Search for Higgs boson decays to beyond-the-Standard-Model light bosons in four-lepton events with the ATLAS detector at the LHC

This thesis presents the search for the dark sector process $h \rightarrow Z_d Z_d \rightarrow 4\ell$ in events collected by the ATLAS detector at the Large Hadron Collider in 2015--2018. In this theorized process, the Standard Model Higgs boson ($h)$ decays to four leptons ($4\ell$) via two intermediate Beyond-the-Standard-Model particles each called $Z_d$. This process arises from interactions of the Standard Model with a dark sector. A dark sector consists of one or more new particles that have limited or zero interaction with the Standard Model, such as the new vector boson $Z_d$ (dark photon). It could have a rich and interesting phenomenology like the visible sector (the Standard Model) and could naturally address many outstanding problems in particle physics. For example, it could contain a particle candidate for dark matter. In particular, Higgs decays to Beyond-the-Standard-Model particles are well-motivated theoretically and are not tightly constrained; current measurements of Standard Model Higgs properties permit the fraction of such decays to be as high as approximately $30\%$. The results of this search do not show evidence for the existence of the $h \rightarrow Z_d Z_d \rightarrow 4\ell$ process and are therefore interpreted in terms of upper limits on the branching ratio $\mathcal{B}(h \rightarrow Z_d Z_d)$ and the effective Higgs mixing parameter $\kappa^\prime$.