Low-energy Universality and the New Charmonium Resonance at 3870 MeV

Abstract

The recently-discovered narrow charmonium resonance near 3870 MeV is interpreted as a hadronic molecule whose constituents are the charm mesons D^0 and \bar D^{*0} or \bar D^0 and D^{*0}. Because of an accidental fine-tuning of the molecule to very near the D^0 \bar D^{*0} threshold, it has some universal properties that are completely determined by the unnaturally large D^0 \bar D^{*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 \bar 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 D^0 \bar D^{*0} threshold.