Longitudinal and azimuthal evolution of two-particle transverse momentum correlations in Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV

TL;DR

This study measures two-particle transverse momentum correlations in Pb-Pb collisions at 2.76 TeV, revealing how these correlations evolve with collision centrality and providing insights into the medium's shear viscosity and flow properties.

Contribution

First measurements of charge independent and dependent transverse momentum correlators in Pb-Pb collisions at 2.76 TeV, showing their evolution with collision centrality and sensitivity to medium properties.

Findings

Correlator $G_{2}^{ m CI}$ broadens longitudinally and narrows azimuthally with centrality.

Correlator $G_{2}^{ m CD}$ narrows along both dimensions as collisions become more central.

Results are consistent with low shear viscosity and stronger transverse flow in central collisions.

Abstract

This paper presents the first measurements of the charge independent (CI) and charge dependent (CD) two-particle transverse momentum correlators $G_{2}^{\rm CI}$ and $G_{2}^{\rm CD}$ in Pb--Pb collisions at $\sqrt{s_{\text{NN}}} = 2.76\;\text{\TeVe}$ by the ALICE collaboration. The two-particle transverse momentum correlator $G_{2}$ was introduced as a measure of the momentum current transfer between neighbouring system cells. The correlators are measured as a function of pair separation in pseudorapidity ($\Delta \eta$) and azimuth ($\Delta \varphi$) and as a function of collision centrality. From peripheral to central collisions, the correlator $G_{2}^{\rm CI}$ exhibits a longitudinal broadening while undergoing a monotonic azimuthal narrowing. By contrast, $G_{2}^{\rm CD}$ exhibits a narrowing along both dimensions. These features are not reproduced by models such as HIJING and AMPT. However, the observed narrowing of the correlators from peripheral to central collisions is expected to result from the stronger transverse flow profiles produced in more central collisions and the longitudinal broadening is predicted to be sensitive to momentum currents and the shear viscosity per unit of entropy density $\eta/s$ of the matter produced in the collisions. The observed broadening is found to be consistent with the hypothesized lower bound of $\eta/s$ and is in qualitative agreement with values obtained from anisotropic flow measurements.