Observation ofρ−ωInterference in the Photoproduction of Electron-Positron Pairs from Carbon and a Measurement of theρ−ωPhase

The electron-positron mass spectrum has been studied in the process $\gamma +\mathrm{C}\to e^{+}+e^{-}+\mathrm{C}$ in the invariant-mass region between 675 and 850 MeV/${\mathit{c}}^2$. An enhancement in the region of the $\omega$ mass due to $\rho -\omega$ interference is clearly observed, and from a fit to the data we find $\frac{{{\gamma }_{\rho }}^2}{4\pi }={0.50}_{-0.10\mathrm{}}^{+0.12\mathrm{}}$ and $\frac{\gamma {\omega }^2}{4\pi }=3.5\mathrm{}\pm 1.2$. The value of the relative phase of the $\rho -\omega$ photoproduction amplitudes is found to be ${100}_{-30\mathrm{}}^{+38\mathrm{}}$ deg. The errors quoted are purely statistical.