COMPASS collaboration (228 authors)Bordalo, P.Franco, C.Nunes, A.S.Quaresma, M.Quintans, C.Ramos, S.Silva, L.Stolarski, M.2019-02-042019-02-042018-08-10http://dx.doi.org/10.1016/j.physletb.2018.07.008http://hdl.handle.net/10400.26/27381We have observed exclusive production of the exotic $X(3872)$ hadron in the reaction $\mu^+~N \rightarrow \mu^+ (J\!/\!\psi\pi^+\pi^-)\pi^{\pm}~N'$ using COMPASS data collected with incoming muons of 160~GeV/$c$ and 200~GeV/$c$ momentum. The statistical significance of the $X(3872)$ signal is 6$\sigma$. The shape of the two-pion mass distribution shows disagreement with previous observations. The product of cross section and branching fraction of the $X(3872)$ decay into $J\!/\!\psi\pi\pi$ is determined to be 71$\pm$28(stat)$\pm$39(syst)~pb.engjournal article2019-02-04