Nuclear Theory·nucl-th

Wednesday·September 13, 2017

10 papers—7 primary·3 cross-listed

01
[Submitted on 11 Sept 2017]
A quasiparticle equation of state with a phenomenological critical point
Hong-Hao Ma🇧🇷 · Wei-Liang Qian🇧🇷
We propose a hybrid parameterization of a quasiparticle equation of state, where a critical point is implemented phenomenologically. In this approach, a quasiparticle model with finite chemical potential is used to describe the quark-gluon plasma phase by fitting to the lattice quantum chromodynamics data at high temperature. On the other hand, the hadronic resonance gas model with excluded volume correction is employed for the hadronic phase. An interpolation scheme is implemented so that the phase transition is a smooth crossover when the chemical potential is smaller than a critical value, or otherwise approximately of first order according to Ehrenfest's classification. Also, the thermodynamic consistency is guaranteed for the equation of state related to both the quasiparticle model and the implementation of the critical point.
Comments:
13 pages, 4 figures
Subjects:
Nuclear Theory (nucl-th)
arXiv:
1709.03586 [pdf]
Braz.J.Phys.(2018)·8 citations
02
[Submitted on 11 Sept 2017]
Quantum Monte Carlo calculations of weak transitions in $A$ =6--10 nuclei
S. Pastore · A. Baroni · J. Carlson · S. Gandolfi · Steven C. Pieper · R. Schiavilla · R.B. Wiringa
Ab initio calculations of the Gamow-Teller (GT) matrix elements in the $β$ decays of $^{6}$ He and $^{10}$ C and electron captures in $^{7}$ Be are carried out using both variational and Green's function Monte Carlo wave functions obtained from the Argonne $v_{18}$ two-nucleon and Illinois-7 three-nucleon interactions, and axial many-body currents derived from either meson-exchange phenomenology or chiral effective field theory. The agreement with experimental data is excellent for the electron captures in $^{7}$ Be, while theory overestimates the $^{6}$ He and $^{10}$ C data by $\sim 2%$ and $\sim 10%$ , respectively. We show that for these systems correlations in the nuclear wave functions are crucial to explain the data, while many-body currents increase by $\sim 2$ -- $3%$ the one-body GT contributions. These findings suggest that the longstanding $g_{A}$ -problem, i.e., the systematic overprediction ( $\sim 20%$ in $A \leq 18$ nuclei) of GT matrix elements in shell-model calculations, may be resolved, at least partially, by correlation effects.
Comments:
6 pages, 2 figures
Subjects:
Nuclear Theory (nucl-th)
arXiv:
1709.03592 [pdf]
PRC(2018)·98 citations
03
[Submitted on 11 Sept 2017]
Photon emission from quark-gluon plasma out of equilibrium
Sigtryggur Hauksson🇨🇦 · Sangyong Jeon🇨🇦 · Charles Gale🇨🇦
The photon emission from a non-equilibrium quark-gluon plasma (QGP) is analyzed. We derive an integral equation that describes photon production through quark-antiquark annihilation and quark bremsstrahlung. It includes coherence between different scattering sites, also known as the Landau-Pomeranchuk-Migdal effect. These leading-order processes are studied for the first time together in an out-of-equilibrium field theoretical treatment that enables the inclusion of viscous corrections to the calculation of electromagnetic emission rates. In the special case of an isotropic, viscous, plasma the integral equation only depends on three constants which capture the non-equilibrium nature of the medium.
Comments:
Discussion of photon rate at higher anisotropy extended, typos corrected, referenced added, published version
Subjects:
Nuclear Theory (nucl-th); High Energy Physics — Phenomenology (hep-ph)
arXiv:
1709.03598 [pdf]
PRC(2018)·33 citations
04
[Submitted on 11 Sept 2017]
Quasiparticle-vibration coupling effects on nuclear transitions of astrophysical interest
Caroline Robin · Elena Litvinova
The relativistic quasiparticle time-blocking approximation (RQTBA) is applied to the description of nuclear excitation modes of astrophysical interest. This method is based on the meson-nucleon Lagrangian and goes beyond the standard relativistic quasiparticle random-phase approximation (RQRPA) by treating the coupling between single quasiparticles and collective vibrations of the nucleus. We calculate electric dipole transitions and Gamow-Teller modes in the (p,n) direction in a few Sn isotopes and obtain the rates of (n, $γ$ ) reaction and $β^{-}$ -decay processes, which govern the r-process nucleosynthesis, in a unified RQTBA framework. Gamow-Teller transitions in the (n,p) branch, which in principle can serve for the modeling of stellar evolution, are also investigated, and $^{90}$ Zr is taken as a study case.
Comments:
10 pages, 8 figures. Contribution to the proceedings of the Fifth Conference on Nuclei and Mesoscopic Physics (NMP17), East Lansing, USA, March 6-10, 2017
Subjects:
Nuclear Theory (nucl-th); Nuclear Experiment (nucl-ex)
arXiv:
1709.03606 [pdf]
AIP Conf.Proc.(2017)·3 citations
05
[Submitted on 12 Sept 2017]
Statistical errors in Weizsaecker-Skyrme mass model
Min Liu · Yu Gao · Ning Wang
The statistical uncertainties of 13 model parameters in the Weizsäcker-Skyrme (WS*) mass model are investigated for the first time with an efficient approach, and the propagated errors in the predicted masses are estimated. The discrepancies between the predicted masses and the experimental data, including the new data in AME2016, are almost all smaller than the model errors. For neutron-rich heavy nuclei, the model errors increase considerably, and go up to a few MeV when the nucleus approaches the neutron drip line. The most sensitive model parameter which causes the largest statistical error is analyzed for all bound nuclei. We find that the two coefficients of symmetry energy term significantly influence the mass predictions of extremely neutron-rich nuclei, and the deformation energy coefficients play a key role for well-deformed nuclei around the $β$ -stability line.
Comments:
5 figrues, 2 tables, to appear in Chinese Physics C
Subjects:
Nuclear Theory (nucl-th); Solar and Stellar Astrophysics (astro-ph.SR)
arXiv:
1709.03703 [pdf]
CPC(2017)·7 citations
06
[Submitted on 12 Sept 2017]
Charge-exchange quasi-elastic process $n d \to p (nn)$ under $0^{\circ}$ in the frame of elastic $n p \to n p$ scattering to $18 0^{\circ}$
R. A. Shindin · D. K. Guriev · A. N. Livanov · I. P. Yudin
It is considered the problem of spin physics related with the difference of representation of the elastic interaction between the neutron and proton. In the first case the charge-exchange reaction $n p \to p n$ under the angle $θ$ is supposed, in the second --- the simple elastic scattering of $n p \to n p$ , when the neutron is going in opposite direction $π - θ$ . The transition from one representation to another is provided by the Majorana operator. In the framework of impulse approximation it is twice calculated the quasi-elastic charge-exchange reaction of a neutron on a deuteron. In the frame of $n d \to p (nn)$ scattering of proton to the angle $θ$ it gives the well-known Dean formula. Using other representation $n d \to (nn) p$ as a neutron elastic scattering under the angle $π - θ$ (together with neutron-spectator in $nn$ -pair) the alternative formula is presented.
Comments:
English
Subjects:
Nuclear Theory (nucl-th); Nuclear Experiment (nucl-ex)
arXiv:
1709.03782 [pdf]
0 citations
07
[Submitted on 12 Sept 2017]
Shear viscosity of a hadron gas and influence of resonance lifetimes on relaxation time
J.-B. Rose🇩🇪 · J. M. Torres-Rincon🇩🇪 · A. Schäfer🇩🇪 · D. R. Oliinychenko🇩🇪 · H. Petersen🇩🇪
We address a discrepancy between different computations of $η / s$ (shear viscosity over entropy density) of hadronic matter. Substantial deviations of this coefficient are found between transport approaches mainly based on resonance propagation with finite lifetime and other (semi-analytical) approaches with energy-dependent cross-sections, where interactions do not introduce a timescale. We provide an independent extraction of this coefficient by using the newly-developed SMASH (Simulating Many Accelerated Strongly interacting Hadrons) transport code, which is an example of a mainly resonance-based approach. We compare the results from SMASH with numerical solutions of the Boltzmann equation for simple systems using the Chapman-Enskog expansion, as well as previous results in the literature. Our conclusion is that the hadron interaction via resonance formation/decay strongly affects the transport properties of the system, resulting in significant differences in $η / s$ with respect to other approaches where binary collisions dominate. We argue that the relaxation time of the system ---which characterizes the shear viscosity--- is determined by the interplay between the mean-free time and the lifetime of resonances. We show how an artificial shortening of the resonance lifetimes, or the addition of a background elastic cross section nicely interpolate between the two discrepant results.
Comments:
15 pages, 16 figures
Subjects:
Nuclear Theory (nucl-th); High Energy Physics — Phenomenology (hep-ph)
arXiv:
1709.03826 [pdf]
PRC(2018)·97 citations
08
[Submitted on 11 Sept 2017] (cross-list from hep-ph)
Approach to equilibrium of quarkonium in quark-gluon plasma
Xiaojun Yao🇺🇸 · Berndt Müller🇺🇸
We calculate the dissociation and recombination rates of $Υ (1 S)$ in quark-gluon plasma by using potential non-relativistic QCD. We then study the dynamical in-medium evolution of the $b b ˉ - Υ$ system in a periodic box via the Boltzmann equation and explore how the system reaches equilibrium. We find that interactions between the free heavy quarks and the medium are necessary for the system to reach equilibrium. We find that the angular distribution of $Υ (1 S)$ probes the stages at which recombination occurs. Finally, we study the system under a longitudinal expansion and show that different initial conditions evolve to distinct final ratios of hidden and open $b$ flavors. We argue that experimental measurements of the ratio could address open questions in the quarkonium production in heavy ion collisions.
Comments:
6 pages, 5 figures, accepted version; a missing factor in Eq.(6) is added
Subjects:
High Energy Physics — Phenomenology (hep-ph); High Energy Physics — Experiment (hep-ex); Nuclear Experiment (nucl-ex); Nuclear Theory (nucl-th)
arXiv:
1709.03529 [pdf]
PRC(2018)·65 citations
09
[Submitted on 11 Sept 2017] (cross-list from nucl-ex)
Collective long-range particle correlations in proton-proton and proton-nucleus collisions at the LHC with the CMS detector
Zhenyu Chen🇺🇸
Measurements of two-particle angular correlations of inclusive charged particles as well as identified strange hadron in pp and pPb collisions are presented over a wide range in pseudorapidity and full azimuth. The data were collected using the CMS detector at the LHC, with nucleon-nucleon center-of-mass energy of 5.02 TeV for pPb collisions and 5, 7, 13 TeV for pp collisions. The results are compared to semi-peripheral PbPb collision data at center-of-mass energy of 2.76 TeV, covering similar charged-particle multiplicity ranges of the events. For high-multiplicity pp and pPb events, a clear particle species dependence of $v_{2}$ is observed for $p_{T} ≲ 2$ GeV/c. When divided by the number of constituent quarks and compared at the same transverse kinetic energy per quark, a universal scaling over a broad range of particle transverse kinetic energy has been observed. For high-multiplicity pp collisions at 13 TeV, the $v_{2}$ values obtained for inclusive charged particles with two-, four- and six-particle correlations are found to be comparable within uncertainties. These measurements provide strong evidence for the collective nature of the long-range correlations observed in pp collisions.
Comments:
Author's Ph.D. thesis (Rice University, May 2017). Adviser: Wei Li
Subjects:
Nuclear Experiment (nucl-ex); High Energy Physics — Experiment (hep-ex); High Energy Physics — Phenomenology (hep-ph); Nuclear Theory (nucl-th)
arXiv:
1709.03567 [pdf]
1 citation
10
[Submitted on 10 Sept 2017] (cross-list from nucl-ex)
Effect of broken axial symmetry on the electric dipole strength and the collective enhancement of level densities in heavy nuclei
Eckart Grosse🇩🇪 · Arnd R. Junghans🇩🇪 · Jon N. Wilson🇫🇷
The basic parameters for calculations of radiative neutron capture , photon strength functions and nuclear level densities near the neutron separation energy are determined based on experimental data without an ad-hoc assumption about axial symmetry - at variance to previous analysis. Surprisingly few global fit parameters are needed in addition to information on nuclear deformation, taken from Hartree Fock Bogolyubov (HFB) calculations with the Gogny force, and the generator coordinator method (GCM) assures properly defined angular momentum. For a large number of nuclei the GDR shapes and the photon strength are described by the sum of three Lorentzians (TLO), extrapolated to low energies and normalized in accordance to the dipole sum rule. Level densities are influenced strongly by the significant collective enhancement based on the breaking of shape symmetry. The replacement of axial symmetry by the less stringent requirement of invariance against rotation by 180 degree leads to a novel prediction for radiative neutron capture. It compares well to recent compilations of average radiative widths and Maxwellian average cross sections for neutron capture by even target nuclei. An extension to higher spin promises a reliable prediction for various compound nuclear reactions also outside the valley of stability. Such predictions are of high importance for future nuclear energy systems and waste transmutation as well as for the understanding of the cosmic synthesis of heavy elements.
Comments:
Pacs. Nos.:21.10.Ma, 24.30.Cz, 24.60.Dr, 25.20.Dc, 25.40.Lw, 26.20.Kn; 25 pages, 12 figures; to appear in Physica Scripta, special issue on "Shapes and symmetries in Nuclei" (2017)
Subjects:
Nuclear Experiment (nucl-ex); Solar and Stellar Astrophysics (astro-ph.SR); Nuclear Theory (nucl-th)
arXiv:
1709.03884 [pdf]
Phys.Scripta(2017)·6 citations

Affiliations

first authorsco-authorsvia INSPIRE