Search for Exotic Higgs Decays to Two Light Scalars in the 6b Final State and Calibration of Soft Secondary Vertices with the ATLAS Experiment

The last building block of the standard model of particle physics was observed in 2012 with the discovery of the Higgs boson. Even with the standard model being one of the most successful theories in physics, there are several questions it can not answer. One type of model that can explain some of the open questions are extensions of the Higgs sector. They can predict the (so far hypothetical) decays of the standard model Higgs boson into non-standard model Higgs-like particles. This thesis presents the first LHC search for exotic decays of the standard model Higgs boson to two new scalars of different mass H → a1 a2 in the full-b final state. Particular focus is put on the cascade decay cases, where the heavier scalar subsequently decays to the lighter scalar, a2 → a1 a1, which then decays to b quarks, a1 → bb̄. This results in a final state of six low-momentum b hadrons. To reconstruct these soft b hadrons, novel reconstruction techniques are employed. These include the double-b-tagging of jets with the DeXTer tagger and the reconstruction of soft secondary vertices. The soft secondary vertices aim to reconstruct secondary vertices origination from b hadron decays that are too low in momentum to be reconstructed as jets. A calibration of the soft secondary vertex reconstruction efficiency is presented in this thesis as well. It is carried out in tt̄ events and corrects the simulated reconstruction efficiency and fake rate. The calibration and the search for exotic Higgs decays are based on the full Run 2 dataset of pp collisions collected with the ATLAS detector at √s = 13 TeV, corresponding to an integrated luminosity of 140 1/fb. In the search, the events are categorized based on the multiplicity of reconstructed b-objects that include the double-b-tagged jets, b-jets and soft secondary vertices. Based on these multiplicity regions, a multi-variate analysis is used to define signal-enriched regions and control regions for the dominant backgrounds. No signal is observed, the 95% CL limits on the branching ratio of exotic Higgs decays to two new scalars undergoing the cascade decay to the 6b final state BR(H → a1 a2 → 6b) range from 11% to 19%.