Search for a light pseudoscalar Higgs boson in the boosted $\mu \mu \tau \tau$ final state in proton-proton collisions at $\sqrt{s}=$ 13 TeV

Abstract

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 $\sqrt{s}=$ 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 $\to$ aa $\to$ $\mu \mu \tau \tau$, and considers pseudoscalar masses in the range 3.6 $<$ $m_{\mathrm{a}}$ $<$ 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 $\mu \mu$ and the $\tau \tau$ pairs have high Lorentz boost and are collimated. The $\tau \tau$ reconstruction efficiency is increased by modifying the standard technique for hadronic $\tau$ 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 $m_{\mathrm{a}}$, on the branching fraction ($\mathcal{B}$) for H $\to$ aa $\to$ $\mu \mu \tau \tau$, down to 1.5 (2.0) $\times$ 10${}^{-4}$ for $m_{\mathrm{H}}=$ 125 (300) GeV. Model-dependent limits on $\mathcal{B}$(H $\to$ 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.

Replaced with the published version. Added the journal reference and the DOI. All the figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/HIG-18-024 (CMS Public Pages)

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