Higgs decay into a lepton pair and a photon: a roadmap to $\mathbf{H\to Z\gamma}$ discovery and probes of new physics

TL;DR

This paper presents a method to disentangle different contributions to Higgs decay into a lepton pair and a photon, enabling the discovery of specific decay modes and probing potential new physics effects.

Contribution

It introduces a gauge-independent approach to separate and measure the $H o Z o ext{leptons}$ and non-resonant contributions in $H o ext{leptons} ext{-} ext{photon}$ decays, facilitating new physics searches.

Findings

Proposes kinematic cuts to isolate decay contributions.

Defines a method to extract $ ext{H} o Z ext{gamma}$ decay rate.

Outlines milestones for discovering $H o Z ext{gamma}$ and $H o ext{muons} ext{-} ext{photon}$.

Abstract

The decay $H\to \ell^+\ell^- \gamma$, $\ell=e,\mu$, receives contributions from $H\to Z[\to \ell^+\ell^-] \gamma$ and a non-resonant contribution, both of which are loop-induced. We describe how one can separate these sub-processes in a gauge-independent way, define the decay rate $\Gamma(H\to Z\gamma)$, and extract the latter from differential $H\to \ell^+\ell^- \gamma$ branching ratios. For $\ell=\mu$ also the tree decay rate, which is driven by the muon Yukawa coupling, is important. We propose kinematic cuts optimized to separate the three contributions, paving the way to the milestones (i) discovery of $H\to Z \gamma$, (ii) discovery of $H\to \mu^+\mu^- \left. \gamma\right|_{\rm tree}$, and (iii) quantification of new physics in both the effective $H$-$Z$-$\gamma$ and non-resonant $H$-$\ell^+$-$\ell^-$-$\gamma$ couplings.