Enhanced rare pion decays from a model of MeV dark matter

A model has been proposed in which neutral scalar particles χ, of mass 1-10 MeV, annihilate through the exchange of a light vector boson U, of mass 10-100 MeV, to produce the 511 keV line observed emanating from the center of the Galaxy. The χ interacts weakly with normal matter and is a viable dark matter candidate. If the U boson couples to quarks as well as to electrons, it could enhance the branching ratio for the rare decay π0→e+e-. A recent measurement by the KTeV Collaboration lies 3 standard deviations above a prediction by Dorokhov and Ivanov, and we relate this excess to the couplings of the U boson. The values are consistent with other constraints and considerations. We make some comments on possible improvements in the data.