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

• Published in: Physical review. C 2021, Vol.104 (1)
• Main Authors: Aad, G., Lund-Jensen, Bengt, Ohm, Christian, Ripellino, Giulia, Shope, David R., Strandberg, Jonas, Zwalinski, L., et.al
• Format: Article
• Language: English
• ISSN: 2469-9985; 2469-9993
• DOI: 10.1103/PhysRevC.104.014903

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 CERN Large Hadron Collider. 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 nonflow 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.