Exotic charmonium-like states have been observed by various experiments over the last 15 years, but their nature is still under discussion. Photo-(muo)production is a new promising instrument to study them. COMPASS, a fixed target experiment at CERN, analyzed the full set of the data collected with a muon beam between 2002 and 2011, covering the range from 7 GeV to 19 GeV in the centre-of-mass energy of the virtual photon-nucleon system. A signal in the mass spectrum of $J/psipi^+pi^-$ with the statistical significance of 4.1 $sigma$ was observed in the reaction $mu^+~N rightarrow mu^+(J/psipi^+pi^- )pi^{pm} N$. 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 $J/psipi^+pi^-$ is different from previous observations for $X(3872)$. The observed signal may be interpreted as possible evidence of a new charmonium state $widetilde{X}(3872)$. It could be associated with a neutral partner of $X(3872)$ with $C = -1$ predicted by a tetraquark model.
تحميل البحث