Nuclear Theory·nucl-th

Tuesday·April 28, 2020

13 papers—8 primary·5 cross-listed

01
Effects of the mean field on fluid dynamics in the relaxation time approximation
Alina Czajka🇵🇱 · Sigtryggur Hauksson🇺🇸 · Chun Shen🇨🇦 · Sangyong Jeon🇨🇦 · Charles Gale🇨🇦
In this paper the nonequilibrium correction to the distribution function containing a time and space dependent mass is obtained. Given that, fully consistent fluid dynamic equations are formulated. Then, the physics of the bulk viscosity is elaborated for Boltzmann and Bose-Einstein gases within the relaxation time approximation. It is found that the parametric form of the ratio $ζ / τ_{R}$ for the quantum gas is affected by the infrared cut-off. This may be an indication that the relaxation time approximation is too crude to obtain a reliable form of bulk viscosity.
nucl-thActa Phys.Polon.Supp.(2021)·2 citations
02
Intrinsic operators for the translationally-invariant many-body problem
M. A. Caprio · A. E. McCoy · P. J. Fasano
The need to enforce fermionic antisymmetry in the nuclear many-body problem commonly requires use of single-particle coordinates, defined relative to some fixed origin. To obtain physical operators which nonetheless act on the nuclear many-body system in a Galilean-invariant fashion, thereby avoiding spurious center-of-mass contributions to observables, it is necessary to express these operators with respect to the translational intrinsic frame. Several commonly-encountered operators in nuclear many-body calculations, including the magnetic dipole and electric quadrupole operators (in the impulse approximation) and generators of SU(3) and Sp(3,R) symmetry groups, are bilinear in the coordinates and momenta of the nucleons and, when expressed in intrinsic form, become two-body operators. To work with such operators in a second-quantized many-body calculation, it is necessary to relate three distinct forms: the defining intrinsic-frame expression, an explicitly two-body expression in terms of two-particle relative coordinates, and a decomposition into one-body and separable two-body parts. We establish the relations between these forms, for general (non-scalar and non-isoscalar) operators bilinear in coordinates and momenta.
nucl-thJ.Phys.G(2020)·19 citations
03
Decoding the QCD critical behaviour in A+A collisions
Nikolaos G. Antoniou🇬🇷 · Nikolaos Davis🇵🇱 · Fotios K. Diakonos🇬🇷 · Georgios Doultsinos🇬🇷 · Nikolaos Kalntis🇬🇷 · Alexandros Kanargias🇬🇷 · Athanasios S. Kapoyannis🇬🇷 · Vitalii Ozvenchuk🇵🇱 · Costas N. Papanicolas🇬🇷 · Andrzej Rybicki🇵🇱 · Efstathios Stiliaris🇬🇷
In a systematic search for the QCD critical point in nuclear collisions, at the CERN SPS, it was found that intermittency measurements in the freeze-out state of central Si+Si collisions, at the maximum SPS energy, provide us with an indication of sizeable critical fluctuations. Also, rather recently, a weaker effect was traced in preliminary data of the Ar+Sc reaction for 10-20% most central collisions at (approximately) the same energy. However, the uncertainties in the analysis and the limitations of the experimental event statistics make the interpretation of the above measurements (NA49, NA61/SHINE) rather inconclusive, inviting for a further, phenomenological investigation with complementary tools and theoretical ideas. To this end, in the present work, we employ intermittency techniques within a model-independent analysis scheme (AMIAS), a novel method from Data Science [arXiv:1205.6505], in order to produce unbiased results for the parameters of the power-laws and in particular for the associated power-law exponent (intermittency index) $ϕ_{2}$ . Using data-sets at different peripheralities, we also study the dependence of the $ϕ_{2}$ -value on the number of wounded nucleons, in order to uncover the approach to the critical point. With these findings and the help of Ising-QCD partition function, the interpretation of SPS intermittency measurements and their links to the critical region, are discussed.
nucl-thhep-phnucl-exNPA(2020)·10 citations
04
The energies and ANCs for 5Li resonances deduced from experimental p- $α$ scattering phase shifts using the effective-range and $Δ$ methods
Yu. V. Orlov
Recently a new $Δ$ method for deducing the energy and asymptotic normalization coefficient (ANC) from phase-shift data has been formulated and applied to resonance states. This differs from the conventional effective-range function (ERF) method by fitting only the nuclear part of the ERF. It also differs from the method which was proposed for bound states by Ramírez Suárez and Sparenberg (see Ref. below) which also named the $Δ$ method where a pole condition defines by the Eq. $Δ_{l} = 0$ ( $Δ_{l}$ is the function in the ERF determined only by the scattering phase shift). Here the standard pole condition, including the Coulomb part into the relate equation, is used for a resonant state. It has been shown that the ERF method does not work for large-charge colliding nuclei. Moreover, even for lower charges it is not clear that the results of the ERF method are accurately enough. The Coulomb part forms a background, which smooths an ERF energy dependence. Therefore, one needs to find when the ERF method becomes inaccurate and this requires recalculating some published results by the $Δ$ method. This project has already been started in a recent paper for resonances in the $α$ - $α$ scattering. Here this method is applied using the $Δ_{l}$ -function fittings to the experimental $p$ - $^{4}$ He scattering phase-shift data in the $P_{3/2}$ and $P_{1/2}$ resonance states. The calculation results are compared with those obtained earlier by the ERF method. The main changes concern resonance energy and width.
nucl-thNPA(2020)·0 citations
05
Size properties of the largest fragments produced in the framework of the statistical multifragmentation model
S.R. Souza · R. Donangelo
We study the size properties of the largest intermediate mass fragments in each partition mode, produced in the prompt statistical breakup of a thermally equilibrated nuclear source, at different temperatures. We find that an appreciable amount of events have primary intermediate mass fragments of similar sizes. Our results suggest that, depending on the temperature of the fragmenting source, their production may be much larger than what would be expected from considerations based on purely combinatorial arrangements of the nucleons in the fragmenting system. We also find that the isospin composition of the largest fragments is sensitive to their rank size within the event. We suggest that experimental analyses, conceived to reconstruct the breakup configuration, should be employed to investigate the validity of our findings.
nucl-th1 citation
06
Are $N Ω ˉ$ bound states?
Hongxia Huang🇨🇳 · Xinmei Zhu🇨🇳 · Jialun Ping🇨🇳 · Fan Wang🇨🇳 · T. Goldman🇺🇸
Inspired by the progress of the experimental search of the $N Ω$ dibaryon by the STAR collaboration, we study $N Ω ˉ$ systems in the framework of quark delocalization color screening model. Our results show that the attraction between $N$ and $Ω ˉ$ is a little bit larger than that between $N$ and $Ω$ , which indicates that it is more possible for the $N Ω ˉ$ than the $N Ω$ system to form bound states. The dynamic calculations state that both the $J^{P} = 1^{+}$ and $2^{+}$ $N Ω ˉ$ systems are bound states. The binding energy of these two states are deeper than that of $N Ω$ systems with $J^{P} = 2^{+}$ , and the $N Ω$ system with $J^{P} = 1^{+}$ is unbound. The calculation of the low-energy scattering phase shifts, scattering length and the effective range also supports the existence of the $N Ω ˉ$ bound states with $J^{P} = 1^{+}$ and $2^{+}$ . So the $N Ω ˉ$ states are better hexaquark states and stronger signals are expected in experiments.
nucl-thhep-ph2 citations
07
Nucleon-Nucleon Scattering with Coupled Nucleon-Delta Channels in Chiral Effective Field Theory
Susanne Strohmeier · Norbert Kaiser
In this work the elastic scattering of two nucleons is calculated in chiral effective field theory at next-to-leading order taking into account the coupled N $Δ$ -, $Δ$ N- and $ΔΔ$ -channels. To solve the coupled channel scattering equation one needs as input the potentials for all combinations of these initial and final states. Up to next-to leading order these (transition) potentials arise from one-pion exchange, two-pion exchange and contact interactions. For the two-pion exchange we give analytic expressions for the spectral functions derived from all contributing one-loop diagrams. The forms of the contact potentials at leading and next-to-leading order are determined. We perform a fit of the low energy constants, that belong to the $N N \to N N$ contact potential and contribute up to next-to-leading order to $S$ - and $P$ -waves of NN scattering only. The influence of the $Δ$ -isobar dynamics entering through the coupled channels is studied in detail.
nucl-thNPA(2020)·7 citations
08
Ab initio multi-shell valence-space Hamiltonians and the island of inversion
T. Miyagi🇨🇦 · S. R. Stroberg🇺🇸 · J. D. Holt🇨🇦 · N. Shimizu🇯🇵
In the shell-model framework, valence-space Hamiltonians connecting multiple major-oscillator shells are of key interest for investigating the physics of neutron-rich nuclei, which have been the subject of intense experimental activity for decades. Here we present an extension of the ab initio valence-space in-medium similarity renormalization group which allows the derivation of such Hamiltonians nonperturbatively. Starting from initial two- and three-nucleon forces from chiral effective field theory, we then calculate properties of nuclei in the important island-of-inversion region above oxygen, so far unexplored with ab initio methods. Our results in the neon and magnesium isotopes indicate the importance of neutron excitation from the $s d$ to $p f$ shells and ground states dominated by intruder configurations around $N = 20$ , consistent with the conclusions from phenomenological studies. We also benchmark the excitation spectrum of $^{16}$ O with coupled-cluster theory, finding generally good agreement, and discuss implications for ground state energies and charge radii in oxygen and calcium isotopes. Finally we outline the proper procedure for treating the long-standing issue of center-of-mass contamination, and show that with a particular choice of valence space, these spurious states can be removed successfully.
nucl-thPRC(2020)·116 citations
09
Threshold Resummation for Hadron Production in the Small- $x$ Region
Hao-Yu Liu🇨🇳 · Zhong-Bo Kang🇺🇸 · Xiaohui Liu🇨🇳
We study the single hadron inclusive production in the forward rapidity region in proton-nucleus collisions. We find the long-standing negative cross section at next-to-leading-order (NLO) is driven by the large negative threshold logarithmic contributions. We established a factorization theorem for resumming these logarithms with systematically improvable accuracy within the color glass condensate formalism. We demonstrate how the threshold leading logarithmic accuracy can be realized by a suitable scale choice in the NLO results. The NLO spectrums with the threshold logarithms resummed remain positive and impressive agreements with experimental data are observed.
↳ hep-phhep-exnucl-exnucl-thPRD(2020)·38 citations
10
Modified Structure of Protons and Neutrons in Correlated Pairs
B. Schmookler · M. Duer · A. Schmidt · O. Hen · S. Gilad · E. Piasetzky · M. Strikman · L.B. Weinstein · The CLAS Collaboration
The atomic nucleus is made of protons and neutrons (nucleons), that are themselves composed of quarks and gluons. Understanding how the quark-gluon structure of a nucleon bound in an atomic nucleus is modified by the surrounding nucleons is an outstanding challenge. Although evidence for such modification, known as the EMC effect, was first observed over 35 years ago, there is still no generally accepted explanation of its cause. Recent observations suggest that the EMC effect is related to close-proximity Short Range Correlated (SRC) nucleon pairs in nuclei. Here we report the first simultaneous, high-precision, measurements of the EMC effect and SRC abundances. We show that the EMC data can be explained by a universal modification of the structure of nucleons in neutron-proton (np) SRC pairs and present the first data-driven extraction of this universal modification function. This implies that, in heavier nuclei with many more neutrons than protons, each proton is more likely than each neutron to belong to an SRC pair and hence to have its quark structure distorted.
↳ nucl-exhep-phnucl-thNature(2019)·197 citations
11
Bulk viscosity of resonating fermions revisited: Kubo formula, sum rule, and the dimer and high-temperature limits
Keisuke Fujii · Yusuke Nishida
The bulk viscosity of two-component fermions with a zero-range interaction is revisited both in two and three dimensions. We first point out that the "standard" Kubo formula employed in recent studies has flaws to give rise to an unphysical divergent bulk viscosity even in a limit where it is supposed to vanish. The corrected Kubo formula as well as the sum rule is then carefully rederived so as to confirm that the bulk viscosity indeed vanishes in the free, unitarity, and dimer limits. We also discuss that the recently found discrepancy between the Kubo formalism and the kinetic theory for the bulk viscosity is attributed to the fact that the quasiparticle approximation assumed by the latter breaks down even in the high-temperature limit.
↳ cond-mat.quant-gascond-mat.stat-mechnucl-thPRA(2020)·15 citations
12
Symmetry restoration and the gluon mass in the Landau gauge
Camille Noûs · Urko Reinosa · Julien Serreau · Rodrigo Carmo Terin · Matthieu Tissier
We investigate the generation of a gluon screening mass in Yang-Mills theory in the Landau gauge. We propose a gauge-fixing procedure where the Gribov ambiguity is overcome by summing over all Gribov copies with some weight function. This can be formulated in terms of a local field theory involving constrained, nonlinear sigma model fields. We show that a phenomenon of radiative symmetry restoration occurs in this theory, similar to what happens in the standard nonlinear sigma model in two dimensions. This results in a nonzero gluon screening mass, as seen in lattice simulations.
↳ hep-thhep-lathep-phnucl-thSciPost Phys.(2021)·17 citations
13
Realistic classical binding energies in the $ω$ -Skyrme model
Sven Bjarke Gudnason🇨🇳 · James Martin Speight🇬🇧
An omega-meson extension of the Skyrme model - without the Skyrme term but including the pion mass - first considered by Adkins and Nappi is studied in detail for baryon numbers 1 to 8. The static problem is reformulated as a constrained energy minimisation problem within a natural geometric framework and studied analytically on compact domains, and numerically on Euclidean space. Using a constrained second-order Newton flow algorithm, classical energy minimisers are constructed for various values of the omega-pion coupling. At high coupling, these Skyrmion solutions are qualitatively similar to the Skyrmions of the standard Skyrme model with massless pions. At sufficiently low coupling, they show similarities with those in the lightly bound Skyrme model: the Skyrmions of low baryon number dissociate into lightly bound clusters of distinct 1-Skyrmions, and the classical binding energies for baryon numbers 2 through 8 have realistic values.
↳ hep-thmath-phmath.MPnucl-thJHEP(2020)·32 citations

Affiliations

first authorsco-authorsvia INSPIRE