Inferring freeze-out parameters from pion measurements at RHIC and LHC

Abstract: We analyze the transverse momentum spectra of charged pions measured in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV and in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV using the Tsallis distribution modified to include transverse flow. All the spectra are well described by the modified Tsallis distribution in an extended transverse momentum range upto 6 GeV/$c$. The kinetic freeze-out temperature ($T$), average transverse flow ($\beta$) and degree of non thermalization ($q$) are obtained as a function of system size for both the energies. With increasing system size $\beta$ shows increasing trend whereas $T$ remains constant. While the systems at RHIC and LHC energies show similar $\beta$ and $q$, the parameter $T$ is higher at LHC as compared to RHIC. The kinetic freeze-out temperature is also extracted using the measured charged particle multiplicity and HBT volume of the system as a function of system size and collision energies.