Search for exotic Higgs boson decays to a pair of pseudoscalars in the $\mu\mu\mathrm{bb}$ and $\tau\tau\mathrm{bb}$ final states in proton-proton collisions with the CMS experiment

A search for the exotic decays of a standard model like Higgs boson (H) with a $125\,\mathrm{GeV}$ mass to a pair of light pseudoscalars $\mathrm{a}_1$ is performed in final states where one pseudoscalar decays to two $\mathrm{b}$ quarks and the other to two $\tau$ leptons or muons. A data sample of proton-proton collisions at $\sqrt{s}=13\,\mathrm{TeV}$ corresponding to an integrated luminosity of $138\,\mathrm{fb}^{-1}$ recorded with the CMS detector is exploited. No statistically significant excess is observed over the standard model backgrounds. Upper limits are set, at $95\%$ confidence level (CL), on the Higgs boson branching fraction to $\ell\ell\mathrm{b}\mathrm{b}$ via a pair of $\mathrm{a}_1$, where $\ell$ stands for muons and tau leptons. The limits depend on the pseudoscalar mass $m_{\mathrm{a}_1}$. The observed limits are in the range $(0.17-3.3)\times 10^{{\rm -4}}$ and $(1.7-7.6)\times 10^{{\rm -2}}$ in the $\mu\mu\mathrm{b}\mathrm{b}$ and $\tau\tau\mathrm{b}\mathrm{b}$ final states, respectively. The two final states are combined to obtain exclusion limits of the branching fraction $\mathrm{B}(\mathrm{H}\to\mathrm{a}_1\mathrm{a}_1\to\ell\ell\mathrm{b}\mathrm{b})$ at $95\%$ CL for a broad class of models of a two Higgs doublet extended with a scalar singlet (2HDM+S). Upper bounds on the Higgs boson branching fraction $\mathrm{B}(\mathrm{H}\to\mathrm{a}_1\mathrm{a}_1)$ are also extracted from the combination. $\mathrm{B}(\mathrm{H}\to\mathrm{a}_1\mathrm{a}_1)$ values above $0.23$ are excluded at $95\%$ CL for most Type-II 2HDM+S models for $m_{\mathrm{a}_1}$ values between 15 and 60 $\mathrm{GeV}$.