Speaker: Christopher McGinn 

Host: Marzia Rosati 

Title: Probing Hot and Cold QCD Matter with Hard Partonic Scatterings in Heavy Ion Collisions

Abstract:
Ultra-relativistic collisions of heavy nuclei, such as Pb ions at the LHC, provide a unique opportunity to experimentally study QCD under extreme conditions otherwise inaccessible in our current, cold universe. In ultraperipheral collisions (UPCs), where there is no nuclear overlap, interactions are initiated by quasireal photons associated with the EM fields of the accelerated nuclear charge. These photons cleanly probe nuclear structure in a manner analogous to deep inelastic scattering experiments. In hadronic collisions of boosted nuclei, where there is nuclear overlap, sufficient temperature and density is achieved to produce a new state of hot QCD matter called the Quark Gluon Plasma (QGP). In this phase, quarks and gluons no longer exist as the colorless composite protons and neutrons making up nuclei; rather, the liberated partons behave collectively as a fluid. Study of both phases of matter and the phase transition between is directly related to the fundamental nature of QCD. In this talk, measurements of experimental probes produced in hard-scatterings such as jets, photons, and open charm will be used to characterize the QGP's parton-medium interactions in the case of hadronic collisions, and constrain the nuclear parton distribution functions of nuclei in the case of UPCs. Future measurements for constraining the short-wavelength properties of hot QCD matter with the sPHENIX experiment at RHIC and searches for novel physics in cold QCD matter, such as gluon saturation, with the ePIC experiment at the future electron-ion collider will be discussed.

Bio: Chris McGinn has been studying QCD experimentally through heavy ion collisions for fifteen years. As a graduate student, he led the software and hardware trigger efforts for the CMS heavy ions group during the Run 2 period at the LHC, spanning 2015 through 2018. In addition, he led the jet reconstruction and high-pT analysis groups, the latter covering all heavy ion analyses incorporating jets, photons, and Z bosons as QGP probes. He was awarded a PhD from Massachusetts Institute of Technology in 2019, defending his thesis titled 'Mapping the Redistribution of Jet Energy in PbPb Collisions at the LHC with CMS'. Upon graduation, he joined the heavy ions group at the University of Colorado Boulder under Jamie Nagle, and switched to the ATLAS and sPHENIX experiments. With ATLAS, he led the heavy ions jet analysis group and pursued analyses of photon+jets in PbPb collisions. With sPHENIX, he led the calorimeter electronics testing team at Boulder. He is currently back at MIT as a senior postdoctoral research scientist with the heavy-ions group under the supervision of Yen-Jie Lee, where he leads the CMS heavy ions effort as the physics analysis group convener. His most recent analysis interest is using ultraperipheral collisions to characterize nuclear parton distribution functions and search for gluon saturation effects in the nucleus. In his free time, he enjoys walking, weight training, and bad movies.