We have searched for exclusive production of exotic charmonia in the reaction $mu^+~N rightarrow mu^+ (J!/!psipi^+pi^-)pi^{pm}~N$ using COMPASS data collected with incoming muons of 160 GeV/$c$ and 200 GeV/$c$ momentum. In the $J!/!psipi^+pi^-$ mass distribution we observe a signal with a statistical significance of 4.1 $sigma$. Its mass and width are consistent with those of the $X(3872)$. The shape of the $pi^+pi^-$ mass distribution from the observed decay into $J!/!psipi^+pi^-$ shows disagreement with previous observations for $X(3872)$. The observed signal may be interpreted as a possible evidence of a new charmonium state. It could be associated with a neutral partner of $X(3872)$ with $C = -1$ predicted by a tetraquark model. The product of cross section and branching fraction of the decay of the observed state into $J!/!psipi^+pi^-$ is determined to be 71$pm$28(stat)$pm$39(syst) pb.
تحميل البحث