Search for rare decays of the Z and Higgs bosons to a J/$\psi$ or $\psi$(2S) meson and a photon in proton-proton collisions at $\sqrt{s}$ = 13 TeV

TL;DR

This paper reports a search for rare Z and Higgs boson decays into a photon and a charmonium state, setting upper limits on their branching fractions and constraining related coupling modifiers using 13 TeV proton-proton collision data.

Contribution

The study provides the first experimental upper limits on these rare decay channels and constrains the Higgs coupling modifiers in proton-proton collisions at 13 TeV.

Findings

No evidence for the predicted rare decays was observed.

Upper limits on branching fractions were established at 95% confidence level.

Constraints on the Higgs coupling modifier ratio were derived.

Abstract

A search is presented for rare decays of the Z and Higgs bosons to a photon and a J/$\psi$ or a $\psi$(2S) meson, with the charmonium state subsequentially decaying to a pair of muons. The data set corresponds to an integrated luminosity of 123 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV collected with the CMS detector at the LHC. No evidence for branching fractions of these rare decay channels larger than predicted in the standard model is observed. Upper limits at 95% confidence level are set: $\mathcal{B}$(H $\to$ J/$\psi \gamma$) $\lt$ 2.6 $\times$ 10$^{-4}$, $\mathcal{B}$(H $\to$ $\psi$(2S)$\gamma$) $\lt$ 9.9 $\times$ 10$^{-4}$, $\mathcal{B}$(Z $\to$ J/$\psi \gamma$) $\lt$ 0.6 $\times$ 10$^{-6}$, and $\mathcal{B}$(Z $\to$ $\psi$(2S)$\gamma$) $\lt$ 1.3 $\times$ 10$^{-6}$. The ratio of the Higgs boson coupling modifiers $\kappa_\mathrm{c} / \kappa_\gamma$ is constrained to be in the interval ($-$157, $+$199) at 95% confidence level. Assuming $\kappa_\gamma = 1$, this interval becomes ($-$166, $+$208).