Resonances in a sudden chemical freeze-out model

Abstract

Abstract. The prediction for p T spectra of various resonances produced in Pb+Pb collisions at 2.76 TeV at the LHC in equilibrium and non-equilibrium models is made. It includes the η, ρ(770), Σ(1385), Λ(1520), and Ξ(1530). The apparent differences may allow to distinguish between the models.A chemical non-equilibrium model [1] with a single freeze-out [2] appeared to be rather successful in describing the LHC ALICE data at 2.76 TeV for various particles [3,4]. In this model the mean multiplicities are described with the use of four thermodynamic parameters: temperature T , volume V, and two non-equilibrium parameters -γ s and γ q . It fixes the area under the curve for the p T spectra. The form of the spectra is best reproduced by the Hubble-like single freeze-out hyper-surface. Then, the slopes of the spectra are described with only one extra parameter -the ratio of the freeze-out time τ f to the freeze-out radius r f , because their combination, the cylinder of volume π * r 2 * τ, is equal to the volume V, which is determined from multiplicities on the previous step.It appears that the p T spectra of pions, kaons, protons, K * (892) 0 , and φ(1020) are described by the same parameters in the single freeze-out model [3,4]. This is very surprising for the K * (892) 0 and the φ(1020), because the first one is short living, while the second one is long living. The description of both of them may question the necessity of the long re-scattering phase, which is also successfully used to describe the ALICE data [5]. It may also indicate that the non-equilibrium, as implemented in [3,4], may effectively include the re-scattering, because γ q and γ s are equivalent to non-equilibrium chemical potentials for each particle, see [3,4]. It is important to differentiate between the equilibrium with the re-scattering, and the single sudden freeze-out in the non-equilibrium, because the nonequilibrium also leads to pion condensation [6,7].A good test for the non-equilibrium single freeze-out scenario [3,4] is the comparison to different resonances, especially strange resonances, because this scenario requires a special relation between the strange and the non-strange chemical potentials, depending on the quark content of a resonance. The heavy Λ, Ξ and Ω can be still described by the non-equilibrium very well, if one assumes a smaller slope for them [4]. This introduces the dependence on the mass of the resonance, but is also supported by smaller flow of heavy particles in other approaches, see e.g. [8]. The parameters obtained in the fit to the 2.76 TeV Pb+Pb LHC data in equilibrium (EQ), non-equilibrium (NEQ) [3,4], and nonequilibrium with the possibility of pion Bose-Einstein condensation (BEC) on the ground state [6,7] in hadron-resonance gas, using correspondingly modified SHARE [9] and THERMINATOR [2] codes, are shown in Fig. 1. One can see that the system is closer to the scenario with the condensate in central collisions. However, the uncertainty is rather large, which means that more mean multiplicities are needed t...

Neutral pion and η meson production at midrapidity in Pb-Pb collisions at sNN=2.76 TeV

“…The difference observed between the NEQ SHM and the data may point towards a different flow profile of the two mesons with a larger flow for the η than the π 0 [74]. Significant differences between the EQ and NEQ predictions are also observed for the ρ 0 , (1385), (1520), and (1530) [73,75].…”

“…The parameters of the model are determined by fits to the measured charged pion and kaon spectra [20]. While only these two particles are considered in the fits, the resulting parameters are used to make predictions for other particles [73], e.g., the η meson, the ρ meson, and the proton. The EPOS generator addresses both low-and high-p T phenomena, where the particle spectra include effects (low p T ) associated to hydrodynamic flow as discussed in Ref.…”