Examination of the sensitivity of the thermal fits to heavy-ion hadron yield data to the modeling of the eigenvolume interactions

The hadron-resonance gas (HRG) model with the mass-proportional eigenvolume (EV) corrections is employed to fit the hadron yield data of the NA49 collaboration for central Pb+Pb collisions at $\sqrt{s_{_{\rm NN}}} = 6.3, 7.6, 8.8, 12.3,$ and $17.3$ GeV, the hadron midrapidity yield data of the STAR collaboration for Au+Au collisions at $\sqrt{s_{\rm NN}} = 200$ GeV, and the hadron midrapidity yield data of the ALICE collaboration for Pb+Pb collisions at $\sqrt{s_{\rm NN}} = 2760$ GeV. At given bombarding energy, for a given set of radii, the EV HRG model fits do not just yield a single $T-\mu_B$ pair, but a whole range of $T-\mu_B$ pairs, each with similarly good fit quality. These pairs form a valley in the $T-\mu_B$ plane along a line of nearly constant entropy per baryon, $S/A$, which increases nearly linearly with bombarding energy $E_{\rm lab}$. The entropy per baryon values extracted from the data at the different energies are a robust observable: it is almost independent of the details of the modeling of the eigenvolume interactions and of the specific $T-\mu_B$ values obtained. These results show that the extraction of the chemical freeze-out temperature and chemical potential is extremely sensitive to the modeling of the short-range repulsion between the hadrons. This implies that the ideal point-particle HRG values are not unique. The wide range of the extracted $T$ and $\mu_B$ values suggested by the eigenvolume HRG fits, as well as the approximately constant $S/A$ at freeze-out, are consistent with a non-equilibrium scenario of continuous freeze-out, where hadrons can be chemically frozen-out throughout the extended space-time regions during the evolution of the system. Even when the EV HRG fits are restricted to modest temperatures suggested by lattice QCD, the strong systematic effects of EV interactions are observed.Comment: 13 pages, 6 figures, new section III.E with fits constrained to low temperatures, to be published in Phys. Rev.

Surprisingly large uncertainties in temperature extraction from thermal fits to hadron yield data at LHC

The conventional hadron-resonance gas (HRG) model with the Particle Data Group (PDG) hadron input, full chemical equilibrium, and the hadron type dependent eigenvolume interactions is employed to fit the hadron mid-rapidity yield data of ALICE Collaboration for the most central Pb+Pb collisions. For the case of point-like hadrons the well-known fit result $T = 154 \pm 2$ MeV is reproduced. However, the situation changes if hadrons have different eigenvolumes. In the case when all mesons are point-like while all baryons have an effective hard-core radius of 0.3 fm the $\chi^2$ temperature dependence of the $\chi^2$ has a broad minimum in the temperature range of $155-210$ MeV, with fit quality comparable to the $T \sim 155$ MeV minimum in the point-particle case. Very similar result is obtained when only baryon-baryon eigenvolume interactions are considered, with eigenvolume parameter taken from previous fit to ground state of nuclear matter. Finally, when we apply the eigenvolume corrections with mass-proportional eigenvolume $v_i \sim m_i$, fixed to particular proton hard-core radius $r_p$, we observe a second minimum in the temperature dependence of the $\chi^2$, located at the significantly higher temperatures. For instance, at $r_p = 0.5$ fm the fit quality is better than in the point-particle HRG case in a very wide temperature range of $170-320$ MeV, which gives an uncertainty in the temperature determination from the fit to the data of 150 MeV. These results show that thermal fits to the heavy-ion hadron yield data are very sensitive to the modeling of the short-range repulsion eigenvolume between hadrons, and that chemical freeze-out temperature can be extracted from the LHC hadron yield data only with sizable uncertainty.Comment: 8 pages, 3 figures, v3: added calculations for baryon-baryon only eigenvolume interactions fitted to nuclear ground state, added table with fitted data, title and discussion modified in order to ensure more clarity about the presented result

Transport model study of the mTm_T-scaling for Λ\Lambda, K, and π\pi HBT-correlations

Based on the microscopic transport model UrQMD in which hadronic and string degrees of freedom are employed, the HBT parameters in the longitudinal co-moving system are investigated for charged pion and kaon, and $\Lambda$ sources in heavy ion collisions (HICs) at SPS and RHIC energies. In the Cascade mode, $R_O$ and the $R_L$ at high SPS and RHIC energies do not follow the $m_T$-scaling, however, after considering a soft equation of state with momentum dependence (SM-EoS) for formed baryons and a density-dependent Skyrme-like potential for ``pre-formed'' particles, the HBT radii of pions and kaons and even those of $\Lambda$s with large transverse momenta follow the $m_T$-scaling function $R=3/\sqrt{m_T}$ fairly well.Comment: 6 pages, 5 fig

Hadron production in relativistic nuclear collisions: thermal hadron source or hadronizing quark-gluon plasma?

Measured hadron yields from relativistic nuclear collisions can be equally well understood in two physically distinct models, namely a static thermal hadronic source vs.~a time-dependent, nonequilibrium hadronization off a quark-gluon plasma droplet. Due to the time-dependent particle evaporation off the hadronic surface in the latter approach the hadron ratios change (by factors of $<\approx 5$) in time. Final particle yields reflect time averages over the actual thermodynamic properties of the system at a certain stage of the evolution. Calculated hadron, strangelet and (anti-)cluster yields as well as freeze-out times are presented for different systems. Due to strangeness distillation the system moves rapidly out of the T, $\mu_q$ plane into the $\mu_s$-sector. Strangeness to baryon ratios f_s=1-2 prevail during a considerable fraction (50%) of the time evolution (i.e. $\Lambda$-droplets or even $\Xi^-$-droplets form the system at the late stage: The possibility of observing this time evolution via HBT correlations is discussed). The observed hadron ratios require $T_c\approx 160 MeV$ and $B^{1/4}>\approx 200 MeV$. If the present model is fit to the extrapolated hadron yields, metastable hypermatter can only be produced with a probability $p< 10^{-8}$ for $A \ge 4$.Comment: Submitted to Z. Phys.

Parity Doublet Model applied to Neutron Stars

The Parity doublet model containing the SU(2) multiplets including the baryons identified as the chiral partners of the nucleons is applied for neutron star matter. The chiral restoration is analyzed and the maximum mass of the star is calculated.Comment: Proceeding to the conference International Symposium on Exotic States of Nuclear Matte

Perturbative QCD Calculations of Elliptic Flow and Shear Viscosity in Au+Au Collisions at sNN=200\sqrt{s_{NN}}=200 GeV

The elliptic flow $v_2$ and the ratio of the shear viscosity over the entropy density, $\eta/s$, of gluon matter are calculated from the perturbative QCD (pQCD) based parton cascade Boltzmann approach of multiparton scatterings. For Au+Au collisions at $\sqrt{s}=200$A GeV the gluon plasma generates large $v_2$ values measured at the BNL Relativistic Heavy Ion Collider. Standard pQCD yields $\eta/s\approx 0.08-0.15$ as small as the lower bound found from the anti-de Sitter/conformal field theory conjecture.Comment: 4 pages, 6 figures, new results added in Figs 1, 2, and 3, version published in PR

Probing the symmetry energy and the degree of isospin equilibrium

The rapidity dependence of the single- and double- neutron to proton ratios of nucleon emission from isospin-asymmetric but mass-symmetric reactions Zr+Ru and Ru+Zr at energy range $100 \sim 800$ A MeV and impact parameter range $0\sim 8$ fm is investigated. The reaction system with isospin-asymmetry and mass-symmetry has the advantage of simultaneously showing up the dependence on the symmetry energy and the degree of the isospin equilibrium. We find that the beam energy- and the impact parameter dependence of the slope parameter of the double neutron to proton ratio ($F_D$) as function of rapidity are quite sensitive to the density dependence of symmetry energy, especially at energies $E_b\sim 400$ A MeV and reduced impact parameters around 0.5. Here the symmetry energy effect on the $F_D$ is enhanced, as compared to the single neutron to proton ratio. The degree of the equilibrium with respect to isospin (isospin mixing) in terms of the $F_D$ is addressed and its dependence on the symmetry energy is also discussed.Comment: 10 pages, 2 figure