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Enhanced Rare Pion Decays from a Model of MeV Dark Matter
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A model has been proposed in which neutral scalar particles chi, 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 chi 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 pi0 -> e+e-. A recent measurement by the KTeV Collaboration lies three 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.
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Improved Standard-Model predictions for $\eta^{(\prime)}\to \ell^+ \ell^-$
Updated SM predictions yield Br(η→e⁺e⁻)=5.37(4)(2)[4]×10⁻⁹, Br(η→μ⁺μ⁻)=4.54(4)(2)[4]×10⁻⁶, Br(η'→e⁺e⁻)=1.80(2)(3)[3]×10⁻¹⁰, and Br(η'→μ⁺μ⁻)=1.22(2)(2)[3]×10⁻⁷, with a mild 1.6σ tension in the η→μ⁺μ⁻ channel.
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