Search for rare decays of Z and Higgs bosons to J$/\psi$ and a photon in proton-proton collisions at $\sqrt{s} =$ 13 TeV

A search is presented for decays of $\mathrm {Z}$ and Higgs bosons to a ${\mathrm {J}/\psi }$ meson and a photon, with the subsequent decay of the ${\mathrm {J}/\psi }$ to $\mathrm {\mu ^+}\mathrm {\mu ^-}$ . 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 $\mathrm {Z}\rightarrow {\mathrm {J}/\psi } \gamma$ decay branching fraction, assuming that the ${\mathrm {J}/\psi }$ 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 $\mathrm {H} \rightarrow {\mathrm {J}/\psi } \gamma$ where the ${\mathrm {J}/\psi }$ 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 $\mathrm {H} \rightarrow {\mathrm {J}/\psi } \gamma$ that is a factor of 220 larger than the standard model value.