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 ${\text{fb}}^{-1}$ at $\sqrt{s}=13\text{TeV}$ 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 $1.4\times {10}^{-6}$ 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 $7.6\times {10}^{-4}$ , 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 $\sqrt{s}=8\text{TeV}$ 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.