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

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 $\psi (2S)\to \mathrm{J}/\psi X$, 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 $\psi (2S)$ 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.

References in corpus (3)

• Search for Higgs and $Z$ Boson Decays to $J/\psi \gamma$ and ${\rm Y}(nS)\gamma$ with the ATLAS Detector
• Measurements of the pp$\to$ZZ production cross section and the Z$\to 4\ell$ branching fraction, and constraints on anomalous triple gauge couplings at $\sqrt{s}=$ 13 TeV
• Search for exclusive Higgs and $Z$ boson decays to $\varphi \gamma$ and $\rho \gamma$ with the ATLAS detector