CMS-PAS-SUS-23-005

CMS-PAS-SUS-23-005
Search for a light pseudoscalar Higgs boson in final states with boosted $ \mu\mu $ and $ \tau\tau $ pairs in proton-proton collisions at $ \sqrt{s}= $ 13 TeV
CMS Collaboration
2026-03-18

Abstract: A search for a light pseudoscalar Higgs boson ($ \mathrm{a} $) is performed using LHC data collected at $ \sqrt{s}= $ 13 TeV by the CMS experiment from 2016 to 2018, targeting the process $ \mathrm{H} \to \mathrm{a}\mathrm{a} \to \mu\mu\tau\tau $, where the scalar H can be either the 125 GeV state or a heavier one. Due to the large mass difference between H and $ \mathrm{a} $, the two tau leptons are highly boosted and collimated, often failing standard CMS tau lepton reconstruction. To address this, dedicated di-tau reconstruction techniques, including a deep neural network, are developed to increase efficiency for cases where at least one tau decays to hadrons. No significant excess over standard model (SM) backgrounds is observed, and upper limits at the 95% confidence level are set on this process. Model-independent upper limits on the branching ratio for $ m_{\mathrm{H}}= $ 125 GeV range from 5 $ \times10^{-5} $ to 3 $ \times10^{-4} $ for pseudoscalar masses between 3.6 and 21 GeV. For the first time using LHC data, upper limits are obtained in final states with tau leptons for hypothetical scalar boson masses up to 1 TeV, with $ m_{\mathrm{H}}= $ 1 TeV limits ranging from 4 $ \times10^{-4} $ to 1.5 $ \times10^{-3} $ for pseudoscalar masses between 10 and 50 GeV. In addition, limits for SM extensions involving two-Higgs-doublet plus singlet models are set for $ m_{\mathrm{H}}= $ 125 GeV and extend previous LHC results.
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