Searches for Axions in Ultra-Peripheral heavy ion collisions

Axions are particles that were postulated to solve the "strong CP problem". However, axion-like particles (ALPs) appear in many beyond Standard Model theories. Their signature couplings are to photons and their mass is unconstrained. The LHC is currently breaking new ground in accessing previously unexplored ALP models using both proton-proton and Pb-Pb collisions. This project is about searching with ALPs using ultra-peripheral Pb-Pb collision (UPC) data recorded with the ATLAS detector at the LHC. In these collisions2, the heavy ions interact electromagnetically without breaking up. They are ideal for the study of photon-photon produced resonances, like ALPs produced by the interaction of two photons. The ALPs then decay to photon-photon pair that can be recorded in the detector. Photon reconstruction and identification in ATLAS has not been optimized for the UPC environment and this opens an excellent opportunity to develop new techniques. A major improvement in the sensitivity of this analysis is expected by studying a photon reconstruction and identification using image analysis with deep neural networks, which will allow us to reconstruct photons with lower transverse momentum and identify them better against backgrounds from electrons or neutral pions than it is now possible using the current algorithms. These techniques combined with the forthcoming LHC heavy ion datasets will enable us to achieve the best possible sensitivity to ALPs leading to a discovery or a most stringent upper limit on their production.