We have measured large-angle electron-positron pairs from the reaction $\gamma +\mathrm{Be}\to \mathrm{Be}+e^{+}+e^{-}$ in the $e^{+}{e}^{-}$ invariant-mass region $610<m<\mathrm{}850\mathrm{} \frac{\mathrm{MeV}}{c^2}$. The phase of the photoproduction amplitude of the $\rho$ meson at 4.1-6.1 GeV was found to deviate from pure imaginary by 11.8°±4.4° which corresponds to a ratio of the real to imaginary $\rho$-nucleon amplitude of $\beta =0.2\mathrm{}\pm 0.1$.

• Received 24 August 1970

DOI:https://doi.org/10.1103/PhysRevLett.25.1377