Rare exclusive decays of the Z-boson revisited

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 $Z \to J/ψ+γ$, $Z \to Υ+γ$, and $Z \to ϕ+γ$ 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 $J/ψ$ and $Υ$ processes. We present numerical predictions for the branching ratios that include a careful accounting of the theoretical uncertainties.