Charged-hadron yield measurements in photo-nuclear collisions using 5.02 TeV Pb+Pb data with ATLAS

This note presents the analysis of charged-hadron measurements in photo-nuclear collisions using 1.73 $\mathrm{nb^{-1}}$ of 5.02 TeV Pb+Pb data collected in 2018 by ATLAS. Candidate photo-nuclear events are selected and background events rejected using a combination of zero degree calorimeter, pseudorapidity gap, and tracking information. The yield of charged hadrons as a function of transverse momentum ($p_{\mathrm{T}}$) and pseudorapidity ($\eta$) is measured in these photo-nuclear collisions as a function of charged-particle multiplicity. The results are compared with 0.10 $\mathrm{nb^{-1}}$ of 5.02 TeV p+Pb data collected in 2016 by ATLAS using similar charged-particle multiplicity selections. These photo-nuclear measurements are important for constraining the photon energy distribution and particle production in Monte Carlo models such as DPMJET-III. Additionally, the results are important to test whether such photo-nuclear collisions may produce small droplets of quark-gluon plasma that flow hydrodynamically.