Balance functions of charged hadron pairs $(\pi,K,p)\otimes (\pi,K,p)$ in Pb--Pb collisions at $\sqrt{s_{_{\rm NN}}} =$2.76 TeV

TL;DR

This study measures charge balance functions for hadron pairs in Pb--Pb collisions at 2.76 TeV, revealing how correlations vary with collision centrality and providing insights into quark production and radial flow effects.

Contribution

First measurement of charge balance functions for $( ext{pi,K,p}) imes ( ext{pi,K,p})$ pairs in Pb--Pb collisions at 2.76 TeV, with detailed differential analysis and new insights into hadron production.

Findings

Azimuthal widths decrease from peripheral to central collisions.

Longitudinal widths of $ ext{pi} ext{pi}$ and cross-species pairs narrow with centrality.

Hadron pairing probabilities show minimal dependence on collision centrality.

Abstract

We present the first balance function (BF) measurement of charged hadron pairs $(\pi,K,p)\otimes (\pi,K,p)$ in Pb--Pb collisions at $\sqrt{s_{_{\rm NN}}} =$2.76 TeV. The BF measurements are carried out as two-dimensional (2D) differential correlators vs. the relative rapidity ($\Delta y$) and azimuthal angle ($\Delta\varphi$) of hadron pairs, and studied as a function of collision centrality. While the BF azimuthal widths of all pairs substantially decrease from peripheral to central collisions, the longitudinal widths exhibit mixed behaviors: BF of $\pi\pi$ and cross-species pairs narrow in more central collisions whereas those of $KK$ and $pp$ are found invariant with collision centrality. This dichotomy is qualitatively consistent with the presence of strong radial flow effects and the existence of two waves of quark production in relativistic heavy ion collisions. We additionally present first measurements of the BF integrals and find that hadron pairing probabilities are very different from single hadron ratios and feature minimal collision centrality dependence. Overall, the results presented provide new and challenging constraints for theoretical models of hadron production and transport in relativistic heavy ion collisions.