Exotic h → Za Higgs decays into τ leptons

A bstract Exotic Higgs decays are among the most promising areas to be explored at the High-Luminosity LHC, given the unprecedentedly large amount (~ 3 × 10 8 ) of 125 GeV Higgs bosons that will be produced. In this context, we propose a new search channel for which the Higgs boson decays to a (leptonically decaying) Z boson and a light BSM pseudoscalar a , which subsequently decays to a pair of τ -leptons ( h → Za → ℓℓττ ). After performing a validation of existing ATLAS and CMS exotic Higgs decay searches in related channels, we analyze the HL-LHC projected sensitivity of our a → ττ search, targeting the kinematic region where the exotic Higgs decay is two-body. We are able to probe pseudoscalar masses m a ∈ [5 , 33] GeV by leveraging both leptonic and hadronic τ decays, and establish model-independent 95% C.L. sensitivity projections on the branching fraction BR( h → Za ) × BR( a → ττ ). These a → ττ projections yield a competitive probe of light pseudoscalars, which depending on the model can become significantly more sensitive than projections from existing experimental searches in a → μμ and a → γγ final states. Finally, we explore the potential of our search to probe an Axion-Like-Particle (ALP) solution to the muon ( g − 2) anomaly (when taken face-value), finding that our proposed h → Za , a → ττ search can provide valuable constraints on such ALP scenario, in complementarity with existing h → Za , a → γγ experimental searches.