Observation of the decay ${\chi }_{c1}(3872)\to J\text{\hspace{0.17em}⁣}\mathrm{/}\psi {\mu }^{+}{\mu }^{-}$

The first observation of the ${\chi }_{c1}(3872)\to J\text{\hspace{0.17em}⁣}\mathrm{/}\psi {\mu }^{+}{\mu }^{-}$ decay is reported using proton-proton collision data recorded with the LHCb detector corresponding to an integrated luminosity of $9f{b}^{-1}$. The decay mode is observed for the first time, with a significance of $6.5\sigma$. Its branching fraction is measured relative to the ${\chi }_{c1}(3872)\to J\text{\hspace{0.17em}⁣}\mathrm{/}\psi {\pi }^{+}{\pi }^{-}$ decay mode \begin{align*} \frac{\cal{BF}(χ_{c1}(3872)\rightarrow J\mskip -3mu/\mskip -2muψμ^+μ^-)}{\cal{BF}(χ_{c1}(3872)\rightarrow J\mskip -3mu/\mskip -2muψπ^+π^-)} = \left(1.68\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.