Studying the emergence of Hot & Cold QCD phenomena in Heavy Ions at the ATLAS experiment

• Riccardo Longo (University of Illinois Urbana-Champaign)

Room: IR-1A (Aula Riunioni) Since the beginning of its operations, the ATLAS experiment has participated in the LHC Heavy-ion program, recording A+A and p+A collisions at different center of mass energies, as well as dedicated low luminosity p+p data to be used as a baseline to study nuclear modification effects. Thanks to the ATLAS detector's versatility, several unique measurements have been performed using Run 1 and 2 data, advancing the understanding of Quantum Chromodynamics (QCD) in both hot and cold temperature regimes and of precision Quantum Electrodynamics. Jet and track-based measurements in A+A collisions provided input to probe the strong coupling behavior of the Quark Gluon Plasma (QGP) over a variety of length scales to explore the emergence of quark and gluon modes at short wavelengths. Meanwhile, similar measurements in smaller systems like p+A, p+p, and photo-nuclear reactions have found collective behavior and, at the same time, no clear evidence of QGP formation, raising several intriguing questions that still need to be answered. p+A and photo-nuclear reactions have also been extensively studied at ATLAS to investigate initial-state effects, shedding light on dynamics related to nuclear structure and color fluctuations in the proton. Several of these measurements rely on the collision geometry characterization that is provided by a high-performance Zero Degree Calorimeter (ZDC) installed for Heavy Ion runs at ±140 meters from the interaction point. This vital piece of instrumentation will need an upgrade for the HL-LHC era, where the radiation environment and space available for detector implementation will place significant challenges that demand the construction of a new HL-ZDC. This talk will feature recent results obtained by the ATLAS Heavy Ion group, review the performance of the existing ZDC, and discuss the future outlook for the HL-ZDC upgrade.