Correlations between azimuthal anisotropy and mean transverse momentum in pp, pPb, and peripheral PbPb collisions

TL;DR

This study investigates the relationship between azimuthal anisotropy and mean transverse momentum in different collision systems at the LHC, revealing multiplicity-dependent correlation sign changes in small systems but not in larger or more complex ones.

Contribution

It provides the first detailed analysis of azimuthal anisotropy and transverse momentum correlations across pp, pPb, and PbPb collisions, highlighting the role of particle multiplicity and pseudorapidity separation.

Findings

Correlation coefficients change from negative to positive with decreasing multiplicity in pp and pPb.

Sign changes in correlations disappear when particles are separated in pseudorapidity.

Models with gluon saturation and hydrodynamics do not predict the observed sign changes.

Abstract

Correlations between azimuthal anisotropy and mean transverse momentum of charged particles in proton-proton (pp), proton-lead (pPb), and peripheral lead-lead (PbPb) collisions are presented as a function of charged particle multiplicity. The pp, pPb and PbPb collision data were collected using the CMS detector at the LHC with a center-of-mass energy per nucleon pair of 13, 8.16 and 5.02 TeV, respectively. The two- and four-particle cumulants for the second- and third-order Fourier anisotropy harmonics are correlated with the mean transverse momentum of charged particles on an event-by-event basis. In pp and pPb systems, the observed correlation coefficients based on two-particle cumulants are found to change from negative to positive values as the charged particle multiplicity decreases. The sign changes disappear when the correlated particles are required to be further apart in pseudorapidity. Additionally, no sign changes in correlation coefficients are observed when employing four-particle cumulants. Models incorporating initial-state gluon saturation and final-state hydrodynamic evolutions are compared to pPb data and the predicted sign changes are not observed.