Exclusive vector-meson production in muon-proton scattering

From a muon-proton scattering experiment with a streamer chamber at the Stanford Linear Accelerator we present results in the ranges $0.3<\mathrm{}{Q}^2<4.7\mathrm{}$ Ge${\mathrm{V}}^2$ and $1.7<W<\mathrm{}4.7\mathrm{}$ GeV for the reactions ${\mu }^{+}p\to \mu \mathrm{pV}$ where $V$ is a vector meson (${\rho }^0$, $\omega$, or $\phi$). It is shown that in $\rho$ production the skewing parameter and the longitudinal-transverse ratio change significantly as $Q^2$ increases above 1 Ge${\mathrm{V}}^2$. The cross section for ${\rho }^0$ production as a function of $Q^2$ falls below the vector-meson-dominance prediction. The ratio of the cross section for exclusive vector-meson production to the total cross section falls by a factor of 10 between photoproduction and a $Q^2$ of 2 Ge${\mathrm{V}}^2$, yet the ratio of $\omega$ to $\rho$ production remains constant at the photoproduction value out to $Q^2>2\mathrm{}$ Ge${\mathrm{V}}^2$.