Search for Light Bosons in Exotic Decays of the 125 GeV Higgs Boson

A search for pairs of light bosons produced from decays of the $125$ GeV Higgs boson is presented. The search covers the light boson mass region where the particle could have an enhanced decay rate to a pair of $\tau$ leptons, according to different models and scenarios. The leptonic decay channels corresponding to the $4\tau$ and $2\mu2\tau$ final states are used. The search is based on proton-proton collisions collected by the CMS experiment in 2016 at a center-of-mass energy of \mbox{$\sqrt{s} = 13$ TeV}. This dataset corresponds to an integrated luminosity of $35.9$~fb\textsuperscript{-1}. The analysis is motivated by many theories beyond the Standard Model that suggest modifications in the Higgs sector. The prevailing tendency among these new models is to extend the group structure of the scalar sector, thus resulting in an increased number of physical states in their spectrum. The additional bosons could couple to both the $125$ GeV Higgs boson and the rest of the particles in the theory, which allows their detection through different production and decay mechanisms. Two different approaches used for the analysis of the data are presented. The first strategy targets boosted event topologies, occurring for light bosons with masses between $4$ and $15$ GeV. The second method employs a modified event selection technique combined with a more powerful final discriminant to be able to extend the mass range up to $21$ GeV. In both approaches, no significant deviation beyond the expectation from the Standard Model is observed. Observed and expected upper limits at $95 \%$ confidence level on the product of the Higgs boson production cross-section relative to that in the Standard Model and the branching fraction into the $4\tau$ final state are set. Interpretations of the experimental results in the context of a specific theoretical model are also provided.