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 hadron input, full chemical equilibrium, and the hadron-type dependent eigenvolume interactions, is employed to fit the hadron midrapidity yield data of the 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\,\mathrm{MeV}

is reproduced. However, the situation changes if the hadrons have different eigenvolumes. In the case when all the mesons are point-like while all the baryons have an effective hardcore radius of 0.3 fm, the temperature dependence of

{\chi }^{2}

has a broad minimum in the temperature range of 155−210 MeV, with a fit quality comparable to the

T\sim 155\,\mathrm{MeV}

minimum in the point-particle case. A very similar result is obtained when only baryon–baryon eigenvolume interactions are considered, with the eigenvolume parameter taken from the previous fit to the ground state of nuclear matter. Finally, when we apply the eigenvolume corrections with a mass-proportional eigenvolume

{v}_{i}\sim {m}_{i}

, fixed to a particular hardcore proton radius r ( )p( ), we observe a second minimum in the temperature dependence of

{\chi }^{2}

, located at 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 creates 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 the chemical freeze-out temperature can only be extracted from the LHC hadron yield data with sizable uncertainty.

Note:

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 results

25.75.Ag
24.10.Pa
ALICE
heavy ion: scattering
hadron: matter
hadron: yield
hadron: thermodynamics
hadron: resonance: gas
freeze-out: chemical
freeze-out: temperature

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Figures(3)

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