Search for the decay of the Higgs boson to a Z boson and a long-lived axion-like particle decaying to two photons with the ATLAS Detector
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Abstract
The Standard Model of particle physics is one of the most successful theories, capable of describing a wide range of observed physical processes. However, some phenomena cannot be explained within the Standard Model, necessitating a theoretical extension. The Large Hadron Collider (LHC) at CERN aids in this effort through precision tests and searches for Beyond Standard Model particles using dedicated analyses. Several well-motivated extensions of the Standard Model include Axion-Like Particles (ALPs), which can couple to photons. This thesis presents two searches for ALPs.
The first search, described in the thesis, uses ATLAS Run 2 proton-proton (pp) data to constrain the exotic decay of the Higgs boson into a Z boson and an ALP, leading to a final state of two leptons and two photons. This measurement focuses on long-lived ALPs, which do not decay promptly, extending the excluded photon-ALP coupling phase space and providing limits for a range of couplings not previously covered by ATLAS.
The second search, described in the thesis, utilizes ATLAS Run 3 lead-lead (PbPb) data to probe the resonant production of an ALP in photon-photon scattering within the ATLAS detector. This is the first ALP measurement in ATLAS at a collision energy of $\sqrt{s} = 5.36$ TeV.