Muon Pair Production by Photon-Photon Interactions ine+e−Storage Rings

The photon-photon interaction has been investigated by $e^{+}$ and $e^{-}$ collisions at about 2.7-GeV total energy. Evidence based on 34 well-identified events has been obtained for the process $e^{+}{e}^{-}\to e^{+}{e}^{-}{\mu }^{+}{\mu }^{-}$, hitherto unobserved. Such a process is found to occur in agreement with theoretical predictions based on the equivalent-photon approximation. Results on 74 events from the process $e^{+}{e}^{-}\to e^{+}{e}^{-}{e}^{+}{e}^{-}$ are also reported.