Investigation of the electromagnetic structure of the η meson in the decay

600 events of the rare electromagnetic decay η → μ+μ−γ have been detected. The branching ratio has been measured as BR(η → μ+μ−γ) = (3.1 ± 0.4) × 10−4. The electromagnetic form factor of the η meson, F(mμμ2;0) = (1 − mμμ2/Λ2)−1, has been determined. Λ = (0.72 ± 0.09) GeV/c2 is in good agreement with the vector meson dominance model.