Nuclear Theory·nucl-th

Wednesday·July 19, 2017

10 papers—5 primary·5 cross-listed

01
Proton emission with a screened electrostatic barrier
R. Budaca · A. I. Budaca
Half-lives of proton emission for Z $\geq$ 51 nuclei are calculated within a simple analytical model based on the WKB approximation for the barrier penetration probability which includes the centrifugal and overlapping effects besides the electrostatic repulsion. The model has a single free parameter associated to a Hulthen potential which emulates a Coulomb electrostatic interaction only at short distance. The agreement with experimental data is very good for most of the considered nuclei. Theoretical predictions are made for few cases with uncertain emitting state configuration or incomplete decay information. The model's assignment of the proton orbital momentum is in agreement with the differentiation of the experimental data by orbital momentum values realized with a newly introduced correlation formula.
nucl-thEPJA(2017)·22 citations
02
Estimating the Charm Quark Diffusion Coefficient and thermalization time from D meson spectra at RHIC and LHC
Francesco Scardina🇮🇹 · Santosh K. Das🇮🇹 · Vincenzo Minissale🇮🇹 · Salvatore Plumari🇮🇹 · Vincenzo Greco🇮🇹
We describe the propagation of charm quarks in the quark-gluon plasma (QGP) by means of a Boltzmann transport approach. Non-perturbative interaction between heavy quarks and light quarks have been taken into account through a quasi-particle approach in which light partons are dressed with thermal masses tuned to lQCD thermodynamics. Such a model is able to describe the main feature of the non-perturbative dynamics: the enhancement of the interaction strength near $T_{c}$ . We show that the resulting charm in-medium evolution is able to correctly predict simultaneously the nuclear suppression factor, $R_{AA}$ , and the elliptic flow, $v_{2}$ , at both RHIC and LHC energies and at different centralities. The hadronization of charm quarks is described by mean of an hybrid model of fragmentation plus coalescence and plays a key role toward the agreeement with experimental data. We also performed calculations within the Langevin approach which can lead to very similar $R_{AA} (p_{T})$ as Boltzmann, but the charm drag coefficient as to be reduced by about a $30%$ and also generates an elliptic flow $v_{2} (p_{T})$ is about a $15%$ smaller. We finally compare the space diffusion coefficient $2 π T D_{s}$ extracted by our phenomenological approach to lattice QCD results, finding a satisfying agreement within the present systematic uncertainties. Our analysis implies a charm thermalization time, in the $p \to 0$ limit, of about $4 - 6 f m / c$ which is smaller than the QGP lifetime at LHC energy.
nucl-thhep-phnucl-exPRC(2017)·199 citations
03
Pion transverse-momentum spectrum and elliptic anisotropy of partially coherent source
Peng Ru🇨🇳 · Ghulam Bary🇨🇳 · Wei-Ning Zhang🇨🇳
In this letter, we study the pion momentum distribution of a coherent source and investigate the influences of coherent emission on the pion transverse-momentum ( $p_{T}$ ) spectrum and elliptic anisotropy. With a partially coherent source, constructed by a conventional viscous hydrodynamics model (chaotic part) and a parameterized expanding coherent source model, we reproduce the pion $p_{T}$ spectrum and elliptic anisotropy coefficient $v_{2} (p_{T})$ in the peripheral Pb-Pb collisions at $s_{N N} = 2.76$ TeV. It is found that the influences of coherent emission on the pion $p_{T}$ spectrum and $v_{2} (p_{T})$ are related to the initial size and shape of the coherent source, largely due to the interference effect. However, the effect of source dynamical evolution on coherent emission is relatively small. The results of the partially coherent source with 33% coherent emission and 67% chaotic emission are consistent with the experimental measurements of the pion $p_{T}$ spectrum, $v_{2} (p_{T})$ , and especially four-pion Bose-Einstein correlations.
nucl-thhep-phnucl-exPLB(2018)·6 citations
04
Probing short-range correlations in asymmetric nuclei with quasi-free pair knockout reactions
Sam Stevens · Jan Ryckebusch · Wim Cosyn · Andreas Waets
Short-range correlations (SRC) in asymmetric nuclei with an unusual neutron-to-proton ratio can be studied with quasi-free two-nucleon knockout processes following the collision between accelerated ions and a proton target. We derive an approximate factorized cross section for those SRC-driven $p (A, p^{'} N_{1} N_{2})$ reactions. Our reaction model hinges on the factorization properties of SRC-driven $A (e, e^{'} N_{1} N_{2})$ reactions for which strong indications are found in theory-experiment comparisons. In order to put our model to the test we compare its predictions with results of $^{12} C (p, p^{'} p n)$ measurements conducted at Brookhaven National Laboratory (BNL) and find a fair agreement. The model can also reproduce characteristic features of SRC-driven two-nucleon knockout reactions, like back-to-back emission of the correlated nucleons. We study the asymmetry dependence of nuclear SRC by providing predictions for the ratio of proton-proton to proton-neutron knockout cross sections for the carbon isotopes $^{9 - 15}$ C thereby covering neutron excess values $(N - Z) / Z$ between -0.5 and +0.5.
nucl-thnucl-exPLB(2018)·13 citations
05
Multinucleon transfer dynamics in heavy-ion collisions near Coulomb barrier energies
Fei Niu · Peng-Hui Chen · Ya-Fei Guo · Chun-Wang Ma · Zhao-Qing Feng
The multinucleon transfer reactions near barrier energies has been investigated with a multistep model based on the dinuclear system (DNS) concept, in which the capture of two colliding nuclei, the transfer dynamics and the de-excitation process of primary fragments are described by the analytical formula, the diffusion theory and the statistical model, respectively. The nucleon transfer takes place after forming the DNS and is coupled to the dissipation of relative motion energy and angular momentum by solving a set of microscopically derived master equations within the potential energy surface. Specific reactions of $^{40, 48}$ Ca+ $^{124}$ Sn, $^{40}$ Ca ( $^{40}$ Ar, $^{58}$ Ni)+ $^{232}$ Th, $^{40}$ Ca ( $^{58}$ Ni)+ $^{238}$ U and $^{40, 48}$ Ca ( $^{58}$ Ni) + $^{248}$ Cm near barrier energies are investigated. It is found that the fragments are produced by the multinucleon transfer reactions with the maximal yields along the $β$ -stability line. The isospin relaxation is particularly significant in the process of fragment formation. The incident energy dependence of heavy target-like fragments in the reaction of $^{58}$ Ni+ $^{248}$ Cm is analyzed thoroughly.
nucl-thPRC(2017)·13 citations
06
Thermal excitation spectrum from entanglement in an expanding quantum string
Jürgen Berges🇩🇪 · Stefan Floerchinger🇩🇪 · Raju Venugopalan🇺🇸
A surprising result in $e^{+} e^{-}$ collisions is that the particle spectra from the string formed between the expanding quark-antiquark pair have thermal properties even though scatterings appear not to be frequent enough to explain this. We address this problem by considering the finite observable interval of a relativistic quantum string in terms of its reduced density operator by tracing over the complement region. We show how quantum entanglement in the presence of a horizon in spacetime for the causal transfer of information leads locally to a reduced mixed-state density operator. For very early proper time $τ$ , we show that the entanglement entropy becomes extensive and scales with the rapidity. At these early times, the reduced density operator is of thermal form, with an entanglement temperature $T_{τ} = ℏ/ (2 π k_{B} τ)$ , even in the absence of any scatterings.
↳ hep-phhep-thnucl-thPLB(2018)·74 citations
07
Tachyonic instability of the scalar mode prior to QCD critical point based on Functional renormalization-group method
Takeru Yokota🇯🇵 · Teiji Kunihiro🇯🇵 · Kenji Morita🇯🇵
We establish and elucidate the physical meaning of the appearance of an acausal mode in the sigma mesonic channel, found in the previous work by the present authors, when the system approaches the $Z_{2}$ critical point. The functional renormalization group method is applied to the two--flavor quark--meson model with varying current quark mass $m_{q}$ even away from the physical value at which the pion mass is reproduced. We first determine the whole phase structure in the three-dimensional space $(T, μ, m_{q})$ consisting of temperature $T$ , quark chemical potential $μ$ and $m_{q}$ , with the tricritical point, $O (4)$ and $Z_{2}$ critical lines being located; they altogether make a wing-like shape quite reminiscent of those known in the condensed matters with a tricritical point. We then calculate the spectral functions $ρ_{σ, π} (ω, p)$ in the scalar and pseudoscalar channel around the critical points. We find that the sigma mesonic mode becomes tachyonic with a superluminal velocity at finite momenta before the system reaches the $Z_{2}$ point from the lower density, even for $m_{q}$ smaller than the physical value. One of the possible implications of the appearance of such a tachyonic mode at finite momenta is that the assumed equilibrium state with a uniform chiral condensate is unstable toward a state with an inhomogeneous $σ$ condensate. No such an anomalous behavior is found in the pseudoscalar channel. We find that the $σ$ -to- $2 σ$ coupling due to finite $m_{q}$ play an essential role for the drastic modification of the spectral function.
↳ hep-phnucl-thPRD(2017)·26 citations
08
Symmetric cumulants as a probe of the proton substructure at LHC energies
Javier L. Albacete🇪🇸 · Hannah Petersen🇩🇪 · Alba Soto-Ontoso🇪🇸
We present a systematic study of the normalized symmetric cumulants, NSC(n,m), at the eccentricity level in proton-proton interactions at $s = 13$ TeV within a wounded hot spot approach. We focus our attention on the influence of spatial correlations between the proton constituents, in our case gluonic hot spots, on this observable. We notice that the presence of short-range repulsive correlations between the hot spots systematically decreases the values of NSC(2,3) and NSC(2,4) in mid-to-ultra central collisions while increases them in peripheral interactions. In the case of NSC(2,3) we find that, as suggested by data, an anti-correlation of $ε_{2}$ and $ε_{3}$ in ultra-central collisions, i.e. NSC(2,3) $< 0$ , is possible within the correlated scenario while it never occurs without correlations. We attribute this fact to the decisive role of correlations on enlarging the probability of interaction topologies that reduce the value of NSC(2,3) and, eventually, make it negative. Further, we explore the dependence of our conclusions on the values of the hot spot radius and the repulsive core distance. Our results add evidence to the idea that considering spatial correlations between the subnucleonic degrees of freedom of the proton may have a strong impact on the initial state properties of proton-proton interactions [1].
↳ hep-phhep-exnucl-thPLB(2018)·35 citations
09
Meson masses in electromagnetic fields with Wilson fermions
Gunnar S. Bali🇩🇪 · Bastian B. Brandt🇩🇪 · Gergely Endrodi🇩🇪 · Benjamin Glaessle🇩🇪
We determine the light meson spectrum in QCD in the presence of background magnetic fields using quenched Wilson fermions. Our continuum extrapolated results indicate a monotonous reduction of the connected neutral pion mass as the magnetic field grows. The vector meson mass is found to remain nonzero, a finding relevant for the conjectured $ρ$ -meson condensation at strong magnetic fields. The continuum extrapolation was facilitated by adding a novel magnetic field-dependent improvement term to the additive quark mass renormalization. Without this term, sizable lattice artifacts that would deceptively indicate an unphysical rise of the connected neutral pion mass for strong magnetic fields are present. We also investigate the impact of these lattice artifacts on further observables like magnetic polarizabilities and discuss the magnetic field-induced mixing between $ρ$ -mesons and pions. We also derive Ward-Takashi identities for QCD+QED both in the continuum formulation and for (order $a$ -improved) Wilson fermions.
↳ hep-lathep-phnucl-thPRD(2018)·149 citations
10
Saturation properties of helium drops from a Leading Order description
A. Kievsky · A. Polls · B. Juliá-Díaz · N. K. Timofeyuk
Saturation properties are directly linked to the short-range scale of the two-body interaction of the particles. The case of helium is particular, from one hand the two-body potential has a strong repulsion at short distances. On the other hand, the extremely weak binding of the helium dimer locates this system very close to the unitary limit allowing for a description based on an effective theory. At leading order of this theory a two- and a three-body term appear, each one characterized by a low energy constant. In a potential model this description corresponds to a soft potential model with a two-body term purely attractive plus a three-body term purely repulsive constructed to describe the dimer and trimer binding energies. Here we analyse the capability of this model to describe the saturation properties making a direct link between the low energy scale and the short-range correlations. We will show that the energy per particle, $E_{N} / N$ , can be obtained with reasonable accuracy at leading order extending the validity of this approximation, characterizing universal behavior in few-boson systems close to the unitary limit, to the many-body system.
↳ cond-mat.quant-gasnucl-thPRA(2017)·20 citations

Affiliations

first authorsco-authorsvia INSPIRE