Direct Measurement of the φ(1020) Leptonic Branching Ratio

The process e⁺e^-→μ⁺μ^- has been studied by the SND detector at the VEPP-2M e⁺e^- collider in the φ(1020)-resonance energy region. The measured effective φ meson leptonic branching ratio B(φ→l⁺l^-)≡sqrt[B(φ→e⁺e^-)B(φ→μ⁺μ^-)]  =  (2.89±0.10±0.06)×10^-4 agrees well with the Particle Data Group value B(φ→e⁺e^-)  =  (2.91±0.07)×10^-4, confirming μ- e universality. Without additional assumption of μ- e universality the branching ratio B(φ→μ⁺μ^-)  =  (2.87±0.20±0.14)×10^-4 was obtained.