Axion searches in rare Higgs decays at ATLAS

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The discovery of a Standard Model like Higgs boson was a major milestone forthe LHC. Despite this, the Standard Model (SM) and the underlying theories sofar are plagued by problems that arise both theoretically and experimentally. Thisincludes the Strong CP-problem, which is the theoretical argument that the SMcould break CP symmetry in strong interactions. However, no evidence of suchhas been experimentally observed. The Higgs boson observation with a mass ofapproximately 125 GeV does leave open the possibility for new particles which willnot violate the elementary gauge symmetries of the Standard Model and as suchcreates the potential to search for these particles at low mass .With this motivation my project will contribute to the searches of such particles.The aim is to initiate the analysis for searching for a pseudo-scalar low mass particlewhich we will call a, from the rare Higgs decay to Za. Particle a could identifyas a possible Axion-like particle (ALP) or Next-to-Minimal Supersymmetry Model(NMSSM) candidate.Axions and ALP's are theoretical particles resulting from the Peccei-Quinnmechanism that describes a possible solution to the CP problem by introducinga dynamic field to explain the CP-violating term, ALP's can couple to all SMparticles, however depending on their coupling and possible mass ranges searchesfor ALP's differ significantly. Theoretical studies by Bauer (2017) show that thereare possible parameter regions present were LHC data could well be utilized tosearch for ALPs in the decay channel H Za for masses in the range 0.1-10 GeVand could be long lived In Higgs physics, ALP's could decay into a pair ofjets or photons alongside a Z-boson decaying into two leptons with a clean signal.Previous searches for ALPs have been conducted at the LHC but have so far notbeen successful.A challenge with this study is the fact that R, the distance the ALP's photonsare from each other, is so small that during photon identification (ID) photons areactually being misidentified. A major task of this project will focus on the analysisof these photons by evaluating the substructure of the photon shower in the 1stlayer of the electromagnetic calorimeter (ECAL) to better distinguish those thatare misidentified and as such improve the overall analysis of this decay channel.Machine learning approaches such as deep neural networks and other multivariateanalysis techniques, complemented by skills gained through working as part of theLiverpool data intensive center for doctoral training (LIV.DAT), will be exhaustivelyattempted for better photon reconstruction. By incrementally improvingissues related to the underlying analysis of this decay channel can pseudo-scalarlow mass particles and such be thoroughly investigated