Two-particle azimuthal correlations in photo-nuclear ultra-peripheral Pb+Pb collisions at 5.02~TeV with ATLAS

This note presents a measurement of two-particle long-range azimuthal correlations in photo-nuclear collisions using 1.73~nb$^{-1}$ of 5.02~TeV~Pb+Pb data collected in 2018 by ATLAS with a dedicated photo-nuclear event trigger. Candidate photo-nuclear events are selected using a combination of the zero-degree calorimeters, forward calorimeters, and reconstructed pseudorapidity gaps constructed from calorimeter clusters and charged-particle tracks. Correlation functions are formed using charged-particle tracks in the event. A template fitting method is employed to subtract the non-flow contribution. Second-order (elliptic) flow coefficients are presented as a function of charged-particle multiplicity and transverse momentum, and significant non-zero values of the flow coefficients are observed. The results are compared to flow coefficients obtained in proton--proton and proton--lead collisions in similar multiplicity ranges.