Photon interactions in ultra-peripheral heavy-ion collisions in the ATLAS detector at the LHC

Two analyses involving photon interactions in ultra-peripheral lead-lead collisions in the ATLAS detector at the LHC are described, namely the study of dijet production in photo-nuclear ultra-peripheral collisions and a measurement of light-by-light scattering. The first, is an important way to probe and constrain nuclear parton distribution functions, which are known to exhibit suppression at low Bjorken-x with respect to proton PDFs, as well as enhancement at higher Bjorken-x. Light-by-light scattering is forbidden in classical electrodynamics as it violates the super-position principle and is a fundamental prediction of quantum mechanics. The first direct evidence for this interaction with two quasi-real initial state photons is presented.