Centrality dependence of pion freeze-out radii in Pb-Pb collisions at $\sqrt{\mathbf{s_{NN}}}$=2.76 TeV

TL;DR

This paper measures pion freeze-out radii in Pb-Pb collisions at 2.76 TeV, showing they decrease with pair transverse momentum and scale with multiplicity, supporting the presence of a collective expanding system.

Contribution

It provides detailed measurements of femtoscopic radii as functions of centrality and transverse momentum at LHC energies, confirming hydrodynamic behavior in heavy-ion collisions.

Findings

Radii decrease with increasing transverse momentum following a power-law.

Radii scale linearly with the cube root of charged particle multiplicity.

Decrease in the R_out/R_side ratio aligns with hydrodynamic model predictions.

Abstract

We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb--Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV as a function of collision centrality and the average transverse momentum of the pair $k_{\rm T}$. Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged one-dimensional radii are extracted. The radii decrease with $k_{\rm T}$, following a power-law behavior. This is qualitatively consistent with expectations from a collectively expanding system, produced in hydrodynamic calculations. The radii also scale linearly with $\left< \mathrm{d}N_{\rm ch}/\mathrm{d}\eta \right>^{1/3}$. This behaviour is compared to world data on femtoscopic radii in heavy-ion collisions. While the dependence is qualitatively similar to results at smaller $\sqrt{s_{\rm NN}}$, a decrease in the $R_{\rm out}/R_{\rm side}$ ratio is seen, which is in qualitative agreement with specific predictions from hydrodynamic models. The results provide further evidence for the production of a collective, strongly coupled system in heavy-ion collisions at the LHC.