Charged-hadron and identified-hadron ( K S 0 ,   Λ ,   Ξ − ) yield measurements in photonuclear Pb + Pb and p + Pb collisions at s N N = 5.02 TeV with ATLAS

Abstract

This paper presents the measurement of charged-hadron and identified-hadron ( K S 0 ,   Λ ,   Ξ − ) yields in photonuclear collisions using 1.7 nb − 1 of s N N = 5.02 TeV   Pb + Pb data collected in 2018 with the ATLAS detector at the Large Hadron Collider. Candidate photonuclear 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 photonuclear collisions as a function of charged-particle multiplicity. These photonuclear results are compared with 0.1 nb − 1 of s N N = 5.02 TeV   p + Pb data collected in 2016 by ATLAS using similar charged-particle multiplicity selections. These photonuclear measurements shed light on potential quark-gluon plasma formation in photonuclear collisions via observables sensitive to radial flow, enhanced baryon-to-meson ratios, and strangeness enhancement. The results are also compared with the Monte Carlo generator and hydrodynamic calculations to test whether such photonuclear collisions may produce small droplets of quark-gluon plasma that flow collectively.