Pion Photoproduction at 0

The special features of forward pion photoproduction at high energies are discussed. It is established that the cross section for single pion production can generally be expected to have a forward dip barring a conspiracy between Regge trajectories. Photoproduction of the A,(l+) meson followed by its decay is suggested as an important source of high energy forward pions. Submitted to Phys. Rev. Letters * Work supported by U. S. Atomic Energy Commission. Forwa,rd photo pion production at high energies has simple features of special interest. Real photons, being transversely polarized, introduce into the production amplitude a unit of (spin) angular momentum along their direction of motion. This unit of spin cannot be carried off _ byi zero spin pion pro?,lced at p-~ecis~ly th.: f?r:ia,'? angle 6 = 0' since its orbital angular momentum is normal to its direction of motion. A unit of spin must therefore be transmitted to the target. This requirement suppresses the contribution at @ = 0' of the t channel exchanges that are normally assumed to dominate the high energy, low momentum transfer beha.vior of this process. We may thus hope to learn interesting, new, and unusual aspects of photoproduction by studying the processes leading to a pion at 6 = 0'. To be more precise we are interested in production angles 0 < 8 P = I-+ which is the characteristic angle for the peripheral photoproduction events leading to pions of mass p and energy UJ; u) = large fraction of the photon energy k. Earlier experiments' on photoproduction up to photon energies k6 BeV have consistently indicated appreciable pion fluxes at angles 0 < BP at variance with all theoretical models2 which predict a dip in the angular distribution in the region of 8 < 8 . P More recently the beam survey measurements' at SLAC have confirmed and extended these observations to higher energies. These results have triggered this investigation 4 which will show that a) In a Regge pole theory analysis, the amplitude for single pion production at high energy will have a