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

Understand

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 $\lt$ $m_\mathrm{a}$ $\lt$ 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.