Production of vector mesons by muons in a hybrid bubble-chamber experiment

We present results on vector-meson leptoproduction in the SLAC fast-cycling 40-in. hydrogen bubble chamber exposed to 16-GeV/c muons. From 3644 inelastic events we obtained 495 events of ${\mu }^{-}p\to {\mu }^{-}{\pi }^{+}{\pi }^{-}p$, of which approximately 160 are $\rho$ events with $W>\mathrm{}2\mathrm{}$ GeV and $Q^2>0.05\mathrm{}$ ${\mathrm{GeV}}^2$. Analyzing these data in the same way as in our previous photoproduction experiment, we find the ${\rho }^0$ contribution to ${\sigma }_{\mathrm{tot}}$ decreases by about 35% between $Q^2=0\mathrm{} \mathrm{and} 1$ ${\mathrm{GeV}}^2$, and the exponential slope parameter becomes approximately 20% smaller, although within errors it is still consistent with the photoproduction value. The decay angular distribution of the ${\rho }^0$ shows a substantial ${cos}^2\theta$ component which can be interpreted as evidence for production by longitudinal photons assuming $s$-channel helicity is conserved. Data from 39 $\omega$-production events are also presented. Upper limits for $\phi$ and ${\rho }^{\prime }$ production are given.