Universal scaling of the pion, kaon and proton pT spectra in PbPb collisions at 2.76 TeV

With the experimental data collected by the ALICE collaboration in PbPb collisions at a center-of-mass energy per nucleon pair 2.76 TeV for six different centralities (0-5%, 5-10%, 10-20%, 20-40%, 40-60% and 60-80%), we investigate the scaling property of the pion, kaon and proton transverse momentum (pT) spectra at these centralities. We show that in the low pT region with pT≤ 2.75 (3.10 and 2.35) GeV/c the pion (kaon and proton) spectra exhibit a scaling behaviour independent of the centrality of the collisions. This scaling behaviour arises when these spectra are presented in terms of a suitable variable, z=pT/K. The scaling parameter K is determined by the quality factor method and is parameterized by a〈Npart〉b, where 〈Npart〉 is the average value of the number of participating nucleons, a and b are free parameters, b characterizes the rate at which lnK changes with ln〈Npart〉. The values of b for pions and kaons are consistent within uncertainties, while they are smaller than that for protons. In the high pT region, due to the suppression of the spectra, a violation of the proposed scaling is observed going from central to peripheral collisions. The more peripheral the collisions are, the more clearly violated the proposed scaling becomes. In the framework of the colour string percolation model, we argue that the pions, kaons and protons originate from the fragmentation of clusters which are formed by strings overlapping and the cluster's fragmentation functions are different for different hadrons. The scaling behaviour of the pion, kaon and proton spectra in the low pT region can be simultaneously explained by the colour string percolation model in a qualitative way.