Nuclear Theory

Liao, Zehong;Zhu, Long;Su, Jun;Li, Cheng

doi: 10.1103/PhysRevC.107.014614pmid: N/A

Abstract: The dynamics of the charge equilibration (CE) and the effects on the production of the neutron-rich isotopes around $N = 126$ in multinucleon transfer reactions are still not well understood. In this Letter, we investigate the mechanism of the CE from different viewpoints by using the extended version of the dinuclear system model (DNS-sysu) and the improved quantum molecular dynamics (ImQMD) model. From the macroscopic and microscopic dynamical viewpoints, we find incomplete CE for the mass asymmetry reaction systems even in the very deep collisions, and the behavior of "inverse CE" that the tendency of the fragments is away from the $N/Z$ value of the compound system in the reaction $^{140}$Xe + $^{198}$Pt. Unlike the slow process presented in the ImQMD model, the behavior of fast equilibration with the characteristic time $\sim$ 0.52 zs is obtained based on the DNS-sysu model, which is consistent with the experimental data. By performing the systematic calculation, the correlation between the CE and the mass asymmetry of the reaction systems is clarified, which not only accounts for the observed intriguing phenomena of the CE but also provides essential information for producing the neutron-rich isotopes around $N = 126$.

Han, Ming-Zhe;Huang, Yong-Jia;Tang, Shao-Peng;Fan, Yi-Zhong

doi: 10.1016/j.scib.2023.04.007pmid: 37080849

Abstract: We investigate the neutron star (NS) equation of state (EOS) by incorporating multi-messenger data of GW170817, PSR J0030+0451, PSR J0740+6620, and state-of-the-art theoretical progresses, including the information from chiral effective field theory ($\chi$EFT) and perturbative quantum chromodynamics (pQCD) calculation. Taking advantage of the various structures sampling by a single-layer feed-forward neural network model embedded in the Bayesian nonparametric inference, the structure of NS matter's sound speed $c_{\rm s}$ is explored in a model-agnostic way. It is found that a peak structure is common in the $c_{\rm s}^2$ posterior, locating at $2.4-4.8\rho_{\rm sat}$ (nuclear saturation density) and $c_{\rm s}^2$ exceeds ${c^{2}}/{3}$ at 90\% credibility. The non-monotonic behavior suggests evidence of the state deviating from hadronic matter inside the very massive NSs. Assuming the new/exotic state is featured as it is softer than typical hadronic models or even with hyperons, we find that a sizable ($\geq 10^{-3}M_\odot$) exotic core, likely made of quark matter, is plausible for the NS with a gravitational mass above about $0.98M_{\rm TOV}$, where $M_{\rm TOV}$ represents the maximum gravitational mass of a non-rotating cold NS. The inferred $M_{\rm TOV} = 2.18^{+0.27}_{-0.13}M_\odot$ (90\% credibility) is well consistent with the value of $2.17^{+0.15}_{-0.12}M_\odot$ estimated independently with GW170817/GRB 170817A/AT2017gfo assuming a temporary supramassive NS remnant formed after the merger. PSR J0740+6620, the most massive NS detected so far, may host an exotic core with a probability of $\approx 0.36$.

Sumiyoshi, Kohsuke;Kojo, Toru;Furusawa, Shun

doi: 10.1007/978-981-15-8818-1_104-1pmid: N/A

Abstract: Neutron stars and supernovae provide cosmic laboratories of highly compressed matter at supra nuclear saturation density which is beyond the reach of terrestrial experiments. The properties of dense matter is extracted by combining the knowledge of nuclear experiments and astrophysical observations via theoretical frameworks. A matter in neutron stars is neutron rich, and may further accommodate non-nucleonic degrees of freedom such as hyperons and quarks. The structure and composition of neutron stars are determined by equations of state of matter, which are the primary subject in this chapter. In case of supernovae, the time evolution includes several dynamical stages whose descriptions require equations of state at finite temperature and various lepton fractions. Equations of state also play essential roles in neutron star mergers which allow us to explore new conditions of matter not achievable in static neutron stars and supernovae. Several types of hadron-to-quark transitions, from first order transitions to crossover, are reviewed, and their characteristics are summarized.

Simenel, Cedric

doi: 10.1007/978-981-15-8818-1_19-1pmid: N/A

Abstract: Nuclear physics is ideal to test and develop techniques to describe the microscopic dynamics of quantum many-body systems. At low energy, nuclear dynamics is described with non-relativistic approaches based on the mean-field approximation and its extensions. Variational principles based on the stationarity of the action are introduced to build theoretical models with different levels of approximation. In particular, the time-dependent Hartree-Fock (TDHF) equation for mean-field dynamics and its linear approximation, also known as the Random Phase Approximation (RPA), are derived. Predictions of vibrational spectra at the RPA level are presented as an application. The inclusion of beyond TDHF correlations and fluctuations are then discussed. In particular, pairing correlations are treated at the BCS and Bogoliubov levels. The Balian-Veneroni variational principle is finally introduced. In addition to provide some insight into mean-field limitations, it offers a possibility to incorporate quantum fluctuations of one-body observables with the time-dependent RPA formalism.