Nuclear Theory·nucl-th

Tuesday·February 9, 2021

16 papers—7 primary·9 cross-listed

01
[Submitted on 5 Feb 2021]
Determining the leading-order contact term in neutrinoless double $β$ decay
Vincenzo Cirigliano🇺🇸 · Wouter Dekens🇺🇸 · Jordy de Vries🇳🇱 · Martin Hoferichter🇨🇭 · Emanuele Mereghetti🇺🇸
We present a method to determine the leading-order (LO) contact term contributing to the $nn \to pp e^{-} e^{-}$ amplitude through the exchange of light Majorana neutrinos. Our approach is based on the representation of the amplitude as the momentum integral of a known kernel (proportional to the neutrino propagator) times the generalized forward Compton scattering amplitude $n (p_{1}) n (p_{2}) W^{+} (k) \to p (p_{1}^{'}) p (p_{2}^{'}) W^{-} (k)$ , in analogy to the Cottingham formula for the electromagnetic contribution to hadron masses. We construct model-independent representations of the integrand in the low- and high-momentum regions, through chiral EFT and the operator product expansion, respectively. We then construct a model for the full amplitude by interpolating between these two regions, using appropriate nucleon factors for the weak currents and information on nucleon-nucleon ( $N N$ ) scattering in the $^{1} S_{0}$ channel away from threshold. By matching the amplitude obtained in this way to the LO chiral EFT amplitude we obtain the relevant LO contact term and discuss various sources of uncertainty. We validate the approach by computing the analog $I = 2$ $N N$ contact term and by reproducing, within uncertainties, the charge-independence-breaking contribution to the $^{1} S_{0}$ $N N$ scattering lengths. While our analysis is performed in the $MS$ scheme, we express our final result in terms of the scheme-independent renormalized amplitude $A_{ν} (∣ p ∣, ∣ p^{'} ∣)$ at a set of kinematic points near threshold. We illustrate for two cutoff schemes how, using our synthetic data for $A_{ν}$ , one can determine the contact-term contribution in any regularization scheme, in particular the ones employed in nuclear-structure calculations for isotopes of experimental interest.
Comments:
64 pages, 20 figures, added text and references in the introduction, version published in JHEP
Subjects:
Nuclear Theory (nucl-th); High Energy Physics — Lattice (hep-lat); High Energy Physics — Phenomenology (hep-ph)
arXiv:
2102.03371 [pdf]
JHEP(2021)·81 citations
02
[Submitted on 6 Feb 2021]
Cross sections for neutron-induced reactions from surrogate data: revisiting the Weisskopf-Ewing approximation for $(n, n^{'})$ and $(n, 2 n)$ reactions
Oliver C. Gorton · Jutta E. Escher
Earlier work has demonstrated that cross sections for neutron-induced fission and radiative neutron capture can be determined from a combination of surrogate reaction data and theory. For the fission case, it was shown that Weisskopf-Ewing approximation, which significantly simplifies the implementation of the surrogate method, can be employed. Capture cross sections cannot be obtained, and require a detailed description of the surrogate reaction process. In this paper we examine the validity of the Weisskopf-Ewing approximation for determining unknown $(n, n^{'})$ and $(n, 2 n)$ cross sections from surrogate data. We find that peak cross sections can be estimated using the Weisskopf-Ewing approximation, but the shape of the $(n, n^{'})$ and $(n, 2 n)$ cross sections, especially for low neutron energies, cannot be reliably determined without accounting for the angular-momentum differences between the neutron-induced and surrogate reaction. To obtain reliable $(n, n^{'})$ and $(n, 2 n)$ cross sections from surrogate reaction data, a detailed description of the surrogate reaction mechanisms is required. To do so for the compound-nucleus energies and decay channels relevant to these reactions, it becomes necessary to extend current modeling capabilities.
Comments:
16 pages, 10 figures
Subjects:
Nuclear Theory (nucl-th); Nuclear Experiment (nucl-ex)
arXiv:
2102.03452 [pdf]
PRC(2023)·5 citations
03
[Submitted on 6 Feb 2021]
Fission fragment distributions and their impact on the r-process nucleosynthesis in neutron star mergers
J.-F. Lemaître🇧🇪 · S. Goriely🇧🇪 · A. Bauswein🇩🇪 · H.-T. Janka🇩🇪
Neutron star (NS) merger ejecta offer a viable site for the production of heavy r-process elements with nuclear mass numbers A >140. The crucial role of fission recycling is responsible for the robustness of this site against many astrophysical uncertainties. Here, we introduce new improvements to our scission-point model, called SPY, to derive the fission fragment distribution for all neutron-rich fissioning nuclei of relevance in r-process calculations. These improvements include a phenomenological modification of the scission distance and a smoothing procedure of the distribution. Such corrections lead to a much better agreement with experimental fission yields. Those yields are also used to estimate the number of neutrons emitted by the excited fragments on the basis of different neutron evaporation models. Our new fission yields are extensively compared to those predicted by the so-called GEF model. The impact of fission on the r-process nucleosynthesis in binary neutron mergers is also reanalyzed. Two scenarios are considered, the first one with low initial electron fraction is subject to intense fission recycling, in contrast to the second one which includes weak interactions on nucleons. The various regions of the nuclear chart responsible for fission recycling during the neutron irradiation as well as after freeze-out are discussed. The contribution fission processes may have to the final abundance distribution is also studied in detail in the light of newly defined quantitative indicators describing the fission recycling, the fission seeds and the fission progenitors. In particular, those allow us to estimate the contribution of fission to the final abundance distribution stemming from specific heavy nuclei. Calculations obtained with SPY and GEF fission fragment distributions are compared for both r-process scenarios.
Subjects:
Nuclear Theory (nucl-th)
arXiv:
2102.03686 [pdf]
PRC(2021)·40 citations
04
[Submitted on 8 Feb 2021]
Variation after projection calculations for high-spin states
Zao-Chun Gao
In this paper, an improvement has been made on the variation after projection (VAP) method, which is crucial in the calculations of high-spin states. It turns out that, the form of the trial VAP wave function with spin $J$ can be simplified by adopting just one projected state rather than previously adopting all $(2 J + 1)$ angular momentum projected states for each selected reference state, $∣Φ ⟩$ . The present calculations show that such simplification still minimizes the angular momentum projected energy up to a very good approximation. In this simplified VAP scheme, one can obtain almost equivalent VAP wave functions starting from different sets of the projected basis states. This clearly shows that a nuclear state can not be identified with a single intrinsic state, while in the traditional nuclear collective models, an intrinsic state is usually assigned to a well deformed rotational band.
Comments:
6 page,3 figures,to be published in Physics Letters B
Subjects:
Nuclear Theory (nucl-th)
arXiv:
2102.04044 [pdf]
PLB(2022)·14 citations
05
[Submitted on 8 Feb 2021]
Rotating neutron stars with quark cores
I. A. Rather🇮🇳 · Usuf Rahaman🇮🇳 · M. Imran🇮🇳 · H. C. Das🇮🇳 · A. A. Usmani🇮🇳 · S. K. Patra🇮🇳
The rotating neutron star properties are studied with a phase transition to quark matter. The density-dependent relativistic mean-field model (DD-RMF) is employed to study the hadron matter, while the Vector-Enhanced Bag model (vBag) model is used to study the quark matter. The star matter properties like mass, radius,the moment of inertia, rotational frequency, Kerr parameter, and other important quantities are studied to see the effect on quark matter. The maximum mass of rotating neutron star with DD-LZ1 and DD-MEX parameter sets is found to be around 3 $M_{⊙}$ for pure hadronic phase and decreases to a value around 2.6 $M_{⊙}$ with phase transition to quark matter, which satisfies the recent GW190814 constraints. For DDV, DDVT, and DDVTD parameter sets, the maximum mass decreases to satisfy the 2 $M_{⊙}$ . The moment of inertia calculated for various DD-RMF parameter sets decreases with the increasing mass satisfying constraints from various measurements. Other important quantities calculated also vary with the bag constant and hence show that the presence of quarks inside neutron stars can also allow us to constraint these quantities to determine a proper EoS. Also, the theoretical study along with the accurate measurement of uniformly rotating neutron star properties may offer some valuable information concerning the high-density part of the equation of state.
Comments:
17 pages, 18 figures, 2 tables, comments are welcome
Subjects:
Nuclear Theory (nucl-th); High Energy Astrophysical Phenomena (astro-ph.HE)
arXiv:
2102.04067 [pdf]
PRC(2021)·39 citations
06
[Submitted on 8 Feb 2021]
Few-nucleon matrix elements in pionless effective field theory in a finite volume
W. Detmold🇺🇸 · P. E. Shanahan🇺🇸
Pionless effective field theory in a finite volume (FVEFT $_{π /}$ ) is investigated as a framework for the analysis of multi-nucleon spectra and matrix elements calculated in lattice QCD (LQCD). By combining FVEFT $_{π /}$ with the stochastic variational method, the spectra of nuclei with atomic number $A \in {2, 3}$ are matched to existing finite-volume LQCD calculations at heavier-than-physical quark masses corresponding to a pion mass $m_{π} = 806$ MeV, thereby enabling infinite-volume binding energies to be determined using infinite-volume variational calculations. Based on the variational wavefunctions that are constructed in this approach, the finite-volume matrix elements of various local operators are computed in FVEFT $_{π /}$ and matched to LQCD calculations of the corresponding QCD operators in the same volume, thereby determining the relevant one and two-body EFT counterterms and enabling an extrapolation of the LQCD matrix elements to infinite volume. As examples, the scalar, tensor, and axial matrix elements are considered, as well as the magnetic moments and the isovector longitudinal momentum fraction.
Subjects:
Nuclear Theory (nucl-th); High Energy Physics — Lattice (hep-lat); High Energy Physics — Phenomenology (hep-ph)
arXiv:
2102.04329 [pdf]
PRD(2021)·35 citations
07
[Submitted on 8 Feb 2021]
Why nuclear forces favor the highest weight irreducible representations of the fermionic SU(3) symmetry
Andriana Martinou🇬🇷 · Dennis Bonatsos🇬🇷 · K.E. Karakatsanis🇬🇷 · S. Sarantopoulou🇬🇷 · I.E. Assimakis🇬🇷 · S.K. Peroulis🇬🇷 · N. Minkov🇧🇬
The consequences of the attractive, short-range nucleon-nucleon (NN) interaction on the wave functions of the Elliott SU(3) and the proxy-SU(3) symmetry are discussed. The NN interaction favors the most symmetric spatial SU(3) irreducible representation, which corresponds to the maximal spatial overlap among the fermions. The percentage of the symmetric components out of the total in an SU(3) wave function is introduced, through which it is found, that no SU(3) irrep is more symmetric than the highest weight irrep for a certain number of valence particles in a three dimensional, isotropic, harmonic oscillator shell. The consideration of the highest weight irreps in nuclei and in alkali metal clusters, leads to the prediction of a prolate to oblate shape transition beyond the mid-shell region.
Comments:
16 pages, 1 figure, 10 tables
Subjects:
Nuclear Theory (nucl-th)
arXiv:
2102.04409 [pdf]
EPJA(2021)·28 citations

Affiliations

first authorsco-authorsvia INSPIRE

Determining the leading-order contact term in neutrinoless double β decay

Cross sections for neutron-induced reactions from surrogate data: revisiting the Weisskopf-Ewing approximation for (n,n′) and (n,2n) reactions

Fission fragment distributions and their impact on the r-process nucleosynthesis in neutron star mergers

Variation after projection calculations for high-spin states

Rotating neutron stars with quark cores

Few-nucleon matrix elements in pionless effective field theory in a finite volume

Why nuclear forces favor the highest weight irreducible representations of the fermionic SU(3) symmetry

Affiliations

Determining the leading-order contact term in neutrinoless double $β$ decay

Cross sections for neutron-induced reactions from surrogate data: revisiting the Weisskopf-Ewing approximation for $(n, n^{'})$ and $(n, 2 n)$ reactions