Search for the exotic decay of the Higgs boson to a pair of light pseudoscalars in the final state with two b quarks and two $\tau$ leptons

A search for the exotic decay of the Higgs boson to a pair of light pseudoscalar bosons is performed in the final state with two b jets and two $\tau$ leptons. The motivation lies in models beyond the standard model (SM), such as two-Higgs-doublet models extended with a complex scalar singlet, which include the next-to-minimal supersymmetric SM (NMSSM). The results are based on an integrated luminosity of $35.9~\mathrm{fb}^{-1}$, accumulated by the CMS experiment at the LHC in 2016 at 13 TeV center-of-mass energy. Masses of the pseudoscalar boson between 15 and 60 GeV are probed, and upper limits as low as $23\%$ are set on the branching fraction of the Higgs boson to two light pseudoscalar bosons in the NMSSM, assuming that the production cross section of the Higgs boson is the one predicted in the SM. No mass constraint is set on the b jet pair, making this search sensitive to Higgs boson decays to two scalars with different masses.