The first observation of the $\chi_{ c1}(3872)\rightarrow J\mskip -3mu/\mskip -2mu\psi \mu^+\mu^-$ decay is reported using proton-proton collision data recorded with the LHCb detector corresponding to an integrated luminosity of $9fb^{ -1}$. The decay mode is observed for the first time, with a significance of $6.5\sigma$. Its branching fraction is measured relative to the $\chi_{ c1}(3872)\rightarrow J\mskip -3mu/\mskip -2mu\psi \pi^+\pi^-$ decay mode \begin{ align*} \frac{ \cal{ BF}(\chi_{ c1}(3872)\rightarrow J\mskip -3mu/\mskip -2mu\psi \mu^+\mu^-)}{ \cal{ BF}(\chi_{ c1}(3872)\rightarrow J\mskip -3mu/\mskip -2mu\psi \pi^+\pi^-)} = \left(1.64\pm 0.32\pm 0.05\right)\times10^{ -3}, \end{ align*} where the first uncertainty includes both statistical contributions and systematic contributions which are uncorrelated between data-taking periods, and the second represents the systematic contributions that are correlated between data-taking periods.
Categories: hep-ex