Universal scaling of the pion, kaon and proton p T spectra in Pb Pb collisions at 2.76 TeV

Abstract

With the experimental data collected by the ALICE collaboration in Pb-Pb collisions at a centerof-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 (p T ) spectra at these centralities. We show that in the low p T region with p T ≤ 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 = p T /K. The scaling parameter K is determined by the quality factor method and is parameterized by a N part b , where N part 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 N part . The values of b for pions and kaons are consistent within uncertainties, while they are smaller than that for protons. In the high p T 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 p T region can be simultaneously explained by the colour string percolation model in a qualitative way.

Universal scaling of meson and baryon spectra in p-Pb collisions at 5.02TeV

“…It was argued that the universal scaling was naturally incorporated in the framework of the colour glass condensate mechanism [12,13]. In our recent work [14], we showed that in the low p T region a scaling behavior independent of the collision centrality was exhibited in the pion, kaon and proton p T spectra in Pb-Pb collisions with √ s NN = 2.76 TeV at the LHC. This scaling behaviour was successfully explained by the CSP model.…”

“…The method to search for the universal scaling of the meson and baryon spectra in p-Pb collisions at different centrality classes is similar to that in ref. [14]. When presented in a suitable variable z = p T /K, the scaled pion p T spectra at different centrality classes, Φ(z) = A( N part 2πp T ) −1 d 2 N/d p T dy| p T =Kz , will exhibit a universal scaling behaviour.…”

“…( 1) is able to describe the strange particles (Λ, Ξ and Ω) spectra, but fails to depict the pion, kaon and proton spectra published by the ALICE collaboration in p-Pb collisions at 5.02 TeV. In order to describe the pion, kaon and proton spectra, a double-Tsallis distribution [14,24],…”

“…This hierarchy of the scaling violation is similar to that observed in Pb-Pb collisions in ref. [14]. As described in ref.…”

“…Moreover, for pions (kaons and protons), the rate is larger in p-Pb collisions than that in Pb-Pb collisions at 2.76 TeV in ref. [14].…”