Neutral-Boson Photoproduction on Hydrogen at High Energies

Abstract

Photoproduction cross sections for neutral $\pi$, $\eta$, $\rho$, and $\phi$ mesons have been measured at the Stanford Linear Accelerator Center for photon energies between 5 and 17.8 GeV, and $t$ (four-momentum transfer squared) between -0.12 and -1.4 ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$, using a missing-mass technique. The pion production at lower energies is characterized by a fast falloff with increasing $\mathrm{}\left|t\right|\mathrm{}$ at small $\mathrm{}\left|t\right|\mathrm{}$ values, with a dip at $t=-0.5\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$ followed by a secondary maximum around $t=-0.9\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$ and a smooth falloff at larger $\mathrm{}\left|t\right|\mathrm{}$ values. As the incident photon energy increases, the dip becomes less pronounced, in contradiction to the expectations of simple Regge theories based only on the exchange of the $\omega$ and $B$ trajectories. $\eta$ photo-production was measured around 6 GeV and at 9 GeV. The cross section decreases smoothly with $t$ and shows no dip at $t=-0.5\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$, in disagreement with predictions based on Reggeized $\rho$ exchange. $\rho$ production rates agree well with predictions assuming diffraction production. The differential cross section varies approximately as $e^{8.5t}$. The total cross section decreases from 16.0 μb at 5.5 GeV to 12.3 μb at 17.8 GeV incident photon energy. A quark-model relation between $\pi -p$ elastic scattering and $\rho$ photoproduction gives a good representation of the data. $\phi$ production also appears consistent with the predictions of the diffraction-dissociation model. We also searched for evidence of photoproduction of other particles with mass up to 2 GeV. Production of one particle of mass 1240±20 MeV and width around 100 MeV was observed. No particles with mass between 1300 and 2000 MeV were found. Any particle with cross section larger than 4% of the $\rho$ cross section would have been visible.