The absolute cross section for the $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}$ reaction was measured in a 12-MeV total c.m. energy range centered at the mass of the $\varphi$ meson. We observe a deviation from the $s^{-1\mathrm{}}$ dependence of the cross section predicted by quantum electrodynamics when hadronic modifications are neglected. A good fit to the data requires that one take into account $\gamma \to \varphi \to \gamma$ virtual transitions. Direct evidence for such a vacuum-polarization effect is thus established with a confidence level higher than 0.99.

• Received 1 December 1972

DOI:https://doi.org/10.1103/PhysRevLett.30.462