Study of the rare decay $J/\psi \to \mu^+\mu^-\mu^+\mu^-$

TL;DR

This paper reports the first observation of the rare electromagnetic decay of the J/psi meson into four muons, measuring its branching fraction using LHCb data at 13 TeV, with implications for QED and meson decay studies.

Contribution

The study provides the first measurement of the branching fraction for the rare decay J/psi to four muons, using proton-proton collision data at 13 TeV from LHCb.

Findings

First observation of J/psi to four muons decay.

Measured branching fraction of (1.13 ± 0.10 ± 0.05 ± 0.01) × 10^{-6}.

Decay rate significantly exceeds previous expectations.

Abstract

The rare electromagnetic $J/\psi \to \mu^+\mu^-\mu^+\mu^-$ decay is observed with a significance greatly exceeding the discovery threshold, using proton-proton collision data collected by the LHCb experiment during 2016-2018 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $5.4\,\text{fb}^{-1}$. The rate of this decay is measured relative to that of the $J/\psi \to \mu^+\mu^-$ mode. Using the QED model for the four-muon decay in the efficiency estimation, its branching fraction is determined to be \begin{equation*} {\mathcal{B}}(J/\psi \to \mu^+\mu^-\mu^+\mu^-) = (1.13\pm0.10\pm0.05\pm0.01)\times 10^{-6}, \end{equation*} where the uncertainties are statistical, systematic and due to the uncertainty on the branching fraction of the $J/\psi \to \mu^+\mu^-$ decay.