Speaker
Description
In ultra-relativistic heavy-ion collisions the intense electromagnetic fields of the nuclei provide a large flux of equivalent photons. This flux leads to photon-photon and photon-nucleus reactions at high center-of-mass energies. In ultra-peripheral collisions (UPCs), the nuclei have large impact parameter, and the dominant interaction mechanism is through these photon-induced processes. This talk presents a series of measurements of such processes performed by the ATLAS Collaboration. It includes new measurements of exclusive UPC dimuon production, which provide detailed constraints on the nuclear photon flux and its dependence on impact parameter and photon energy. This production mechanism may also occur in events with smaller impact parameter, resulting in dimuons produced in the same events in which a hot nuclear medium is formed. Measurements of $\gamma+\gamma\rightarrow\mu\mu$ in non-UPC collisions are also presented; the dimuons exhibit a centrality-dependent broadening of their azimuthal angle correlations suggesting that such muons provide a new probe of the medium. Finally, measurements of jet production in photon-nucleus collisions will be presented; such processes provide a clean probe of the nuclear parton distributions.