Search for muoproduction of the X(3872) at COMPASS

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/ψπ + π− with the statistical significance of 4.1 σ was observed in the reaction µ + N → µ +(J/ψπ + π− )π± N ′. Its mass and width are consistent with those of the X(3872). The shape of the π + π− mass distribution from the observed decay into J/ψπ + π− is different from previous observations for X(3872). The observed signal may be interpreted as possible evidence of a new charmonium state (3872). It could be associated with a neutral partner of X(3872) with C = −1 predicted by a tetraquark model.