Observation of the decay $\mathrm{Z}\to\psi\,\ell^+\ell^-\to\mu^+\mu^-\ell^+\ell^-$ with the CMS detector

TL;DR

This paper reports the first observation of the rare Z boson decay to a J/psi meson and a pair of same-flavor leptons using CMS data, with a significance over 5 sigma, and measures its relative branching fraction.

Contribution

It provides the first experimental evidence of the Z to J/psi lepton pair decay, expanding understanding of Z boson rare decay modes.

Findings

Signal observed with >5 sigma significance.

Measured relative branching fraction: 0.70 ± 0.18 (stat) ± 0.05 (syst).

Results assume no J/psi polarization, with polarization effects potentially altering the measurement by -24% to +22%.

Abstract

The observation of the $\mathrm{Z}$ boson rare decay to a $\psi$ meson and two oppositely charged same-flavour leptons, $\ell^+ \ell^-$, where $\psi$ represents the sum of $\mathrm{J}/\psi$ and $\psi(\mathrm{2S})\to\mathrm{J}/\psi\, X$, and $\ell=\mu,\mathrm{e}$, is presented. The data sample used corresponds to an integrated luminosity of $35.9\,\mathrm{fb}^{-1}$ of proton-proton collisions at a center-of-mass energy of $13\,\mathrm{TeV}$ accumulated by the CMS experiment at the LHC. The signal is observed with a significance in excess of 5 standard deviations. Removing contributions from $\psi(\mathrm{2S})$ decays to $\mathrm{J}/\psi$, the signal is interpreted as being entirely from $\mathrm{Z}\to\mathrm{J}/\psi\,\ell^+\ell^-$, with its fiducial branching fraction relative to that of the decay $\mathrm{Z}\to\mu^+\mu^-\mu^+\mu^-$ measured to be $$\frac{\mathcal{B}(\mathrm{Z}\to\mathrm{J}/\psi\,\ell^+\ell^-)}{\mathcal{B}(\mathrm{Z}\to\mu^+\mu^-\mu^+\mu^-)}=0.70\pm 0.18\, \mathrm{(stat)} \pm 0.05\, \mathrm{(syst)}.$$ This result is obtained with the assumption of no $\mathrm{J}/\psi$ polarisation, where extreme polarisation scenarios can create $-24\%$ to $+22\%$ variations.