Nuclear Theory·nucl-th

Tuesday·September 22, 2015

20 papers—13 primary·7 cross-listed

01
Exact solutions of the Boltzmann equation and optimized hydrodynamic approaches for relativistic heavy-ion collisions
U. Heinz (Ohio State)🇺🇸 · D. Bazow (Ohio State)🇺🇸 · G.S. Denicol (McGill)🇨🇦 · M. Martinez (Ohio State)🇺🇸 · M. Nopoush (Kent State)🇺🇸 · J. Noronha (U. Sao Paulo)🇧🇷 · R. Ryblewski (Krakow)🇵🇱 · M. Strickland (Kent State)🇺🇸
Several recent results are reported from work aiming to improve the quantitative precision of relativistic viscous fluid dynamics for relativistic heavy-ion collisions. The dense matter created in such collisions expands in a highly anisotropic manner. Due to viscous effects this also renders the local momentum distribution anisotropic. Optimized hydrodynamic approaches account for these anisotropies already at leading order in a gradient expansion. Recently discovered exact solutions of the relativistic Boltzmann equation in anisotropically expanding systems provide a powerful testbed for such improved hydrodynamic approximations. We present the latest status of our quest for a formulation of relativistic viscous fluid dynamics that is optimized for applications to relativistic heavy-ion collisions.
nucl-thhep-phnucl-exNucl.Part.Phys.Proc.(2016)·26 citations
02
Surface Delta Interaction and g factors
Xiaofei Yu · Larry Zamick
Using an attractive surface delta interaction we obtain wave functions for 2 neutrons (or neutron holes) in the model space of 2 orbits (l=4, j=7/2) and (l=2, j=5/2). If we take the single particle energies to be degenerate we find that the g factors for I=2, 4 and 6 are all the same -namely the orbital g factor of the single nucleon. For a free neutron this quantity zero all 2particle or 2 hole g factors are equal to zero as well.. Only the orbital part of the g -factors contribute - the spin part cancels out. We then consider the effects of introducing a single energy splititng between the 2 orbits. We speculate that g factors for all n can be obtained by a linear approximation.
nucl-thnucl-exNPA(2016)·1 citation
03
Enhancement factor for two-neutron transfer reactions with a schematic coupled-channels model
K. Hagino · G. Scamps
Probabilities for two-neutron transfer reactions, $P_{2n}$ , are often discussed in comparison with the square of the corresponding probabilities for one-neutron transfer process, $(P_{1n})^{2}$ , implicitly assuming that $(P_{1n})^{2}$ provides the probability of two-neutron transfer reactions in the absence of the pairing correlation. We use a schematic coupled-channels model, in which the transfers are treated as effective inelastic channels, and demonstrate that this model leads to $P_{2n} = (P_{1n})^{2} /4$ , rather than $P_{2n} = (P_{1n})^{2}$ , in the pure sequential limit. We argue that a simple model with spin-up and spin-down neutrons in a single-particle orbital also leads to the same conclusion.
nucl-thnucl-exPRC(2015)·8 citations
04
Nuclear annihilation by antinucleons
Teck-Ghee Lee🇺🇸 · Cheuk-Yin Wong🇺🇸
We examine the momentum dependence of $p ˉ$ $p$ and $n ˉ$ $p$ annihilation cross sections by considering the transmission through a nuclear potential and the $p ˉ p$ Coulomb interaction. Compared to the $n ˉ p$ annihilation cross section, the $p ˉ p$ annihilation cross section is significantly enhanced by the Coulomb interaction for projectile momenta below $p_{lab} <$ 500 MeV/ $c$ , and the two annihilation cross sections approach the Pomeranchuk's equality limit [JETP Lett. {\bb 30}, 423 (1956)] at $p_{lab} \sim 500$ MeV/ $c$ . Using these elementary cross sections as the basic input data, the extended Glauber model is employed to evaluate the annihilation cross sections for $n ˉ$ and $p ˉ$ interaction with nuclei and the results compare well with experimental data.
nucl-thPRC(2016)·11 citations
05
Estimating the relevance of predictions from nuclear mean-field models
P.-G. Reinhard
This contribution reviews the present status of the Skyrme-Hartree-Fock (SHF) approach as one of the leading self-consistent mean-field models in the physics of atomic nuclei. It starts with a brief summary of the formalism and strategy for proper calibration of the SHF functional. The main emphasis lies on an exploration of the reliability of predictions, particularly in the regime of extrapolations. Various strategies are discussed to explore the statistical and systematic errors of SHF. The strategies are illustrated on examples from actual applications. Variations of model and fit data are used to get an idea about systematic errors. The statistical error is evaluated in straightforward manner by statistical analysis based on $χ^{2}$ fits. This also allows also to evaluate the correlations (covariances) between observables which provides useful insights into the structure of the model and of the fitting strategy.
nucl-thPhys.Scripta(2016)·18 citations
06
Factorization of event-plane correlations over transverse momentum in relativistic heavy ion collisions in a multi-phase transport model
Kai Xiao · Feng Liu · Fuqiang Wang
Momentum-space azimuthal harmonic event planes (EP) are constructed from final-state midrapidity particles binned in transverse momentum (pT ) in sqrt(s_NN) = 200 GeV Au+Au collisions in a multi-phase transport (AMPT) model. The EP correlations between pT bins, corrected by EP resolutions, are smaller than unity. This indicates that the EP's decorrelate over pT in AMPT, qualitatively consistent with data and hydrodynamic calculations. It is further found that the EP correlations approximately factorize into single pT-bin EP correlations to a common plane. This common plane appears to be the momentum-space EP integrated over all pT, not the configuration space participant plane (PP).
nucl-thnucl-exPRC(2016)·8 citations
07
Production of fragments and hyperfragments in antiproton-nucleus collisions
Zhao-Qing Feng🇨🇳
Formation mechanism of fragments with strangeness in collisions of antiprotons on nuclei has been investigated within the Lanzhou quantum molecular dynamics (LQMD) transport approach combined with a statistical model (GEMINI) for describing the decays of excited fragments. Production of strange particles in the antiproton induced nuclear reactions is modeled within the LQMD model, in which all possible reaction channels such as elastic scattering, annihilation, charge exchange and inelastic scattering in antibaryon-baryon, baryon-baryon and meson-baryon collisions have been included. A coalescence approach is developed for constructing hyperfragments in phase space after de-excitation of nucleonic fragments. The combined approach could describe the production of fragments in low-energy antiproton induced reactions. Hyperfragments are formed within the narrower rapidities and lower kinetic energies. It has advantage to produce heavier hyperfragments and hypernuclides with strangeness s=-2 (double- $Λ$ fragments) and s=1 ( $Λ$ -fragments) in antiproton induced reactions.
nucl-thPRC(2016)·14 citations
08
Chiral symmetry aspects in the open charm sector
Thomas Buchheim🇩🇪 · Thomas Hilger🇦🇹 · Burkhard Kampfer🇩🇪
QCD sum rules serve as tools to investigate changing hadronic properties in a hot and/or dense nuclear medium. The role of chiral symmetry breaking and restoration effects in a medium can be addressed also in the heavy-light meson sector. Thus, we consider Weinberg sum rules which refer to chiral partner mesons composed of a light and a heavy quark.
nucl-thhep-phJ.Phys.Conf.Ser.(2016)·9 citations
09
The $K^{-} p \to f_{1} (1285) Λ$ reaction within an effective Lagrangian approach
Ju-Jun Xie🇨🇳
The production of the $f_{1} (1285)$ resonance in the reaction of $K^{-} p \to f_{1} (1285) Λ$ is studied within an effective Lagrangian approach. The production process is described by the $t$ channel $K^{*+}$ meson exchange. My theoretical approach is based on the results of chiral unitary theory where the $f_{1} (1285)$ resonance is dynamically generated from the single channel $K ˉ K^{*} - c . c .$ interaction. The total and differential cross sections of the $K^{-} p \to f_{1} (1285) Λ$ reaction are evaluated. Within the coupling constant of the $f_{1} (1285)$ to $K ˉ K^{*}$ channel obtained from the chiral unitary theory and a cut off parameter $Λ_{c} \sim 1.5$ GeV in the form factors, the experimental measurement can be reproduced. This production process would provide further evidence for the $K ˉ K^{*} - c . c .$ nature of the $f_{1} (1285)$ state.
nucl-thhep-exhep-phnucl-exPRC(2015)·15 citations
10
Relativistic three-nucleon calculations within the Bethe-Salpeter approach
S.G. Bondarenko · V.V. Burov · S.A. Yurev
The relativistic properties of the three-nucleon system are investigated using the Faddeev equations within the Bethe-Salpeter approach. The nucleon-nucleon interaction is chosen in a separable form. The Gauss quadrature method is used to calculate the integrals. The system of the integral equations are solving by iterations method. The binding energy and the partial-wave amplitudes (1S 0 and 3S 1) of the triton are found.
nucl-thEPJ Web Conf.(2016)·4 citations
11
Importance-Truncated Large-Scale Shell Model
Christina Stumpf · Jonas Braun · Robert Roth
We propose an importance-truncation scheme for the large-scale nuclear shell model that extends its range of applicability to larger valence spaces and mid-shell nuclei. It is based on a perturbative measure for the importance of individual basis states that acts as an additional truncation for the many-body model space in which the eigenvalue problem of the Hamiltonian is solved numerically. Through a posteriori extrapolations of all observables to vanishing importance threshold, the full shell-model results can be recovered. In addition to simple threshold extrapolations, we explore extrapolations based on the energy variance. We apply the importance-truncated shell model for the study of 56-Ni in the pf valence space and of 60-Zn and 64-Ge in the pfg9/2 space. We demonstrate the efficiency and accuracy of the approach, which pave the way for future shell-model calculations in larger valence spaces with valence-space interactions derived in ab initio approaches.
nucl-thPRC(2016)·36 citations
12
Toward a simultaneous description of $R_{AA}$ and $v_{2}$ for heavy quarks
Santosh K. Das🇮🇹 · Francesco Scardina🇮🇹 · Salvatore Plumari🇮🇹 · Vincenzo Greco🇮🇹
The two key observables related to heavy quarks that have been measured in RHIC and LHC energies are the nuclear suppression factor $R_{AA}$ and the elliptic flow $v_{2}$ . Simultaneous description of these two observables is a top challenge for all the existing models. We have highlighted how a consistent combination of four ingredients i.e the temperature dependence of the energy loss, full solution of the Boltzmann collision integral for the momentum evolution of heavy quark, hadronization by coalescence, then the hadronic rescattering, responsible to address a large part of such a puzzle. We have considered four different models to evaluate the temperature dependence of drag coefficients of the heavy quark in QGP. All these four different models are set to reproduce the same $R_{AA}$ as of the experiments. We have shown that for the same $R_{AA}$ , the $v_{2}$ could be quite different depending on the interaction dynamics as well as other ingredients.
nucl-thhep-phJ.Phys.Conf.Ser.(2016)·5 citations
13
Hyperon puzzle and the RMF model with scaled hadron masses and coupling constants
E.E. Kolomeitsev🇸🇰 · K.A. Maslov🇷🇺 · D.N. Voskresensky🇷🇺
The equation of state of cold baryonic matter is studied within a relativistic mean-field model with hadron masses and coupling constants depending on a scalar field. We demonstrate that if the effective nucleon mass stops to decrease with a density increase at densities $n > n_{*} > n_{0}$ , where $n_{0}$ is the nuclear saturation density, the equation of state stiffens for these densities and the limiting neutron star mass increases. The stabilization of the nucleon mass can be realised if in the equation of motion for the scalar mean-field there appear a term sharply varying in a narrow vicinity of the field value corresponding to the density $n_{*}$ . We show several possible realizations of this mechanism getting sufficiently stiff equations of state. The appearance of hyperons in dense neutron star interiors is accounted for. The obtained equations of state remain sufficiently stiff if the reduction of the $ϕ$ meson mass is incorporated. Thereby, the hyperon puzzle can be resolved.
nucl-thastro-ph.HEJ.Phys.Conf.Ser.(2016)·7 citations
14
Higher order fermion effective polynomial interactions
Fabio L. Braghin🇧🇷
Three different fermion effective potentials given by series of bilinears, $\sum_{j}^{N} (ψ ˉ_{a} ψ_{a})^{2^{j}}$ , $\sum_{j}^{N} (ψ ˉ_{a} ψ_{a})^{j}$ and also $\sum_{j}^{N} (ψ ˉ_{a} γ_{μ} ψ_{a})^{2 j}$ where $a = 1, ... N_{r}$ and integer $j$ are investigated by introducing sets of auxiliary fields. A mininal procedure is adopted to deal with the auxiliary fields and an effective bosonized model in each case is found by assuming weak field fluctuations, i.e. weak enough when compared to (normalized) coupling constants. Different fermion condensates are considered for the ground state in the first two series analysed and the factorization of all higher order condensates into the lowest order one is found in most cases, i.e. in general $< (ψ_{a} ψ_{a})^{n} >\propto< ψ ˉ_{a} ψ_{a} >^{n}$ . For the case of the third series built with vector-type bilinears no condensation is assumed to occur. The corresponding (weak) scalar fields effective models for the three cases are expanded in polynomial interactions. The resulting low energy effective boson model may a exhibit new approximate symmetry depending on the terms present in the original series-model and on the values of the coupling constants.
↳ hep-thhep-phnucl-thEur.Phys.J.Plus(2015)·2 citations
15
The curvature of the freeze-out line in heavy ion collisions
A. Bazavov🇺🇸 · H.-T. Ding🇨🇳 · P. Hegde🇨🇳 · O. Kaczmarek🇩🇪 · F. Karsch🇩🇪 · E. Laermann🇩🇪 · Swagato Mukherjee🇺🇸 · H. Ohno🇺🇸 · P. Petreczky🇺🇸 · C.Schmidt🇩🇪 · S. Sharma🇺🇸 · W. Soeldner🇩🇪 · M. Wagner🇺🇸
We calculate the mean and variance of net-baryon number and net-electric charge distributions from Quantum Chromodynamics (QCD) using a next-to-leading order Taylor expansion in terms of temperature and chemical potentials. We compare these expansions with experimental data from STAR and PHENIX, determine the freeze-out temperature in the limit of vanishing baryon chemical potential, and, for the first time, constrain the curvature of the freeze-out line through a direct comparison between experimental data on net-charge fluctuations and a QCD calculation. We obtain a bound on the curvature coefficient, kappa_2^f < 0.011, that is compatible with lattice QCD results on the curvature of the QCD transition line.
↳ hep-lathep-phnucl-thPRD(2016)·45 citations
16
Dileptons in a coarse-grained transport approach
H. van Hees🇩🇪 · S. Endres🇩🇪 · J. Weil🇩🇪 · M. Bleicher🇩🇪
We calculate dilepton spectra in heavy-ion collisions using a coarse-graining approach to the simulation of the created medium with the UrQMD transport model. This enables the use of dilepton-production rates evaluated in equilibrium quantum-field theory at finite temperatures and chemical potentials.
↳ hep-phnucl-thJ.Phys.Conf.Ser.(2016)·0 citations
17
Electromagnetic triangle anomaly and neutral pion condensation in QCD vacuum
Gaoqing Cao🇨🇳 · Xu-Guang Huang🇨🇳
We study the QCD vacuum structure under the influence of an electromagnetic field with a nonzero second Lorentz invariant $I_{2} = E \cdot B$ . We show that the presence of $I_{2}$ can induce neutral pion ( $π^{0}$ ) condensation in the QCD vacuum through the electromagnetic triangle anomaly. Within the frameworks of chiral perturbation theory at leading small-momenta expansion as well as the Nambu--Jona-Lasinio model at leading $1/ N_{c}$ expansion, we quantify the dependence of the $π^{0}$ condensate on $I_{2}$ . The stability of the $π^{0}$ -condensed vacuum against the Schwinger charged pair production due to electric field is also discussed.
↳ hep-phhep-latnucl-thPLB(2016)·46 citations
18
The pion quasiparticle in the low-temperature phase of QCD
Bastian B. Brandt🇩🇪 · Anthony Francis🇨🇦 · Harvey B. Meyer🇩🇪 · Daniel Robaina🇩🇪
We investigate the properties of the pion quasiparticle in the low-temperature phase of two-flavor QCD on the lattice with support from chiral effective theory. We find that the pion quasiparticle mass is significantly reduced compared to its value in the vacuum, in contrast to the static screening mass, which increases with temperature. By a simple argument, the two masses are expected to determine the quasiparticle dispersion relation near the chiral limit. Analyzing two-point functions of the axial charge density at non-vanishing spatial momentum, we find that the predicted dispersion relation and the residue of the pion pole are simultaneously consistent with the lattice data at low momentum. The test, based on fits to the correlation functions, is confirmed by a second analysis using the Backus-Gilbert method.
↳ hep-lathep-phnucl-thPoS(2016)·1 citation
19
A Recursive Method for Computing Certain Bessel Function Integrals
Jeremiah Birrell
We investigate a family of integrals involving modified Bessel functions that arise in the context of neutrino scattering. Recursive formulas are derived for evaluating these integrals and their asymptotic expansions are computed. We prove in certain cases that the asymptotic expansion yields the exact result after a finite number of terms. In each of these cases we derive a formula that bounds the order at which the expansion terminates. The method of calculation developed in this paper is applicable to similar families of integrals that involve Bessel or modified Bessel functions.
↳ math.CAnucl-th0 citations
20
Neutrons on a surface of liquid helium
P. D. Grigoriev · O. Zimmer · T. Ziman · A. D. Grigoriev
We investigate the possibility of ultracold neutron (UCN) storage in quantum states defined by the combined potentials of the Earth's gravity and the neutron optical repulsion by a horizontal surface of liquid helium. We analyse the stability of the lowest quantum state, which is most susceptible to perturbations due to surface excitations, against scattering by helium atoms in the vapor and by excitations of the liquid, comprised of ripplons, phonons and surfons. This is an unusual scattering problem since the kinetic energy of the neutron parallel to the surface may be much greater than the binding energies perpendicular. The total scattering time constant of these UCNs at 0.7 K is found to exceed one hour, and rapidly increasing with decreasing temperature. Such low scattering rates should enable high-precision measurements of the scheme of discrete energy levels, thus providing improved access to short-range gravity. The system might also be useful for neutron beta-decay experiments. We also sketch new experimental concepts for level population and trapping of UCNs above a flat horizontal mirror.
↳ nucl-excond-mat.othernucl-thPRC(2016)·6 citations

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