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

Abstract

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 ± 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 χ 2 temperature dependence of the χ 2 has a broad minimum in the temperature range of 155 − 210 MeV, with fit quality comparable to the T ∼ 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 vi ∼ mi, fixed to particular proton hard-core radius rp, we observe a second minimum in the temperature dependence of the χ 2 , located at the significantly higher temperatures. For instance, at rp = 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.

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

“…1 for the ALICE energy are consistent with the recent Ref. [16]. Small quantitative differences are attributed to the use of the Boltzmann approximation in the present work.…”

“…For this parametrization, however, the eigenvolume corrections essentially cancel out in the ratios of yields and, thus, have a negligible effect on the values of the extracted chemical freeze-out temperatures and chemical potentials. If, however, one considers hadrons with different hard-core radii, then the ratios may change, and the fit quality can be improved [16,24,33].…”

“…The HRG model with repulsive interactions have been compared with the lattice QCD data [13][14][15], and it has recently been shown in Ref. [16] that the inclusion of the repulsive interactions into the HRG model in the form of a multi-component eigenvolume procedure can significantly change the values of chemical freeze-out temperature at µ B = 0 while improving the agreement of the fit with the ALICE hadron yield data as compared to the point-particle HRG. The analysis reveals a surprisingly strong sensitivity of the thermal fits at LHC to the modeling of eigenvolume interactions, leading to a large systematic uncertainty in the determination of chemical freeze-out temperature.…”

Excluded-volume effects for a hadron gas in Yang-Mills theory

“…Remarkably, in Ref. [26] it has been demonstrated that the chemical freeze-out temperature obtained in fits of experimental hadron yields is strongly sensitive to the hadronvolume parameters, i.e., to the details of the short-range repulsion between hadrons. The issue has been studied further in Ref.…”

“…We emphasize that the primary goal of this paper is not to propose a new way to describe Yang-Mills lattice data, but to test the existing excluded-volume description for hadronic interactions in QCD [23][24][25][26][27][28], in theories with a different particle content. As we discuss in detail below, while purely gluonic SU (2) and SU(3) theories have many qualitative similarities with QCD, in some respects they are also remarkably different from it.…”