Search for light bosons in the final state with muons and tau leptons with CMS Run II data

A search for a pair of light bosons produced in decays of the 125 GeV Higgs boson, with one of the light states decaying into a pair of muons and the other into a pair of tau leptons, is presented. The search is based on a data sample corresponding to an integrated luminosity of 137.2 fb$^{-1}$, collected with the CMS detector at the CERN Large Hadron Collider in the years 2016, 2017, and 2018 at a center-of-mass energy of 13 TeV. An extended Higgs sector is well motivated in a vast set of Beyond the Standard Model theories such as the two Higgs doublets plus one additional singlet (2HDM+S) and the Dark Photon Model. In the context of these models, the 125 GeV Higgs boson can decay into a pair of light bosons, which subsequently decay to pairs of Standard Model particles. Considering the enhanced coupling of the light bosons to leptons for some of the scenarios within these models, the final state considered in this work results of particular interest. Masses of the light boson between 3.6 and 21 GeV are probed, which leads to an experimental signature in the detector with both the muon pair and visible decay products from the tau pair being highly collimated. The analysis benefits from the efficient identification and reconstruction of muons by the CMS detector. Using Multivariate Analysis Techniques, the information on several kinematic variables is exploited to enhance the sensitivity to the targeted topology. No significant excess of events is found above the Standard Model expectation. Therefore, model-independent upper limits at 95% confidence level on the 125 GeV Higgs boson production cross-section times the branching fraction into the studied final state are set. Model-specific upper bounds are obtained as constraints on the parameter spaceof the different benchmark scenarios within the 2HDM+S and the Dark Photon Model.