ηand Pion Decays into Lepton Pairs

Abstract

A recent measurement of the branching ratio $\frac{\Gamma (\eta \to {\mu }^{+}{\mu }^{-})}{\Gamma (\eta \to 2\gamma )}$ yielded the value (5.9±2.2) × ${10}^{-5\mathrm{}}$. We examine here the implication of this result on theoretical predictions for the real part of the $\eta \to {\mu }^{+}{\mu }^{-}$ decay amplitude and conclude that the branching ratio is uncomfortably large for most of the models proposed so far. We then evaluate the $\eta \to {\mu }^{+}{\mu }^{-}$ decay rate in the baryon loop model. Predictions are also made for the associated decays $\eta \to \gamma {\mu }^{+}{\mu }^{-}$, ${\pi }^0\to e^{+}{e}^{-}$, and ${\pi }^0\to \gamma e^{+}{e}^{-}$.