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A search for a light pseudoscalar Higgs boson (a) decaying from the 125 GeV (or a heavier) scalar Higgs boson (H) is performed using the 2016 LHC proton-proton collision data at √ s = 13 TeV, corresponding to an integrated luminosity of 35.9 fb −1 , collected by the CMS experiment. The analysis considers gluon fusion and vector boson fusion production of the H, followed by the decay H → aa → µµτ τ , and considers pseudoscalar masses in the range 3.6 < m a < 21 GeV. Because of the large mass difference between the H and the a bosons and the small masses of the a boson decay products, both the µµ and the τ τ pairs have high Lorentz boost and are collimated. The τ τ reconstruction efficiency is increased by modifying the standard technique for hadronic τ lepton decay reconstruction to account for a nearby muon. No significant signal is observed. Model-independent limits are set at 95% confidence level, as a function of m a , on the branching fraction (B) for H → aa → µµτ τ , down to 1.5 (2.0) × 10 −4 for m H = 125 (300) GeV. Model-dependent limits on B(H → aa) are set within the context of two Higgs doublets plus singlet models, with the most stringent results obtained for Type-III models. These results extend current LHC searches for heavier a bosons that decay to resolved lepton pairs and provide the first such bounds for an H boson with a mass above 125 GeV.

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