Searching for Flavor-Violating ALPs in Higgs Decays

TL;DR

This paper explores the potential for detecting flavor-violating axion-like particles (ALPs) in Higgs decays at the LHC, focusing on models where ALPs couple to the Higgs and induce distinctive tau-lepton signatures.

Contribution

It introduces a comprehensive analysis of flavor-violating ALPs with Higgs couplings, providing constraints and projections for their detection at collider experiments.

Findings

Potential for large branching ratios of Higgs to ALP pairs decaying into tau and lepton.

Constraints derived from existing prompt and long-lifetime LHC searches.

Projections for future measurements and implications of muon g-2 results.

Abstract

Pseudo-scalar particles, often referred to as axion-like-particles (ALPs), arise in a variety of theoretical contexts. The phenomenology of such states is typically studied assuming flavor-conserving interactions, yet they can in principle have flavor-violating (FV) couplings to fermions. We consider this general possibility, 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 $m_a \gtrsim 2$ GeV, $a\to \tau \ell$, where $\ell\neq \tau$ is a charged lepton, could have $\mathcal{O}(1)$ branching fraction, leading to potentially detectable $h \to a a \to \tau \ell \tau \ell$ 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 $g-2$ measurement at Fermilab for the ALP interactions considered in our work are also briefly discussed.