Neutral-Boson Photoproduction on Hydrogen at High Energies

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Abstract

Photoproduction cross sections for neutral $\ensuremath{\pi}$, $\ensuremath{\eta}$, $\ensuremath{\rho}$, and $\ensuremath{\varphi}$ 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 $|t|$ at small $|t|$ values, with a dip at $t=\ensuremath{-}0.5$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$ followed by a secondary maximum around $t=\ensuremath{-}0.9$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$ and a smooth falloff at larger $|t|$ 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 $\ensuremath{\omega}$ and $B$ trajectories. $\ensuremath{\eta}$ photo-production was measured around 6 GeV and at 9 GeV. The cross section decreases smoothly with $t$ and shows no dip at $t=\ensuremath{-}0.5$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$, in disagreement with predictions based on Reggeized $\ensuremath{\rho}$ exchange. $\ensuremath{\rho}$ 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 \ensuremath{\mu}b at 5.5 GeV to 12.3 \ensuremath{\mu}b at 17.8 GeV incident photon energy. A quark-model relation between $\ensuremath{\pi}\ensuremath{-}p$ elastic scattering and $\ensuremath{\rho}$ photoproduction gives a good representation of the data. $\ensuremath{\varphi}$ 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\ifmmode\pm\else\textpm\fi{}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 $\ensuremath{\rho}$ cross section would have been visible.

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