Rare exclusive decays of the Z-boson revisited

TL;DR

This paper provides a detailed theoretical analysis of rare Z-boson decays into a meson and a photon, offering predictions that can serve as benchmarks for similar Higgs decays and exploring the underlying QCD effects.

Contribution

It offers the first comprehensive calculation of Z-boson rare decays into mesons and a photon, including QCD and relativistic corrections, with detailed uncertainty estimates.

Findings

Predicted branching ratios for Z → J/ψ+γ, Z → Υ+γ, Z → φ+γ.

Inclusion of direct-production and fragmentation contributions.

Quantified theoretical uncertainties in decay predictions.

Abstract

The realization that first- and second-generation Yukawa couplings can be probed by decays of the Higgs boson to a meson in association with a photon has renewed interest in such rare exclusive decays. We present here a detailed study of the rare $Z$-boson processes $Z \to J/\psi+\gamma$, $Z \to \Upsilon+\gamma$, and $Z \to \phi+\gamma$ that can serve as benchmarks for the analogous Higgs-boson decays. We include both direct-production and fragmentation contributions to these decays, and consider the leading QCD corrections and the relativistic corrections to the $J/\psi$ and $\Upsilon$ processes. We present numerical predictions for the branching ratios that include a careful accounting of the theoretical uncertainties.