Azimuthal anisotropies of charged particles with high transverse momentum in Pb+Pb collisions at $\sqrt{s_{_\text{NN}}} = 5.02$ TeV with the ATLAS detector

TL;DR

This paper measures azimuthal anisotropies ($v_2$ and $v_3$) of charged particles in Pb+Pb collisions at 5.02 TeV over a wide transverse momentum range, revealing insights into jet quenching and the quark-gluon plasma.

Contribution

It provides the first detailed measurement of azimuthal anisotropies at high transverse momentum using advanced methods sensitive to fluctuations and nonflow effects.

Findings

Positive $v_2$ observed up to 100 GeV for all centralities

Positive $v_3$ observed up to 25 GeV, with method-dependent variations

Measurements sensitive to path-length dependence of parton energy loss

Abstract

A measurement is presented of elliptic ($v_2$) and triangular ($v_3$) azimuthal anisotropy coefficients for charged particles produced in Pb+Pb collisions at $\sqrt{s_{_\text{NN}}} = 5.02$ TeV using a dataset corresponding to an integrated luminosity of $0.44$ nb$^{-1}$ collected with the ATLAS detector at the LHC in 2018. The values of $v_2$ and $v_3$ are measured for charged particles over a wide range of transverse momentum ($p_\text{T}$), 1-400 GeV, and Pb+Pb collision centrality, 0-60%, using the scalar product and multiparticle cumulant methods. These methods are sensitive to event-by-event fluctuations and nonflow effects in the measurements of azimuthal anisotropies. Positive values of $v_2$ are observed up to a $p_{\text{T}}$ of approximately 100 GeV from both methods across all centrality intervals. Positive values of $v_3$ are observed up to approximately 25 GeV using both methods, though the application of the three-subevent technique to the multiparticle cumulant method leads to significant changes at the highest $p_{\text{T}}$. At high $p_{\text{T}}$ ($p_{\text{T}} \gtrapprox 10$ GeV), charged particles are dominantly from jet fragmentation. These jets, and hence the measurements presented here, are sensitive to the path-length dependence of parton energy loss in the quark-gluon plasma produced in Pb+Pb collisions.