Evidence for the phi-meson contribution to vacuum polarization obtained with the Orsay e+e- colliding-beam ring

The absolute cross section for the e⁺e⁻→μ⁺μ⁻ reaction was measured in a 12-MeV total c.m. energy range centered at the mass of the φ meson. We observe a deviation from the s⁻¹ 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 γ→φ→γ virtual transitions. Direct evidence for such a vacuum-polarization effect is thus established with a confidence level higher than 0.99.