Searching for flavor-violating ALPs in Higgs boson decays
Hooman Davoudiasl, Roman Marcarelli, Nicholas Miesch, Ethan T. Neil
Abstract
Pseudoscalar particles, often referred to as axionlike-particles (ALPs), arise in a variety of theoretical contexts. The phenomenology of such states is typically studied while assuming flavor-conserving interactions, yet they can in principle have flavor-violating (FV) couplings to fermions. We consider this general possibility while focusing on models where the ALP has non-negligible coupling to the Standard Model Higgs boson $h$. For a lepton FV ALP $a$ of mass ${m}_{a}\ensuremath{\gtrsim}2\text{ }\text{ }\mathrm{GeV}$, $a\ensuremath{\rightarrow}\ensuremath{\tau}\ensuremath{\ell}$, where $\ensuremath{\ell}\ensuremath{\ne}\ensuremath{\tau}$ is a charged lepton, could have an $\mathcal{O}(1)$ branching fraction, leading to a potentially detectable $h\ensuremath{\rightarrow}aa\ensuremath{\rightarrow}\ensuremath{\tau}\ensuremath{\ell}\ensuremath{\tau}\ensuremath{\ell}$ at the LHC and its future program. We examine this possibility, in light of existing bounds on FV processes, in a general effective theory. We obtain constraints on the effective couplings from both prompt and long-lifetime searches at the LHC; some projections for envisioned measurements are also provided. The implications of the recently announced first results of the muon $g\ensuremath{-}2$ measurement at Fermilab for the ALP interactions considered in our work are also briefly discussed.