Search for Light Pseudoscalars in Exotic Decays of the Higgs Boson with the CMS Experiment

Abstract

This thesis presents a search for a pair of light pseudoscalar bosons produced from the decays of the 125 GeV Higgs boson, focusing on final states with four tau leptons. This search is motivated by numerous beyond the standard model theories that predict an extended scalar sector, introducing a wide array of new physical states beyond the single Higgs boson observed in the standard model. Such theories, including the Two-Higgs-Doublet Model with an additional singlet (2HDM+S), suggest new interactions and couplings, enabling the 125 GeV Higgs to decay into lighter pseudoscalar bosons. These exotic decays serve as direct probes of new physics beyond the standard model. Given the enhanced couplings of light pseudoscalars to leptons in various scenarios within these models, the final states analyzed in this study are of particular relevance.

The search is based on data from proton-proton collisions at a center-of-mass energy of 13 TeV collected by the CMS detector at the LHC during the Run 2 data-taking period, corresponding to an integrated luminosity of 138 fb-1. The analysis targets pseudoscalars with masses between 4 and 15 GeV, which result in highly collimated decay products and present a challenging final state with overlapping fermion signatures. A specialized strategy, deviating from standard reconstruction and selection techniques, is implemented by utilizing muons and tracks for event identification.

No significant deviation of data with respect to the standard model expectation is observed in this search. Consequently, model-independent upper limits are set at 95 % confidence level on the cross section times branching ratio for the 4τ final state, relative to the SM Higgs production cross section. Additionally, exclusion limits are established for various benchmark scenarios within the 2HDM+S framework, further constraining the parameter space of this model.