Role of system size in freeze-out conditions extracted from transverse momentum spectra of hadrons

Ajay Kumar Dash,Sandeep Chatterjee,Chitrasen Jena,Bedangadas Mohanty,Ranbir Singh

doi:10.1103/physrevc.98.064902

Abstract

The data on hadron transverse momentum spectra in different centrality classes of p+Pb collisions at $\sqrt{s}_{NN} = 5.02$ TeV has been analysed to extract the freezeout hypersurface within a simultaneous chemical and kinetic freezeout scenario. The freezeout hypersurface has been extracted for three different freezeout schemes that differ in the way strangeness is treated: i. unified freezeout for all hadrons in complete thermal equilibrium (1FO), ii. unified freezeout for all hadrons with an additional parameter $\gamma_S$ which accounts for possible out-of-equilibrium production of strangeness (1FO$+\gamma_S$), and iii. separate freezeout for hadrons with and without strangeness content (2FO). Unlike in heavy ion collisions where 2FO performs best in describing the mean hadron yields as well as the transverse momentum spectra, in p+Pb we find that 1FO$+\gamma_S$ with one less parameter than 2FO performs better. This confirms expectations from previous analysis on the system size dependence in the freezeout scheme with mean hadron yields: while heavy ion collisions that are dominated by constituent interactions prefer 2FO, smaller collision systems like proton + nucleus and proton + proton collisions with lesser constituent interaction prefer a unified freezeout scheme with varying degree of strangeness equilibration.

Highlights

Unlike in heavy-ion collisions where 2FO performs best in describing the mean hadron yields as well as the transverse momentum spectra, with p + Pb we find that 1FO + γS with one fewer parameter than 2FO performs better
This confirms expectations based on previous analysis of system size dependence in the freeze-out scheme with mean hadron yields: while heavy-ion collisions that are dominated by constituent interactions prefer 2FO, smaller collision systems like proton + nucleus and proton +
Knowledge of the surface of last scattering of hadrons produced in a heavy-ion collision event is of utmost significance, as it contributes to the calibration of the hadronic physics baseline to be contrasted with data to extract information on the quark gluon plasma phase [1,2] as well as on the QCD critical point [3,4]

Summary

Pb collisions at

37] shown for three centralities—0-5%, 10–20%, and 60–80%—in three FO schemes at 0 < ycm < 0.5. The improvement of 1FO + γS over 1FO and 2FO increases as one goes from central to peripheral collisions This is driven by the strange sector; it is more sensitive to the three freeze-out schemes studied here, which differ in the treatment of the freeze-out of strange hadrons. The yield in the nonstrange sector receives a partial contribution from the decays of strange resonances This leads to a small sensitivity in the fit quality of the nonstrange sector to the different freeze-out schemes studied here. We use similar errors in T, ρmax, and τf as for Pb + Pb results [14] since the errors are mostly system size independent

INTRODUCTION

Published by the American Physical Society

RESULTS

SUMMARY AND OUTLOOK