Search for the Decay of the Standard Model Higgs Boson to Muon Pairs with the ATLAS Detector at the LHC

The Standard Model predicts the Higgs boson to interact with all mas- sive particles. At present, experimental evidence exists for the inter- actions with bosons and third generation fermions. This thesis focuses on the search for the decay of the Higgs boson to muon pairs, the most sensitive way of probing the interactions with second generation fermions. The search uses proton-proton collision data collected at the centre-of-mass energy √s = 13 TeV collected by the ATLAS experi- ment between 2015 and 2018 and corresponds to an integrated lumi- nosity of 140 fb−1. The analysis selects events with oppositely charged muon pairs and splits them in categories using a gradient boosting clas- sifier to maximise the sensitivity. A simultaneous maximum-likelihood fit is performed in all analysis categories to extract the signal strength and evaluate the compatibility of the data with the background-only hypothesis. The signal strength is measured to be 0.5 ± 0.7, where the uncertainty is dominated by the statistical uncertainty on the data. The observed (expected) significance is found to be 0.8 (1.5) σ. The sensitivity projection for the 3000 fb−1 dataset collected at 14 TeV esti- mates that the signal strength will be measured to 13% and still limited by the statistical uncertainty on the data.