Rare exclusive decays of theZboson 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 → J/ψ + γ, Z → Υ+γ, and Z → 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. *

Searches for exclusive Higgs and Z boson decays into J/ψ γ, ψ(2S) γ, and ϒ(nS) γ at s=13TeV with the ATLAS detector

“…The Higgs and Z boson decays were simulated as a cascade of two-body decays, accounting for effects of the quarkonium helicity on the µ + µ − kinematics. The quarkonium state is simulated to be transversely polarised in the case of the Higgs boson decay and longitudinally polarised in the case of the Z boson decay [34]. The branching fractions for the decays Q → µ + µ − are taken from Ref.…”

“…In addition, the ATLAS Collaboration searched for the rare Higgs decays H → φ γ and H → ρ γ [30,31].Owing to the large Z boson production cross section at the LHC, rare Z boson decays can be probed at much lower rates than for Higgs boson decays into the same final state. Branching fractions for Z → Q γ decays have been calculated to be between 10 −8 and 10 −7 for both the Z → J/ψ γ and Z → Υ(nS) γ decays [32][33][34]. Measurements of the branching fractions for these decays would provide a sensitive test of the SM and the factorisation approach in quantum chromodynamics (QCD), since the power corrections in terms of the ratio of the QCD energy scale to the vector-boson mass are small [33].…”

Prospects for detecting light bosons at the FCC-ee and CEPC

“…Note that the vertex admits the Bose symmetry under Z(p 1 , ǫ 1 ) ⇔ Z(p 2 , ǫ 2 ) exchange. By using the values [44,46]: M J/ψ = 3.0969 GeV, M Υ = 9.4603 GeV, φ J/ψ 0 = 0.270(20) (GeV) 3/2 , and φ Υ 0 = 0.715(24) (GeV) 3/2 , we have A c ZZV = 0.0866(64) (GeV) 2 and A b ZZV = 0.723(24) (GeV) 2 . Now we can calculate the SM background Z(P Z ) → V Q (P V )ν(k 1 )ν(k 2 ).…”

Exposing dark sector with futureZ-factories

“…There have been previous work on constraining dark sector related new physics from Z properties at future e + e − colliders, including dark photon [41,42], sterile neutrino model [43,44], Z invisible width e.g. [45], rare SM Z decays [46][47][48][49][50], light CP-odd Higgs bosons and supersymmetric models [51][52][53][54][55], . Recently, this topic has been addressed for some specific models [56][57][58][59][60].…”