Photon–photon fusion and ultra-peripheral physics with ATLAS

Photon-photon fusion is a rare process at hadron and ion colliders. It is particularly interesting as a remarkably clean interaction with little (if any) remnant activity from the interacting particles. In this talk we present the status of photon-photon fusion measurements at the ATLAS detector. We present differential measurements of the light-by-light scattering process, γγ → γγ, in lead-lead collisions. In addition, we present photon-photon fusion measurements using events that contain two charged leptons in the final state. The scattered proton is detected by the ATLAS Forward Proton spectrometer while the leptons are reconstructed by the central ATLAS detector. Finally, we highlight the observation of photon-induced WW production. This talk also gives an overview of the latest ultra-peripheral physics measurements performed with the ATLAS detector at the LHC. These include differential measurements of the exclusive di-muon production cross-section, which are crucial for setting constraints on the initial photon spectrum for all UPC measurements at the LHC; measurements of light-by-light scattering, which result in an observation of this elusive Standard Model process and set competitive limits on the parameter space for axion-like particles; measurements of electromagnetic di-muon production in non-UPC Pb+Pb collisions, which are sensitive to the structure of the initial EM fields and possibly EM content of the created Quark-Gluon Plasma; and measurements of collective behavior in high-multiplicity photo-nuclear collisions.