Speaker
Description
Ultraperipheral collisions of relativistic heavy ion beams lead to a diverse set of photon-nucleus (photonuclear) interactions. Measurements of particles produced in photonuclear reactions can shed light on the QCD dynamics of these novel, extremely asymmetric colliding systems, with energies between those available at RHIC and the LHC. Previous studies by ATLAS have characterized photonuclear collisions through inclusive charged hadron measurements, and have observed elliptic and triangular flow coefficients in these events. This talk presents a measurement of charged hadron production and, for the first time, measurements of identified hadron yields in high-multiplicity photonuclear collisions using 5.02 TeV Pb+Pb data collected in 2018 by ATLAS, with a dedicated photo-nuclear event trigger. The yields of charged hadrons and, for the first time, identified strange hadrons ($K^0_S$, $\Lambda$, and $\Xi$) are shown differentially in rapidity, transverse momentum, and event multiplicity. These new results shed light on potential QGP formation in ultraperipheral collisions via observables sensitive to radial flow, enhanced baryon-to-meson ratios, and strangeness enhancement. These results are compared to 5.02 TeV p+Pb data collected by ATLAS in 2016, at the same event multiplicities. The results are also compared with calculations from DPMJET and hydrodynamic-based models predicting flow even in these ultraperipheral collisions.
