras-Haddad,
F. D. Smit
Abstract:
The fragmentation of the Isoscalar Giant Quadrupole Resonance (ISGQR) in 40Ca has been investigated in high energy-resolution experiments using proton inelastic scattering at E_p = 200 MeV. Fine structure is observed in the region of the ISGQR and its characteristic energy scales are extracted from the experimental data by means of a wavelet analysis. The experimental scales are well described by…
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The fragmentation of the Isoscalar Giant Quadrupole Resonance (ISGQR) in 40Ca has been investigated in high energy-resolution experiments using proton inelastic scattering at E_p = 200 MeV. Fine structure is observed in the region of the ISGQR and its characteristic energy scales are extracted from the experimental data by means of a wavelet analysis. The experimental scales are well described by Random Phase Approximation (RPA) and second-RPA calculations with an effective interaction derived from a realistic nucleon-nucleon interaction by the Unitary Correlation Operator Method (UCOM). In these results characteristic scales are already present at the mean-field level pointing to their origination in Landau damping, in contrast to the findings in heavier nuclei and also to SRPA calculations for 40Ca based on phenomenological effective interactions, where fine structure is explained by the coupling to two-particle two-hole (2p-2h) states.
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Submitted 4 March, 2011;
originally announced March 2011.
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Isoscalar dipole coherence at low energies and forbidden E1 strength
Authors:
P. Papakonstantinou,
V. Yu. Ponomarev,
R. Roth,
J. Wambach
Abstract:
In 16O and 40Ca an isoscalar, low-energy dipole transition (IS-LED) exhausting approximately 4% of the isoscalar dipole (ISD) energy-weighted sum rule is experimentally known, but conspicuously absent from recent theoretical investigations of ISD strength. The IS-LED mode coincides with the so-called isospin-forbidden E1 transition. We report that for N=Z nuclei up to 100Sn the fully self-consiste…
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In 16O and 40Ca an isoscalar, low-energy dipole transition (IS-LED) exhausting approximately 4% of the isoscalar dipole (ISD) energy-weighted sum rule is experimentally known, but conspicuously absent from recent theoretical investigations of ISD strength. The IS-LED mode coincides with the so-called isospin-forbidden E1 transition. We report that for N=Z nuclei up to 100Sn the fully self-consistent Random-Phase-Approximation with finite-range forces, phenomenological and realistic, yields a collective IS-LED mode, typically overestimating its excitation energy, but correctly describing its IS strength and electroexcitation form factor. The presence of E1 strength is solely due to the Coulomb interaction between the protons and the resulting isospin-symmetry breaking. The smallness of its value is related to the form of the transition density, due to translational invariance. The calculated values of E1 and ISD strength carried by the IS-LED depend on the effective interaction used. Attention is drawn to the possibility that in N-not-equal-Z nuclei this distinct mode of IS surface vibration can develop as such or mix strongly with skin modes and thus influence the pygmy dipole strength as well as the ISD strength function. In general, theoretical models currently in use may be unfit to predict its precise position and strength, if at all its existence.
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Submitted 4 November, 2010;
originally announced November 2010.