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

TL;DR

This study shows that uncertainties in modeling hadron eigenvolumes lead to large variations in the extracted chemical freeze-out temperature from LHC heavy-ion collision data, questioning the precision of such thermal fits.

Contribution

It demonstrates that different assumptions about hadron eigenvolumes cause broad temperature ranges in thermal fits, highlighting the sensitivity of freeze-out temperature extraction to eigenvolume modeling.

Findings

Eigenvolume effects cause broad minima in temperature fits.

Uncertainty in temperature extraction can be as large as 150 MeV.

Modeling short-range repulsion significantly impacts thermal fit results.

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 $T = 154 \pm 2$ 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.