Thermal hadron production in relativistic nuclear collisions: The hadron mass spectrum, the horn, and the QCD phase transition

Abstract

We present, using the statistical model, a new analysis of hadron production in central collisions of heavy nuclei. This study is motivated by the availability of final measurements both for the SPS (beam energies 20-160 AGeV) and for the RHIC energies ( √ s N N =130 and 200 GeV) and by updates in the hadron mass spectrum, which is a crucial input for statistical models. Extending previous studies by inclusion of very high-mass resonances (m> 2 GeV), and the up-to-now neglected scalar σ meson leads to an improved description of the data. In particular, the hitherto poorly reproduced energy dependence of the K + /π + ratio at SPS energies ("the horn") is now well described through the connection to the hadronic mass spectrum and, implicitly, Hagedorn's limiting temperature. We thereby demonstrate the intimate connection between the horn and the QCD phase transition.

Hadron yields and the phase diagram of strongly interacting matter

“…The largest contribution to the χ 2 /NDF comes from the low yield of protons relative to pions. This conclusion persists for the three different thermal model implementations which were used by the ALICE collaboration [4,16,30], indicating that the residual differences in those models (minor difference in the hadron list, treatment of charm and of the hadron excluded volume) have a second order effect. The temperature obtained, T ch = 156 ± 2 MeV, is lower than at RHIC.…”

“…Equilibrium thermal models were found to give an excellent description of particle yields measured in Au-Au collisions at top RHIC energy ( √ s NN = 200 GeV, χ 2 /NDF 1 when analyzing data from the same experiment, χ 2 /NDF ∼ 2 when fitting all RHIC results simultaneously [30]). The analysis of data as a function of √ s NN , moreover, allowed to extrapolate the parameters to the limiting case of very high energy, which is essentially reached at the LHC [30,31]. Therefore, the observation of an anomalously low p/π = 0.046 ± 0.003 ratio in central collisions at the LHC (a factor ∼ 1.5 lower then expectations based on T ch = 164 MeV) [32,33] came as a surprise and triggered considerable theoretical discussions.…”

Pion, Kaon, and Proton Production in Central Pb-Pb Collisions atsNN=2.76 TeV

“…The observed particle abundances were described in terms of thermal models. Relative particle abundances in thermal and chemical equilibrium are governed mainly by two parameters, the chemical freeze-out temperature, T ch and the baryochemical potential B , where the latter describes the net baryon content of the system [6][7][8][9]. Measured particle yields in heavy-ion collisions at RHIC, as well as SPS and AGS, are consistent with equilibrium populations, allowing the extraction of both model parameters from fits to the measured particle ratios [6,7,9,10].…”

“…Relative particle abundances in thermal and chemical equilibrium are governed mainly by two parameters, the chemical freeze-out temperature, T ch and the baryochemical potential B , where the latter describes the net baryon content of the system [6][7][8][9]. Measured particle yields in heavy-ion collisions at RHIC, as well as SPS and AGS, are consistent with equilibrium populations, allowing the extraction of both model parameters from fits to the measured particle ratios [6,7,9,10]. It has been argued that interactions modifying the relative abundances of particle species are negligible in the hadronic phase [11,12], and that T ch can be linked to the phase transition temperature [13].…”

Strangeness production as a function of charged particle multiplicity in proton–proton collisions

“…We observe a rising trend which is identical to the one observed in p-Pb collisions. In the case of Ξ ± produced in high multiplicity events, the grand-canonical saturation limit calculated with two different implementations of the statistical model (THERMUSV3.0 [14] and GSI-Heidelberg model [15]) is reached, while this is not the case for Ω ± . The results are compared to MC calculations performed using three different versions of the PYTHIA generator.…”