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

TL;DR

This paper searches for rare Z and Higgs boson decays to J/ψ and a photon in proton-proton collisions at 13 TeV, setting upper limits on their branching fractions and comparing with Standard Model predictions.

Contribution

It provides the first limits on Z and Higgs boson decays to J/ψ and a photon at 13 TeV, including polarization considerations, and combines results with previous data for improved constraints.

Findings

Observed limit on Z → J/ψ γ branching fraction is 1.4×10⁻⁶.

Observed limit on H → J/ψ γ branching fraction is 7.6×10⁻⁴.

Limits are significantly above Standard Model predictions.

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 fb$^{-1}$ at $\sqrt{s} =$ 13 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 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.