Analysis of Vector-Meson-Photoproduction Experiments in theρ−ωInterference Region

Abstract

Vector-meson photoproduction on nuclei in the ${\ensuremath{\rho}}^{0}\ensuremath{-}\ensuremath{\omega}$ interference region has been observed both by means of experiments detecting photoproduction of ${e}^{+}{e}^{\ensuremath{-}}$ pairs and by means of experiments detecting photoproduction of ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ pairs. These two phenomena are closely related to each other and to the corresponding interference phenomenon in ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$. It is shown how the data on all such phenomena can be expressed in terms of three model-independen complex parameters: $\ensuremath{\epsilon}$, the intrinsic $\ensuremath{\rho}\ensuremath{-}\ensuremath{\omega}$ mixing parameter, ${r}_{\ensuremath{\omega}}$, the ratio of $\ensuremath{\omega}$-photon to $\ensuremath{\rho}$-photon coupling at an energy corresponding to the mass of the $\ensuremath{\omega}$, and $\frac{{P}_{\ensuremath{\omega}}}{{P}_{\ensuremath{\rho}}}$, the ratio of nuclear photoproduction amplitudes for $\ensuremath{\omega}$ and $\ensuremath{\rho}$. Existing data on the interference phenomena are combined with data on the ratios of $\ensuremath{\rho}$ and $\ensuremath{\omega}$ production cross sections to determine these parameters. It is found that $|\ensuremath{\epsilon}|=0.034\ifmmode\pm\else\textpm\fi{}0.004$, $|{r}_{\ensuremath{\omega}}|\ensuremath{\equiv}\frac{{\ensuremath{\gamma}}_{\ensuremath{\rho}}}{{\ensuremath{\gamma}}_{\ensuremath{\omega}}}=0.34\ifmmode\pm\else\textpm\fi{}0.07$, and $|\frac{{P}_{\ensuremath{\rho}}}{{P}_{\ensuremath{\omega}}|}=3.44\ifmmode\pm\else\textpm\fi{}0.19$ is the most consistent representation of the data. Because present experiments have uncertainties at the 10% level, it turns out that only relative phases can be determined. If ${r}_{\ensuremath{\omega}}$ is assumed to be real, the most consistent representation yields $\mathrm{arg}\ensuremath{\epsilon}={95}^{\ensuremath{\circ}}\ifmmode\pm\else\textpm\fi{}{15}^{\ensuremath{\circ}}$ and $\mathrm{arg}(\frac{{P}_{\ensuremath{\omega}}}{{P}_{\ensuremath{\rho}}})={0}^{\ensuremath{\circ}}\ifmmode\pm\else\textpm\fi{}{21}^{\ensuremath{\circ}}$. The former result is in agreement with the expectations from mixing theory. The latter is interpreted in terms of a simple vector-dominance model and is shown to agree with simple peripheralism, that is, equality of the $\ensuremath{\rho}$ and $\ensuremath{\omega}$ forward scattering amplitudes. Because of disagreements in the data from different laboratories, there may be some doubt about the reality of ${r}_{\ensuremath{\omega}}$. It appears that the phase of ${r}_{\ensuremath{\omega}}$ could be determined in a model-independent way by an electron-positron colliding-beam experiment on interference in ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ production at the 1% level.