A.Tsatsos: Exploring ALPs under the Alps: The Search for a Diphoton Resonance with the CMS Experiment in an Unexplored Mass Range

The ATLAS and CMS experiments at the Large Hadron Collider (LHC) have confirmed the existence of a particle that coincides with the Standard Model (SM) Higgs boson, creating an avenue for the LHC to explore new physics beyond the Standard Model (BSM). Several theoretical models such as the axion-like particle (ALP) predict a Higgs-like particle with a mass lower than the SM Higgs. ALPs may even solve symmetry problems such as the strong charge-parity (CP) problem, which addresses the lack of CP violation in strong force interactions. The diphoton final state at CMS is sensitive to many of these BSM models, presenting new prospects for an ALP search. In this thesis defense, I will present the motivation, analysis, and results on the production cross-section times branching ratio of diphoton resonances in the invariant mass range of 10-70 GeV using Run 2 data from the CMS experiment. A dataset with an integrated luminosity of 54.4/fb is utilized, applying analysis and fitting strategies to obtain first results for CMS in this phase space. No significant excess above the expected background is observed, so upper limits on the cross-section times branching ratio are derived. These limits are then interpreted in the context of an ALP effective field theory model.