Measurement ofρoandφ-Meson Electroproduction

Abstract

We report measurements of ${\rho }^0$ and $\phi$ electroproduction at 19.5 GeV in a wide-aperture spectrometer which detected the scattered electron and the decay products of the vector mesons. Data are presented for the region of virtual-photon mass squared $q^2=-0.25\mathrm{} \mathrm{to} -2.0\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$ and virtual-photoproduction center-of-mass energy squared $s=10\mathrm{} \mathrm{to} 30$ Ge${\mathrm{V}}^2$. As $\left|q^2\right|$ increases, the $\rho$ mass spectrum shape changes and the momentum-transfer distribution broadens. The ratio of the $\rho$ cross section to the total cross section decreases from 11% at $q^2=0\mathrm{}$ to about 6% at $\left|q^2\right|={m_{\rho }}^2$. The ratio for the production of longitudinally to transversely polarized ${\rho }^0$ mesons is ${0.45}_{-0.10\mathrm{}}^{+0.15\mathrm{}}$ at $\left|q^2\right|={m_{\rho }}^2$, and the interference between longitudinal and transverse amplitudes is almost maximal. The relative $\phi$-meson cross section also decreases as $\left|q^2\right|$ increases. Unitary lower bounds on the longitudinal-transverse ratio in inelastic electron scattering are derived.