Measurement of $\eta$ meson decays into lepton-antilepton pairs
M. Ber{\l}owski \fIet al.\fP

A search for rare lepton decays of the $\eta$ meson was performed using the WASA detector at CELSIUS. Two candidates for double Dalitz decay $\eta\to e^+e^-e^+e^-$ events are reported with a background of 1.3$\pm$0.2 events. This allows to set an upper limit to the branching ratio of 9.7$\times$10$^{-5}$ (90\% CL). The branching ratio for the decay $\eta\to e^+e^-\gamma$ is determined to $(7.8\pm0.5_{stat}\pm0.8_{syst})\times10^{-3}$ in agreement with world average value. An upper limit (90\% CL) for the branching ratio for the $\eta\to e^+e^-$ decay is $2.7\times10^{-5}$ and a limit for the sum of the $\eta\to \mu^+\mu^-\mu^+\mu^-$ and $\eta\to \pi^+\pi^-\mu^+\mu^-$ decays is $3.6\times10^{-4}$.

© 2008 The American Physical Society.