Bubble-Chamber Study of Photoproduction by 2.8- and 4.7-GeV Polarized Photons. I. Cross-Section Determinations and Production ofρ0andΔ++in the Reactionγp→pπ+π−

Abstract

Photoproduction is studied at 2.8 and 4.7 GeV using a linearly polarized monoenergetic photon beam in a hydrogen bubble chamber. We discuss the experimental procedure, the determination of channel cross sections, and the analysis of the channel $\ensuremath{\gamma}p\ensuremath{\rightarrow}p{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$. A model-independent analysis of the ${\ensuremath{\rho}}^{0}$-decay angular distribution allows us to measure nine independent density-matrix elements. From these we find that the reaction $\ensuremath{\gamma}p\ensuremath{\rightarrow}p{\ensuremath{\rho}}^{0}$ proceeds almost completely through natural parity exchange for squared momentum transfers $|t|<1$ Ge${\mathrm{V}}^{2}$ and that the $\ensuremath{\rho}$ production mechanism is consistent with $s$-channel c.m. helicity conservation for $|t|<0.4$ Ge${\mathrm{V}}^{2}$. A cross section for the production of ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ pairs in the $s$-channel c.m. helicity-conserving $p$-wave state is determined. The $\ensuremath{\rho}$ mass shape is studied as a function of momentum transfer and is found to be inconsistent with a $t$-independent Ross-Stodolsky factor. Using a $t$-dependent parametrization of the ${\ensuremath{\rho}}^{0}$ mass shape we derive a phenomenological ${\ensuremath{\rho}}^{0}$ cross section. We compare our phenomenological ${\ensuremath{\rho}}^{0}$ cross section with other experiments and find good agreement for $0.05<|t|<1$ Ge${\mathrm{V}}^{2}$. We discuss the discrepancies in the various determinations of the forward differential cross section. We study models for ${\ensuremath{\rho}}^{0}$ photoproduction and find that the S\"oding model best describes the data. Using the S\"oding model we determine a ${\ensuremath{\rho}}^{0}$ cross section. We determine cross sections and nine density-matrix elements for $\ensuremath{\gamma}p\ensuremath{\rightarrow}{\ensuremath{\Delta}}^{++}{\ensuremath{\pi}}^{\ensuremath{-}}$. The parity asymmetry for ${\ensuremath{\Delta}}^{++}$ production is incompatible with simple one-pion exchange. We compare ${\ensuremath{\Delta}}^{++}$ production with models.