Mon·Apr 19, 2021Monday·April 19, 2021

1. 01*

   The X17 boson and the ${}^3$H$(p,e^{+}{e}^{-}{)}^4$He and ${}^3$He$(n,e^{+}{e}^{-}{)}^4$He processes: a theoretical analysis

   M. Viviani🇮🇹 · E. Filandri🇮🇹 · L. Girlanda🇮🇹 · C. Gustavino🇮🇹 · A. Kievsky🇮🇹 · L.E. Marcucci🇮🇹 · R. Schiavilla🇺🇸

   The present work deals with $e^{+}$-$e^{-}$ pair production in the four-nucleon system. We first analyze the process as a purely electromagnetic one in the context of a state-of-the-art approach to nuclear strong-interaction dynamics and nuclear electromagnetic currents, derived from chiral effective field theory ($\chi$EFT). Next, we examine how the exchange of a hypothetical low-mass boson would impact the cross section for such a process. We consider several possibilities, that this boson is either a scalar, pseudoscalar, vector, or axial particle. The ab initio calculations use exact hyperspherical-harmonics methods to describe the bound state and low-energy spectrum of the $A$=$4$ continuum, and fully account for initial state interaction effects in the $3+1$ clusters. While electromagnetic interactions are treated to high orders in the chiral expansion, the interactions of the hypothetical boson with nucleons are modeled in leading-order $\chi$EFT (albeit, in some instances, selected subleading contributions are also accounted for). We also provide an overview of possible future experiments probing pair production in the $A$=$4$ system at a number of candidate facilities.facilities.

   ↳ nucl-thnucl-exPRC(2022)·45 citations
2. 02*

   Anisotropic flow decorrelation in heavy-ion collisions with event-by-event viscous hydrodynamics

   Jakub Cimerman🇨🇿 · Iurii Karpenko🇨🇿 · Boris Tomášik🇸🇰 · Barbara Antonina Trzeciak🇨🇿

   Decorrelation of the elliptic flow in rapidity is calculated within a hybrid approach which includes event-by-event viscous fluid dynamics and final state hadronic cascade model. The simulations are performed for Au+Au collisions at center-of-mass collision energies of 27 and 200 GeV per nucleon pair, as well as various colliding systems at 72 GeV per nucleon pair. Initial conditions determined by an extended Monte Carlo Glauber model show better agreement with experimental data than initial conditions from the UrQMD transport model. We show how the observed decorrelation is connected with the decorrelation of initial state spatial anisotropies. We also study how the effect is increased by the final state hadronic cascade.

   ↳ nucl-thnucl-exPRC(2021)·9 citations
3. 03*

   Estimation of nuclear matrix elements of double-$\beta$ decay from shell model and quasiparticle random-phase approximation

   J. Terasaki🇨🇿 · Y. Iwata🇯🇵

   The nuclear matrix element (NME) of the neutrinoless double-$\beta$ ($0\nu \beta \beta$) decay is an essential input for determining the neutrino effective mass, if the half-life of this decay is measured. The reliable calculation of this NME has been a long-standing problem because of the diversity of the predicted values of the NME depending on the calculation method. In this paper, we focus on the shell model and the QRPA. The shell model have a rich amount of the many-particle many-hole correlations, and the QRPA can obtain the convergence of the result of calculation with respect to the extension of the single-particle space. It is difficult for the shell model to obtain the convergence of the $0\nu \beta \beta$ NME with respect to the valence single-particle space. The many-body correlations of the QRPA are insufficient depending on nuclei. We propose a new method to modify phenomenologically the results of the shell model and the QRPA compensating the insufficient point of each method by using the information of other method complementarily. Extrapolations of the components of the $0\nu \beta \beta$ NME of the shell model are made toward a very large valence single-particle space. We introduce a modification factor to the components of the $0\nu \beta \beta$ NME of the QRPA. Our modification method gives similar values of the $0\nu \beta \beta$ NME of the two methods for ${}^{48}$Ca. The NME of the two-neutrino double-$\beta$ decay is also modified in a similar but simpler manner, and the consistency of the two methods is improved.

   ↳ nucl-thnucl-exEur.Phys.J.Plus(2021)·9 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.