Diffractive production of vector mesons in muon-proton scattering at 150 and 100 GeV

Abstract

We have studied the production of $\ensuremath{\rho}$ and $\ensuremath{\varphi}$ mesons from muon scattering on a liquidhydrogen target at 150 and 100 GeV. For the $\ensuremath{\rho}$ we observe a skewed mass distribution which becomes somewhat more normal with increasing ${Q}^{2}$ (the square of the four-momentum transferred from the muon), and an exponential distribution in $t$ (the square of the four-momentum transferred to the target proton) with a slope which is consistent with a slight decrease as ${Q}^{2}$ increases. The dependence of the cross section on ${Q}^{2}$ follows that of the square of the $\ensuremath{\rho}$ propagator with little contribution from longitudinal $\ensuremath{\rho}$ production. The angular distribution of the $\ensuremath{\rho}$ decay confirms the smallness of the contribution from longitudinal $\ensuremath{\rho}$ production at our energies. The cross section when extrapolated to ${Q}^{2}=0$ agrees with that measured in real photoproduction. The decay angular distribution of the $\ensuremath{\rho}$ decay shows that $s$-channel helicity is largely conserved, although we detect a helicity-single-flip contribution at the 10-15% level. Natural-parity exchange in the $t$ channel dominates, and the transverse and longitudinal amplitudes are found to be in phase. These characteristics are consistent with the diffractive nature of the vectordominance model. The $t$ distribution of $\ensuremath{\varphi}$ production is also exponential, although less steep than that for the $\ensuremath{\rho}$. We observe an elastically produced four-pion state at a mass of approximately 1600 MeV. We identify this state with the ${\ensuremath{\rho}}^{\ensuremath{'}}(1600)$, and find it to be produced with a distribution exponential in $t$.