Exotic $h\to Za$ Higgs decays into $\ddot{I}„$ 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 $\ddot{I}„$-leptons ($h\to Za\to \ell \ell \ddot{I}„\ddot{I}„$). 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 $a\to \ddot{I}„\ddot{I}„$ 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 $\ddot{I}„$ decays, and establish model-independent 95\% C.L. sensitivity projections on the branching fraction $\mathrm{BR}(h\to Za)\times \mathrm{BR}(a\to \ddot{I}„\ddot{I}„)$. These $a\to \ddot{I}„\ddot{I}„$ 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 $a\to \hat{I}¼\hat{I}¼$ and $a\to {\hat{I}}^3{\hat{I}}^3$ 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 $h\to Za$, $a\to \ddot{I}„\ddot{I}„$ search can provide valuable constraints on such ALP scenario, in complementarity with existing $h\to Za$, $a\to {\hat{I}}^3{\hat{I}}^3$ experimental searches.

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