Colloquium – Peter Steinberg

Peter Steinberg, Brookhaven National Laboratory

Collisions with light at the LHC: photon-photon and photonuclear physics with ATLAS.

Heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory and the Large Hadron Collider (LHC) at CERN are rich laboratories to study the physics of quantum chromodynamics (QCD) under extreme conditions, with temperatures of the quark-gluon plasma reaching trillions of Kelvin and the system behaving like a near ideal fluid. However, the same nuclei are surrounded by dense, highly-Lorentz-contracted electromagnetic fields that act as sources of nearly-real high energy photons. Photons emitted from one nucleus can interact with the other nucleus, leading to photoproduction processes similar to those seen at electron-hadron machines. The photons can also collide with each other, producing particles through photon-photon interactions. Since 2015, the ATLAS experiment at the LHC has measured a wide variety of photonuclear and photon-photon processes that have explored aspects of QCD as well as quantum electrodynamics (QED). Electron and muon pair production provides a detailed understanding of the initial photon fluxes, especially how they vary when changing the impact parameter between the nuclei — selected using the presence of forward neutrons measured in the ATLAS zero degree calorimeters. Tau lepton production is potentially sensitive to physics beyond the standard model via the influence of its anomalous magnetic moment, and ATLAS has set the most stringent limits on this quantity since LEP2. ATLAS has also observed the once-elusive QED process of light-by-light scattering, the elastic scattering of two photons off of each other, which is also sensitive to new physics processes. Photonuclear jet production will provide detailed access to the partonic structure of nuclei, particularly the gluon structure, giving a preview of the physics of the upcoming Electronic Ion Collider (EIC). Quark-antiquark fluctuations of the incoming photon interact like a vector meson, and collective behavior is observed in these processes, similar to that seen for proton-proton, proton-nucleus and nucleus-nucleus systems, giving further insight into the nature of the quark-gluon plasma. Prospects for LHC Run 3, now underway, will also be discussed.

Dec 7, 2023 @ 4:00pm Central in Stevenson 4327; reception beforehand at 3:30pm in Stevenson 6333