Search for rare decays of Z and Higgs bosons to J/ψ and a photon in proton-proton collisions at √s = 13 TeV

Abstract

A search is presented for decays of \(Z\) and Higgs bosons to a \(J/\psi \) meson and a photon, with the subsequent decay of the \(J/\psi \) to \({\mu }^{+}{\mu }^{-}\). The analysis uses data from proton-proton collisions with an integrated luminosity of 35.9 \(\phantom{\rule{0.166667em}{0ex}}{\text{fb}}^{-1}\) at \(\sqrt{s}=13\phantom{\rule{0.166667em}{0ex}}\text{TeV}\) collected with the CMS detector at the LHC. The observed limit on the \(Z\to J/\psi \gamma \) decay branching fraction, assuming that the \(J/\psi \) meson is produced unpolarized, is \(1.4×{10}^{-6}\) at 95% confidence level, which corresponds to a rate higher than expected in the standard model by a factor of 15. For extreme-polarization scenarios, the observed limit changes from \(-13.6\) to \(+8.6%\) with respect to the unpolarized scenario. The observed upper limit on the branching fraction for \(H\to J/\psi \gamma \) where the \(J/\psi \) meson is assumed to be transversely polarized is \(7.6×{10}^{-4}\), a factor of 260 larger than the standard model prediction. The results for the Higgs boson are combined with previous data from proton-proton collisions at \(\sqrt{s}=8\phantom{\rule{0.166667em}{0ex}}\text{TeV}\) to produce an observed upper limit on the branching fraction for \(H\to J/\psi \gamma \) that is a factor of 220 larger than the standard model value.