Diffractive production of vector mesons in muon-proton scattering at 150 and 100 GeV
W. D. ShambroomRichard WilsonB. A. GordonW. A. LoomisF. M. PipkinS. PordesLynn VerheyH. L. AndersonR. FineR. H. HeisterbergH. S. MatisL. W. MoL. C. MyrianthopoulosS. C. WrightW. R. FrancisR. G. HicksT. KirkV. BharadwajN. E. BoothG. I. KirkbrideT. W. QuirkAndris SkujaW.S.C. Williams
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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 $t$.Keywords:
Helicity
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