Search for muoproduction of X(3872) at COMPASS and indication of a new state X˜(3872)

We have searched for exclusive production of exotic charmonia in the reaction μ⁺N→μ⁺(J/ψπ⁺π⁻)π^±N^′ using COMPASS data collected with incoming muons of 160 GeV/c and 200 GeV/c momentum. In the J/ψπ⁺π⁻ mass distribution we observe a signal with a statistical significance of 4.1 σ. 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⁻ 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/ψπ⁺π⁻ is determined to be 71±28(stat)±39(syst) pb.