Azimuthal anisotropy of dijet events in PbPb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV

TL;DR

This study measures azimuthal anisotropies of high-energy dijet events in PbPb collisions at 5.02 TeV to understand parton energy loss in the quark-gluon plasma, revealing a significant elliptic flow component.

Contribution

It provides the first detailed measurement of azimuthal anisotropies of dijets at this energy, highlighting the path-length dependence of parton energy loss in heavy-ion collisions.

Findings

Positive $v_2$ values increase from central to peripheral collisions

$v_3$ and $v_4$ are consistent with zero within uncertainties

Long-range correlations effectively suppress jet fragmentation background

Abstract

The path-length dependent parton energy loss within the dense partonic medium created in lead-lead collisions at a nucleon-nucleon center-of-mass energy of $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV is studied by determining the azimuthal anisotropies for dijets with high transverse momentum. The data were collected by the CMS experiment in 2018 and correspond to an integrated luminosity of 1.69 nb$^{-1}$. For events containing back-to-back jets, correlations in relative azimuthal angle and pseudorapidity ($\eta$) between jets and hadrons, and between two hadrons, are constructed. The anisotropies are expressed as the Fourier expansion coefficients $v_n$, $n = $ 2-4 of these azimuthal distributions. The dijet $v_n$ values are extracted from long-range (1.5 $\lt$ $\vert\Delta\eta\vert$ $\lt$ 2.5) components of these correlations, which suppresses the background contributions from jet fragmentation processes. Positive dijet $v_2$ values are observed which increase from central to more peripheral events, while the $v_3$ and $v_4$ values are consistent with zero within experimental uncertainties.