Low-energy universality and the new charmonium resonance at 3870 MeV

The recently discovered narrow charmonium resonance near 3870 MeV is interpreted as a hadronic molecule whose constituents are the charm mesons D⁰ and D*0 or D⁰ and D*0. Because of an accidental fine-tuning of the molecule to very near the D⁰D*0 threshold, it has some universal properties that are completely determined by the unnaturally large D⁰D*0 scattering length a. Its narrow width can be explained by the suppression by a factor of $1/a$ of decay modes other than the decay of a constituent D*0 or D*0. Its production rates are also suppressed by a factor of $1/a.$ A particularly predictive mechanism for generating the large scattering length is the accidental fine-tuning of a P-wave charmonium state to the D⁰D*0 threshold.