Charged-hadron and identified-hadron (𝐾⁰_𝑆, 𝛬, 𝛯⁻) yield measurements in photonuclear Pb+Pb and 𝑝+Pb collisions at √𝑠_𝑁⁢𝑁=5.02 TeV with ATLAS

This paper presents the measurement of charged-hadron and identified-hadron (𝐾⁰_𝑆, 𝛬, 𝛯⁻) yields in photo-nuclear collisions using 1.7 𝑛⁢𝑏⁻¹ of √𝑠_𝑁⁢𝑁 = 5.02 TeV Pb+Pb data collected in 2018 with the ATLAS detector at the Large Hadron Collider. Candidate photo-nuclear events are selected using a combination of tracking and calorimeter information, including the zero-degree calorimeter. The yields as a function of transverse momentum and rapidity are measured in these photo-nuclear collisions as a function of charged-particle multiplicity. These photo-nuclear results are compared with 0.1 𝑛⁢𝑏⁻¹ of √𝑠_𝑁⁢𝑁 = 5.02 TeV p+Pb data collected in 2016 by ATLAS using similar charged-particle multiplicity selections. These photo-nuclear measurements shed light on potential quark-gluon plasma formation in photo-nuclear collisions via observables sensitive to radial flow, enhanced baryon-to-meson ratios, and strangeness enhancement. The results are also compared with the Monte Carlo DPMJET-III generator and hydrodynamic calculations to test whether such photo-nuclear collisions may produce small droplets of quark-gluon plasma that flow collectively.