Nuclear Theory·nucl-th

Thursday·December 30, 2021

27 papers—13 primary·14 cross-listed

01
Transverse mass scaling of dilepton radiation off a quark-gluon plasma
Maurice Coquet🇫🇷 · Xiaojian Du🇩🇪 · Jean-Yves Ollitrault🇫🇷 · Soeren Schlichting🇩🇪 · Michael Winn🇫🇷
The spectrum of dileptons produced by the quark-gluon plasma in an ultrarelativistic nucleus-nucleus collision depends only, to a good approximation, on the transverse mass M_t of the dilepton. This scaling is exact as long as transverse flow is negligible, and the system is in local thermal equilibrium. We implement a state-of-the-art modelization of kinetic and chemical equilibration in the early stages of the evolution to study the modifications of the spectrum. Violations of M_t scaling resulting from these effects are evaluated as a function of the shear viscosity to entropy ratio (eta/s) that controls the equilibration time. We determine the dependence of the spectrum on system size, centrality, rapidity, and collision energy. We show that the quark-gluon plasma produces more dileptons than the Drell-Yan process up to invariant masses of order M = 4 GeV. Due to different kinematics, for a given M_t , the dependence of the dilepton yield on M is opposite for the two processes, so that experiment alone can in principle determine which process dominates.
nucl-thhep-exhep-phnucl-exNPA(2023)·19 citations
02
Machine learning phase transitions of the three-dimensional Ising universality class
Xiaobing Li🇨🇳 · Ranran Guo🇨🇳 · Yu Zhou🇺🇸 · Kangning Liu🇨🇳 · Jia Zhao🇨🇳 · Fen Long🇨🇳 · Yuanfang Wu🇨🇳 · Zhiming Li🇨🇳
Exploration of the QCD phase diagram and critical point is one of the main goals in current relativistic heavy-ion collisions. The QCD critical point is expected to belong to a three-dimensional (3D) Ising universality class. Machine learning techniques are found to be powerful in distinguishing different phases of matter and provide a new way to study the phase diagram. We investigate phase transitions in the 3D cubic Ising model using supervised learning methods. It is found that a 3D convolutional neural network can be trained to effectivelly predict physical quantities in different spin configurations. With a uniform neural network architecture, it can encode phases of matter and identify both second- and first-order phase transitions. The important features that discriminate different phases in the classification processes are investigated. These findings can help study and understand QCD phase transitions in relativistic heavy-ion collisions.
nucl-thhep-phnucl-exCPC(2023)·8 citations
03
Nuclear level densities: from empirical models to microscopic methods
Y. Alhassid🇺🇸
The level density is among the most important statistical nuclear properties. It appears in Fermi's golden rule for transition rates and is an important input to the Hauser-Feshbach theory of compound nucleus reactions. We discuss empirical models of level densities and summarize the main experimental methods used to determine them. The microscopic calculation of level densities in the presence of correlations is a challenging many-body problem. We review recent microscopic approaches to calculate level densities. Mean-field and combinatorial methods have been applied across the nuclear chart, but often need to be augmented with empirical collective enhancement factors. The moment method and the auxiliary-field quantum Monte Carlo (AFMC) method are formulated in the context of the configuration-interaction shell model approach, and include correlations beyond the mean-field approximation.
nucl-thcond-mat.mes-hallSpringer Proc.Phys.(2020)·0 citations
04
Prediction of (p,n) Charge-Exchange Reactions with Uncertainty Quantification
T. R. Whitehead · T. Poxon-Pearson · F. M. Nunes · G. Potel
Background: Charge-exchange reactions are a powerful tool for exploring nuclear structure and nuclear astrophysics, however, a robust charge-exchange reaction theory with quantified uncertainties is essential to extracting reliable physics. Purpose: The goal of this work is to determine the uncertainties due to optical potentials used in the theory for charge-exchange reactions to isobaric analogue states. Method: We implement a two-body reaction model to study (p,n) charge-exchange transitions and perform a Bayesian analysis. We study the (p,n) reaction to the isobaric analog states of $^{14}$ C, $^{48}$ Ca, and $^{90}$ Zr targets over a range of beam energies. We compare predictions using standard phenomenological optical potentials with those obtained microscopically. Results: Charge-exchange cross sections are reasonably reproduced by modern optical potentials. However, when uncertainties in the optical potentials are accounted for, the resulting predictions of charge-exchange cross sections have very large uncertainties. Conclusions: The charge-exchange reaction cross section is strongly sensitive to the input interactions, making it a good candidate to further constrain nuclear forces and aspects of bulk nuclear matter. However, further constraints on the optical potentials are necessary for a robust connection between this tool and the underlying isovector properties of nuclei.
nucl-thnucl-exPRC(2022)·18 citations
05
Time-dependent extension of the self-consistent band theory for neutron star matter: Anti-entrainment effects in the slab phase
Kazuyuki Sekizawa · Sorataka Kobayashi · Masayuki Matsuo
Background: In the solid crust of neutron stars, a variety of crystalline structure may exist. Recently the band theory of solids has been applied to the inner crust of neutron stars and significance of the entrainment between dripped neutrons and the solid crust was advocated. Since it influences interpretations of various phenomena of neutron stars, it has been desired to develop deeper understanding of the microphysics behind. Purpose: The purpose of the present article is to propose a fully self-consistent microscopic framework for describing time-dependent dynamics of neutron star matter, which allows us to explore diverse properties of nuclear matter, including the entrainment effect. Results: As the first application of the time-dependent self-consistent band theory for nuclear systems, we investigate the slab phase of nuclear matter with various proton fractions. From a dynamic response of the system to an external force, we extract the collective mass of a slab, associated with entrained neutrons as well as bound nucleons. We find that the extracted collective mass is smaller than a naive estimation based on a potential profile and single-particle energies. We show that the reduction is mainly caused by "counterflow" of dripped neutrons towards the direction opposite to the motion of the slabs. We interpret it as an "anti-entrainment" effect. As a result, the number of effectively bound neutrons is reduced, indicating an enhancement of the number density of conduction neutrons. We demonstrate that those findings are consistent with a static treatment in the band theory of solids. *shortened due to the arXiv's word limit.
nucl-thastro-ph.HEcond-mat.quant-gasPRC(2022)·30 citations
06
Single-state or low-lying-states dominance mechanism of $2 ν β β$ -decay nuclear matrix elements
W. L. Lv🇨🇳 · Y. F. Niu🇨🇳 · D. L. Fang🇨🇳 · C. L. Bai🇨🇳
The $2 ν β β$ -decay nuclear matrix elements (NMEs) for 11 nuclei are studied with the self-consistent quasiparticle random phase approximation (QRPA) based on Skyrme Hartree-Fock-Bogoliubov (Skyrme HFB) model. As a common feature pointed out in https://journals.aps.org/prc/abstract/10.1103/PhysRevC.98.064325 Phys. Rev. C 98, 064325 (2018), negative contributions in the running sums of NMEs are found, and play important roles in the fulfillment of the single-state dominance or low-lying-states dominance hypothesis. By comparing the results of QRPA model and quasiparticle Tamm-Dancoff approximation (QTDA) model, we find that the negative contributions are due to the enhanced ground-state correlations, which are brought by the backward amplitude in QRPA model and tuned by strong isoscalar pairing interaction. The enhancement of ground-state correlations will change the signs of GT $^{+}$ transition amplitudes of higher-lying states and leads to the negative contributions in the running sum.
nucl-thPRC(2022)·8 citations
07
Evolution of $Λ$ polarization in the hadronic phase of heavy-ion collisions
Yifeng Sun🇮🇹 · Zhen Zhang🇨🇳 · Che Ming Ko🇺🇸 · Wenbin Zhao🇺🇸
Using the AMPT + MUSIC+UrQMD hybrid model, we study the global and local spin polarizations of $Λ$ hyperons as functions of the freeze-out temperature of the spin degree of freedom in the hadronic phase of Au+Au collisions at $s_{N N} = 19.6$ GeV. Including contributions from both the thermal vorticity and thermal shear of the hadronic matter, we find that, with the spin freeze-out temperature dropping from the hadronization temperature of 160 MeV to 110 MeV at the kinetic freeze-out, both the global and local spin polarizations of $Λ$ hyperons due to the thermal vorticity decrease by a factor of two, while those due to the thermal shear decrease quickly and become negligibly small at 140 MeV. Our results suggest the importance of understanding the dynamical evolution of the spin degree of freedom in the hadronic stage in relativistic heavy-ion collisions.
nucl-thhep-phPRC(2022)·34 citations
08
Exact expressions for the number of levels in single-\textit{j} orbits for three, four and five fermions
Michel Poirier🇫🇷 · Jean-Christophe Pain🇫🇷
We propose closed-form expressions of the distributions of magnetic quantum number $M$ and total angular momentum $J$ for three and four fermions in single- $j$ orbits. The latter formulas consist of polynomials with coefficients satisfying congruence properties. Such results, derived using doubly-recursive relations over $j$ and the number of fermions, enable us to deduce explicit expressions for the total number of levels in the case of three-, four- and five-fermion systems. We present applications of these formulas, such as sum rules for six- $j$ and nine- $j$ symbols, obtained from the connection with fractional-parentage coefficients, an alternative proof of the Ginocchio-Haxton relation or cancellation properties of the number of levels with a given angular momentum.
nucl-thPRC(2021)·2 citations
09
Extended relaxation time approximation and relativistic dissipative hydrodynamics
Dipika Dash🇮🇳 · Samapan Bhadury🇮🇳 · Sunil Jaiswal🇮🇳 · Amaresh Jaiswal🇮🇳
Development of a new framework for derivation of order-by-order hydrodynamics from Boltzmann equation is necessary as the widely used Anderson-Witting formalism leads to violation of fundamental conservation laws when the relaxation-time depends on particle energy, or in a hydrodynamic frame other than the Landau frame. We generalize an existing framework for consistent derivation of relativistic dissipative hydrodynamics from the Boltzmann equation with a energy-dependent relaxation-time by extending the Anderson-Witting relaxation-time approximation. We argue that the present framework is compatible with conservation laws and derive first-order hydrodynamic equations in landau frame. Further, we show that the transport coefficients, such as shear and bulk viscosity as well as charge and heat diffusion currents, have corrections due to the energy dependence of relaxation time compared to what one obtains from the Anderson-Witting approximation of the collision term. The ratio of these transport coefficients are studied using a parametrized relaxation-time, and several interesting scaling features are reported.
nucl-thhep-phhep-thPLB(2022)·38 citations
10
Resummed relativistic dissipative hydrodynamics
Huda Alalawi🇺🇸 · Mubarak Alqahtani🇸🇦 · Michael Strickland🇺🇸
In this review, we present the motivation for using relativistic anisotropic hydrodynamics to study the physics of ultrarelativistic heavy-ion collisions. We then highlight the main ingredients of the 3+1D quasiparticle anisotropic hydrodynamics model and present phenomenological comparisons with experimental data at different collision energies. These comparisons show that anisotropic hydrodynamics can describe many bulk observables of the quark-gluon plasma.
nucl-thSymmetry(2022)·19 citations
11
Two-body weak currents in heavy nuclei
E.M. Ney · J. Engel · N. Schunck
In light and medium-mass nuclei, two-body weak currents from chiral effective field theory account for a significant portion of the phenomenological quenching of Gamow-Teller transition matrix elements. Here we examine the systematic effects of two-body axial currents on Gamow-Teller strength and $β$ -decay rates in heavy nuclei within energy-density functional theory. Using a Skyrme functional and the charge-changing finite amplitude method, we add the contributions of two-body currents to the usual one-body linear response in the Gamow-Teller channel, both exactly and though a density-matrix expansion. The two-body currents, as expected, usually quench both summed Gamow-Teller strength and decay rates, but by an amount that decreases as the neutron excess grows. In addition, they can enhance individual low-lying transitions, leading to decay rates that are quite different from those that an energy-independent quenching would produce, particularly in neutron-rich nuclei. We show that both these unexpected effects are related to changes in the total nucleon density as the number of neutrons increases.
nucl-thPRC(2022)·12 citations
12
Thermalization of Nuclear Matter in Heavy-Ion Collisions at Fermi-Energies
Thomas Onyango · Aldo Bonasera · Ralf Rapp
We analyze the time evolution of the kinetic properties of nuclear matter produced in heavy-ion collisions at Fermi energies. The collision system is simulated using Constrained Molecular Dynamics (CoMD) transport calculations whose output is the isospin, position, and momentum of the nucleons. Focusing on central 35A $\cdot$ MeV $^{40}$ Ca+ $^{40}$ Ca collisions we utilize this information to extract localized momentum distributions in volume elements of 8fm $^{3}$ and time steps of 5fm/ $c$ . We then parameterize the single-particle momentum distributions with thermally motivated fit functions in the local rest frame of each cell. While the transverse-momentum distributions are well reproduced by thermal ones, the longitudinal ones carry a marked imprint of the initial nuclear motion which we capture by introducing a centroid motion into our fit functions. In particular, we find that Fermi distributions yield significantly better fits than Boltzmann ones, a consequence of the Pauli blocking implemented in CoMD. From the fits we extract the time dependence of the thermodynamic and collective properties of the excited nuclear medium. We find that the transverse temperature gradually rises to about 6MeV, which is accompanied by a dissipation of the initial centroid motion of the incoming nuclei which vanishes at about 100fm/ $c$ after initial impact. We are therefore able to track the transition of beam energy into random kinetic energy for nucleons, suggesting a three-dimensional equilibration of energy in the late stages of the collision.
nucl-thNPA(2022)·2 citations
13
Characteristic momentum of Hydro+ and a bound on the speed of sound near the QCD critical point
Navid Abbasi🇨🇳 · Matthias Kaminski🇺🇸
Near the critical point in the QCD phase diagram, hydrodynamics breaks down at a momentum where the frequency of the fastest hydrodynamic mode becomes comparable with the decay rate of the slowest non-hydrodynamic mode. Hydro+ was developed as a framework which extends the range of validity of hydrodynamics beyond that momentum value. This was achieved through coupling the hydrodynamic modes to the slowest non-hydrodynamic mode. In this work, analyzing the spectrum of linear perturbations in Hydro+, we find that a slow mode falls out of equilibrium if its momentum is greater than a characteristic momentum value. That characteristic momentum turns out to be set by the branch points of the dispersion relations. These branch points occur at the critical momenta of so-called spectral curves and are related to the radius of convergence of the derivative expansion. The existence of such a characteristic momentum scale suggests that a particular class of slow modes has no remarkable effect on the flow of the plasma. Based on these results and previously derived relations to the stiffness of the equation of state, we find a temperature-dependent upper bound for the speed of sound near the critical point in the QCD phase diagram.
nucl-thhep-phhep-thPRD(2022)·11 citations

Affiliations

first authorsco-authorsvia INSPIRE