LARGE ANGLE HIGH ENERGY PHOTOPRODUCTION r [ + MESONS FROM LIQUID HYDROGEN OF SINGLE

We have measured angular distributions for single photoproduced r[' mesons at 4.0, 5.0, and 7.5 GeV incident photons energies and at lab angles from XL0 to 66' with the S.L.A .C. 8 GeV spectrometer. Combined with previous S.L.A.C. results, this gives complete angular coverages for this range of energies. The data show the usual "t" 2nd "u" diffraction peeks and a flcentrsl plateau" region dropping as S-7.3. * Work supported by the U.S. Atomic Energy Commission, SC* Work supported in part by the U.S. Atomic Energy Commission under Contract No. AT(ll-1) -881. *Present address: Northeastern University, Boston, Mass. 02115 (Submitted to Phys. Rev. Letters) Previous experiments on the photoproduction of single charged pion production from hydrogen via y + P --sn + + N have been made over the range of It/ values from 0 to 3.0(GeV/c)2 and /ul values from 0 to l.7(GeV/c)2.'1'2) The cross sections in da/dt can be approximated by the formulae da/at a S e -' -3'31tl for 1 < ItI < 3(GeV/c)2 and do/dt CX S -3,I4 for .5 < ]ul < 1.5(GeV/c)2. It seemed of interest to obtain complete engular distributions over several energies, whim could be compared with the elastic SP scattering distributions3. The experiment was a standard single arm "missing mass" identification of the process using an analysis of the kinematic step to provide cross section measurements1'2. The SLAC 8 GeV Spectrometer' was used to analyze pions (and protons) produced by the SLAC bremsstrahlung beam passing through a liquid hydrogen target. The beam layout is shown in Fig. 1. The bremsstrahlung beam could be prepared either with a radiator 52 meters upstream from the liquid hydrogen target (distant-targeting) which was then followed by a sweep magnet to remove the main electron beam, or in order to obtain maximum photon beam intensities could be prepared by placing a radiator 2 meters in front of the hydrogen target and allowing both the photons and electrons to proceed through the target. The pure photon beam obtained with the distant targeting could be monitored with a secondary emission quantameter. The close up targeting was monitored with the standard SLAC toroids to give the number of electrons in the beam and effective photon fluxes were calculated from the amount of radiator in the beam line. These calculations were cross checked by changing the amount of radiator in the beam line, and by comparing the observed rates with the standard bremsstrahlung beam. The rates used for these cross comparisons were measured with the 1.6 GeV spectrometer set to detect low