Search for a light pseudoscalar Higgs boson in final states with boosted muon pairs and tau lepton pairs in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Abstract

A search is presented for a light pseudoscalar Higgs boson, a, using LHC proton-proton data at $\sqrt{s}$ = 13 TeV collected by the CMS experiment from 2016 to 2018, corresponding to an integrated luminosity of 138 fb$^{-1}$. The analysis targets the process H $\to$ aa $\to μμττ$, where the scalar H can be either the 125 GeV Higgs boson or a heavier state. Because of the large mass difference between H and a, the two tau leptons are highly Lorentz boosted and collimated, often failing standard CMS tau lepton reconstruction. To address this, dedicated ditau reconstruction techniques, including a deep neural network, are developed to improve the efficiency for events in which at least one tau lepton decays to hadrons. No significant excess over the standard model (SM) background is observed. Model-independent upper limits at the 95% confidence level are set on the product of the Higgs boson production cross section and branching fraction, divided by the SM cross section. For a Higgs boson mass ($m_\mathrm{H}$) equal to 125 GeV, these limits range from 5$\times$10$^{-5}$ to 2.7$\times$10$^{-4}$ for pseudoscalar masses between 3.6 and 21 GeV. For the first time using LHC data, upper limits are also obtained in final states with tau leptons for a hypothetical scalar boson with a mass up to 1 TeV. For $m_\mathrm{H}$ = 1 TeV, the limits range from 3.5$\times$10$^{-4}$ to 1.5$\times$10$^{-3}$ for pseudoscalar masses between 3.6 and 50 GeV. In addition, limits are set for SM extensions involving two-Higgs-doublet plus singlet models for $m_\mathrm{H}$ = 125 GeV, extending previous LHC results.