Two-particle azimuthal correlations in photonuclear ultraperipheral Pb+Pb collisions at 5.02 TeV with ATLAS

TL;DR

This paper reports the measurement of long-range azimuthal correlations in photonuclear Pb+Pb collisions at 5.02 TeV, revealing collective behavior and flow coefficients that offer new insights into the initial conditions of such collisions.

Contribution

It presents the first measurement of azimuthal correlations in photonuclear collisions at the LHC, using a novel event selection and analysis method to explore collective effects.

Findings

Significant second- and third-order flow coefficients observed.

Flow coefficients depend on charged-particle multiplicity and transverse momentum.

Results compared with proton-proton and proton-lead collisions.

Abstract

Two-particle long-range azimuthal correlations are measured in photonuclear collisions using 1.7 nb$^{-1}$ of 5.02 TeV Pb+Pb collision data collected by the ATLAS experiment at the LHC. Candidate events are selected using a dedicated high-multiplicity photonuclear event trigger, a combination of information from the zero-degree calorimeters and forward calorimeters, and from pseudorapidity gaps constructed using calorimeter energy clusters and charged-particle tracks. Distributions of event properties are compared between data and Monte Carlo simulations of photonuclear processes. Two-particle correlation functions are formed using charged-particle tracks in the selected events, and a template-fitting method is employed to subtract the non-flow contribution to the correlation. Significant nonzero values of the second- and third-order flow coefficients are observed and presented as a function of charged-particle multiplicity and transverse momentum. The results are compared with flow coefficients obtained in proton-proton and proton-lead collisions in similar multiplicity ranges, and with theoretical expectations. The unique initial conditions present in this measurement provide a new way to probe the origin of the collective signatures previously observed only in hadronic collisions.