Search for exotic Higgs Boson decay to multiple b-quarks with the ATLAS detector at LHC using machine learning methods

The discovery of the Higgs boson has opened up new possibilities for investigating physics beyond the Standard Model (SM). New particles may interact with the SM through the Higgs boson, and deviations from SM predictions can indicate the presence of new physics. This dissertation focuses on the search for exotic Higgs decay, $H\rightarrow aa \rightarrow (b\bar{b})(b\bar{b})$ where a is a new scalar boson and focuses on the case where the Higgs boson is produced in association with a Z boson. The data were collected by the ATLAS detector at the Large Hadron Collider at center-of-mass energy $\sqrt{s} =~13~\text{TeV}$ from 2015 to 2018 with a total integrated luminosity of 140 $\text{fb}^{-1}$. A new general-purpose low-mass double-b tagging algorithm was developed and applied in the search. With new machine-learning reconstruction techniques, the search is able to set a strong limit on the branching ratio of Br($H\rightarrow aa \rightarrow (b\bar{b})(b\bar{b})$). Furthermore, the search also considers models with two new scalars of different masses, $a_{1}$ and $a_{2}$, and sets the limits on the branching ratio of Br($H\rightarrow a_{1}a_{2} \rightarrow (b\bar{b})(b\bar{b})$)