Nuclear Theory·nucl-th

Wednesday·December 23, 2015

17 papers—8 primary·9 cross-listed

01
[Submitted on 22 Dec 2015]
Exploring the Beam Energy Dependence of Flow-Like Signatures in Small System $d +$ Au Collisions
J.D. Orjuela Koop🇺🇸 · R. Belmont🇺🇸 · P. Yin🇺🇸 · J.L. Nagle🇺🇸
Recent analyses of small collision systems, namely $p + p$ and $p +$ Pb at the LHC and $p +$ Au, $d +$ Au and $^{3}$ He+Au at RHIC, have revealed azimuthal momentum anisotropies commonly associated with collective flow in larger systems. Viscous hydrodynamics and parton cascade calculations have proved successful at describing some flow-like observables in these systems. These two classes of calculations also confirm these observables to be directly related to the initial geometry of the created medium. However, the question of whether equilibrium dynamics is the dominant driver of the signal remains open, given the short lifetime of small systems. In this regime, pre-equilibrium dynamics and late stage hadronic interactions are expected to play a significant role. Hence, a beam energy scan of small systems---that amounts to varying the initial temperature and the lifetime of the medium---can provide valuable information to shed light on these issues. In this paper, we present predictions from viscous hydrodynamics (SONIC), partonic (AMPT) and hadronic (UrQMD) cascade calculations for elliptic $v_{2}$ and triangular $v_{3}$ anisotropy coefficients in $d$ +Au at $s_{N N}$ = 7.7, 20, 39, 62.4 and 200 GeV, corresponding to the expected running at RHIC in 2016. We also present predictions for $d$ +Pb at $s_{N N}$ = 5.02 TeV, an interesting system to compare to existing $p +$ Pb data taken at the LHC.
Comments:
Added new figure comparing cumulant and event-plane methods
Subjects:
Nuclear Theory (nucl-th)
arXiv:
1512.06949 [pdf]
PRC(2016)·27 citations
02
[Submitted on 22 Dec 2015]
Recent developments in heavy-ion fusion reactions around the Coulomb barrier
K. Hagino · N. Rowley🇫🇷 · J.M. Yao🇯🇵
The nuclear fusion is a reaction to form a compound nucleus. It plays an important role in several circumstances in nuclear physics as well as in nuclear astrophysics, such as synthesis of superheavy elements and nucleosynthesis in stars. Here we discuss two recent theoretical developments in heavy-ion fusion reactions at energies around the Coulomb barrier. The first topic is a generalization of the Wong formula for fusion cross sections in a single-channel problem. By introducing an energy dependence to the barrier parameters, we show that the generalized formula leads to results practically indistinguishable from a full quantal calculation, even for light symmetric systems such as $^{12}$ C+ $^{12}$ C, for which fusion cross sections show an oscillatory behavior. We then discuss a semi-microscopic modeling of heavy-ion fusion reactions, which combine the coupled-channels approach to the state-of-the-art nuclear structure calculations for low-lying collective motions. We apply this method to subbarrier fusion reactions of $^{58}$ Ni+ $^{58}$ Ni and $^{40}$ Ca+ $^{58}$ Ni systems, and discuss the role of anharmonicity of the low-lying vibrational motions.
Comments:
10 pages, 6 figures. A talk given at the 5th International Workshop on Compound-Nuclear Reactions and Related Topics (CNR*15), October 9-23, 2015, Tokyo, Japan
Subjects:
Nuclear Theory (nucl-th); Nuclear Experiment (nucl-ex)
arXiv:
1512.06954 [pdf]
EPJ Web Conf.(2016)·0 citations
03
[Submitted on 22 Dec 2015]
The In-Medium Similarity Renormalization Group: A Novel Ab Initio Method for Nuclei
H. Hergert · S. K. Bogner · T. D. Morris · A. Schwenk · K. Tsukiyama
We present a comprehensive review of the In-Medium Similarity Renormalization Group (IM-SRG), a novel ab inito method for nuclei. The IM-SRG employs a continuous unitary transformation of the many-body Hamiltonian to decouple the ground state from all excitations, thereby solving the many-body problem. Starting from a pedagogical introduction of the underlying concepts, the IM-SRG flow equations are developed for systems with and without explicit spherical symmetry. We study different IM-SRG generators that achieve the desired decoupling, and how they affect the details of the IM-SRG flow. Based on calculations of closed-shell nuclei, we assess possible truncations for closing the system of flow equations in practical applications, as well as choices of the reference state. We discuss the issue of center-of-mass factorization and demonstrate that the IM-SRG ground-state wave function exhibits an approximate decoupling of intrinsic and center-of-mass degrees of freedom, similar to Coupled Cluster (CC) wave functions. To put the IM-SRG in context with other many-body methods, in particular many-body perturbation theory and non-perturbative approaches like CC, a detailed perturbative analysis of the IM-SRG flow equations is carried out. We conclude with a discussion of ongoing developments, including IM-SRG calculations with three-nucleon forces, the multi-reference IM-SRG for open-shell nuclei, first non-perturbative derivations of shell- model interactions, and the consistent evolution of operators in the IM-SRG. We dedicate this review to the memory of Gerry Brown, one of the pioneers of many-body calculations of nuclei.
Comments:
92 pages, 33 figures, to appear in Physics Reports
Subjects:
Nuclear Theory (nucl-th)
arXiv:
1512.06956 [pdf]
Phys.Rept.(2016)·550 citations
04
[Submitted on 22 Dec 2015]
Calculation of nuclear matter in the presence of strong magnetic field using LOCV technique
G. H. Bordbar · Z. Rezaei
In the present work, we are interested in the properties of nuclear matter at zero temperature in the presence of strong magnetic fields using the lowest order constraint variational (LOCV) method employing $A V_{18}$ nuclear potential. Our results indicate that in the absence of a magnetic field, the energy per particle is a symmetric function of the spin polarization parameter. This shows that for the nuclear matter, the spontaneous phase transition to a ferromagnetic state does not occur. However, we have found that for the magnetic fields $B ≳ 1 0^{18} G$ , the symmetry of energy is broken and the energy has a minimum at a positive value of the spin polarization parameter. We have also found that the effect of magnetic field on the value of energy is more significant at the low densities. Our calculations show that at lower densities, the spin polarization parameter is more sensitive to the magnetic field.
Comments:
14 pages, 5 figures Romanian J. Phys. 61 (2016) in press
Subjects:
Nuclear Theory (nucl-th); Solar and Stellar Astrophysics (astro-ph.SR)
arXiv:
1512.06993 [pdf]
Rom.J.Phys.(2016)·2 citations
05
[Submitted on 22 Dec 2015]
Separable Representation of Energy-Dependent Optical Potentials
Linda Hlophe · Charlotte Elster
Background. One important ingredient for many applications of nuclear physics to astrophysics, nuclear energy, and stockpile stewardship are cross sections for reactions of neutrons with rare isotopes. Since direct measurements are often not feasible, indirect methods, e.g. (d,p) reactions, should be used. Those (d,p) reactions may be viewed as three-body reactions and described with Faddeev techniques. Purpose. Faddeev equations in momentum space have a long tradition of utilizing separable interactions in order to arrive at sets of coupled integral equations in one variable. Optical potentials representing the effective interactions in the neutron (proton) nucleus subsystem are usually non-Hermitian as well as energy-dependent. Potential matrix elements as well as transition matrix elements calculated with them must fulfill the reciprocity theorem. The purpose of this paper is to introduce a separable, energy-dependent representation of complex, energy-dependent optical potentials that fulfill reciprocity exactly. Results. Starting from a separable, energy-independent representation of global optical potentials based on a generalization of the Ernst-Shakin-Thaler (EST) scheme, a further generalization is needed to take into account the energy dependence. Applications to n $+^{48}$ Ca, n $+^{208}$ Pb, and p $+^{208}$ Pb are investigated for energies from 0 to 50~MeV with special emphasis on fulfilling reciprocity. Conclusions. We find that the energy-dependent separable representation of complex, energy-dependent phenomenological optical potentials fulfills reciprocity exactly. In addition, taking into account the explicit energy dependence slightly improves the description of the $S$ matrix elements.
Subjects:
Nuclear Theory (nucl-th)
arXiv:
1512.07084 [pdf]
PRC(2016)·7 citations
06
[Submitted on 22 Dec 2015]
Noncongruence of phase transitions in strongly interacting matter
Matthias Hempel · Veronica Dexheimer · Stefan Schramm · Igor Iosilevskiy
First-order phase transitions (PTs) with more than one globally conserved charge, so-called noncongruent PTs, have characteristic differences compared to congruent PTs (e.g., dimensionality of phase diagrams and location of critical points and endpoints). Here we discuss the noncongruent features of the QCD PT and compare it with the nuclear liquid-gas (LG) PT, for symmetric and asymmetric matter in heavy-ion collisions and neutron stars. In addition, we have identified a principle difference between the LG and the QCD PT: they have opposite slopes in the pressure-temperature plane.
Comments:
7 pages, 2 figures, Proceeding of the 14th Marcel Grossmann Meeting 2015
Subjects:
Nuclear Theory (nucl-th)
arXiv:
1512.07116 [pdf]
4 citations
07
[Submitted on 22 Dec 2015]
The unreasonable effectiveness of hydrodynamics in heavy ion collisions
Jacquelyn Noronha-Hostler🇺🇸 · Jorge Noronha🇧🇷 · Miklos Gyulassy🇺🇸
Event-by-event hydrodynamic simulations of AA and pA collisions involve initial energy densities with large spatial gradients. This is associated with the presence of large Knudsen numbers ( $K_{n} \approx 1$ ) at early times, which may lead one to question the validity of the hydrodynamic approach in these rapidly evolving, largely inhomogeneous systems. A new procedure to smooth out the initial energy densities is employed to show that the initial spatial eccentricities, $ε_{n}$ , are remarkably robust with respect to variations in the underlying scale of initial energy density spatial gradients, $λ$ . For $s_{N N} = 2.76$ TeV LHC initial conditions generated by the MCKLN code, $ε_{n}$ (across centralities) remains nearly constant if the fluctuation scale varies by an order of magnitude, i.e., when $λ$ varies from 0.1 to 1 fm. Given that the local Knudsen number $K_{n} \approx \frac{1}{λ}$ , the robustness of the initial eccentricities with respect to changes in the fluctuation scale suggests that the vn's cannot be used to distinguish between events with large $K_{n}$ from events where $K_{n}$ is in the hydrodynamic regime. We use the 2+1 Lagrangian hydrodynamic code v-USPhydro to show that this is indeed the case: anisotropic flow coefficients computed within event-by-event viscous hydrodynamics are only sensitive to long wavelength scales of order $\frac{1}{Λ _{QC D}} \approx 1$ fm and are incredibly robust with respect to variations in the initial local Knudsen number. This robustness can be used to justify the somewhat unreasonable effectiveness of the nearly perfect fluid paradigm in heavy ion collisions.
Comments:
Proceedings of the 25th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2015), Kobe, Japan,4 pages, 4 figures
Subjects:
Nuclear Theory (nucl-th); High Energy Physics — Phenomenology (hep-ph)
arXiv:
1512.07135 [pdf]
NPA(2016)·18 citations
08
[Submitted on 22 Dec 2015]
Long wavelength perfect fluidity from short distance jet transport in quark-gluon plasmas
Jiechen Xu🇺🇸 · Jinfeng Liao🇺🇸 · Miklos Gyulassy🇺🇸
We build a new phenomenological framework that bridges the long wavelength bulk viscous transport properties of the strongly-coupled quark-gluon plasma (sQGP) and short distance hard jet transport properties in the QGP. The full nonperturbative chromo-electric (E) and chromo-magnetic (M) structure of the near "perfect fluid" like sQGP in the critical transition region are integrated into a semi-Quark-Gluon-Monopole Plasma (sQGMP) model lattice-compatibly and implemented into the new CUJET3.0 jet quenching framework. All observables computed from CUJET3.0 are found to be consistent with available data at RHIC and LHC simultaneously. A quantitative connection between the shear viscosity and jet transport parameter is rigorously established within this framework. We deduce the $T = 160 - 600$ MeV dependence of the QGP's $η / s$ : its near vanishing value in the near $T_{c}$ regime is determined by the composition of E and M charges, it increases as $T$ rises, and its high $T$ limit is fixed by color screening scales.
Comments:
4 pages excluding references, 3 figures, elsarticle style; contributed talk by Jiechen Xu at Quark Matter 2015, Kobe
Subjects:
Nuclear Theory (nucl-th); High Energy Physics — Phenomenology (hep-ph); Nuclear Experiment (nucl-ex)
arXiv:
1512.07137 [pdf]
NPA(2016)·0 citations

Affiliations

first authorsco-authorsvia INSPIRE