Hadron multiplicities and chemical freeze-out conditions in proton-proton and nucleus-nucleus collisions

New results of the NA61/SHINE Collaboration at the CERN SPS on mean hadron multiplicities in proton-proton (p+p) interactions are analyzed within the transport models and the hadron resonance gas (HRG) statistical model. The chemical freeze-out parameters in p+p interactions and central Pb+Pb (or Au+Au) collisions are found and compared with each other in the range of the center-of-mass energy of the nucleon pair $\sqrt{s_{NN}}=3.2–17.3$ GeV. The canonical ensemble formulation of the HRG model is used to describe mean hadron multiplicities in p+p interactions and the grand canonical ensemble in central Pb+Pb and Au+Au collisions. The chemical freeze-out temperatures in p+p interactions are found to be larger than the corresponding temperatures in central nucleus-nucleus collisions.

Figure 1

Mean hadron multiplicities in inelastic p+p reactions as functions of the center-of-mass collision energy $\sqrt{s_{NN}}$. The full circles are the data [14] of NA61/SHINE Collaboration. The solid and dashed lines correspond to the results of the HSD 2.5 and UrQMD 3.4 model simulations, respectively. The crosses show the fit in the statistical hadron-resonance gas model in the canonical ensemble (see Sec. 3).

Figure 2

(a) Temperature $T$ as a function of baryon chemical potential in central Pb+Pb and Au+Au collisions. Temperature $T$ (b), strangeness saturation factor ${\gamma }_S$ (c), radius of the system $R$ (d) in p+p inelastic reactions and central heavy ion collisions as functions of the collision energy; see text for more explanations.