Exotic $h\to Za$ Higgs decays into $\tau$ leptons

Abstract

Exotic Higgs decays are among the most promising areas to be explored at the High-Luminosity LHC, given the unprecedentedly large amount $(\sim 3\times 10^8)$ 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 $\tau$-leptons ($h\to Za\to \ell \ell \tau \tau$). 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 $a\to \tau \tau$ search, targeting the kinematic region where the exotic Higgs decay is two-body. We are able to probe pseudoscalar masses $m_a\in [5,33]$ GeV by leveraging both leptonic and hadronic $\tau$ decays, and establish model-independent 95\% C.L. sensitivity projections on the branching fraction $\mathrm{BR}(h\to Za)\times \mathrm{BR}(a\to \tau \tau )$. These $a\to \tau \tau$ 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 $a\to \mu \mu$ and $a\to \gamma \gamma$ 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 $h\to Za$, $a\to \tau \tau$ search can provide valuable constraints on such ALP scenario, in complementarity with existing $h\to Za$, $a\to \gamma \gamma$ experimental searches.

26 pages, 13 figures, references added. Published in JHEP

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