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

Searches for the exclusive decays of the Higgs and Z bosons into a J/ψ, ψ(2S), or ϒ(nS) (n=1,2,3) meson and a photon are performed with a pp collision data sample corresponding to an integrated luminosity of 36.1fb⁻¹ collected at √s=13TeV with the ATLAS detector at the CERN Large Hadron Collider. No significant excess of events is observed above the expected backgrounds, and 95% confidence-level upper limits on the branching fractions of the Higgs boson decays to J/ψ γ, ψ(2S)γ, and ϒ(nS)γ of 3.5×10⁻⁴, 2.0×10⁻³, and (4.9,5.9,5.7)×10⁻⁴, respectively, are obtained assuming Standard Model production. The corresponding 95% confidence-level upper limits for the branching fractions of the Z boson decays are 2.3×10⁻⁶, 4.5×10⁻⁶ and (2.8,1.7,4.8)×10⁻⁶, respectively.