The differential cross section has been measured for the reaction γ +p→p+ π o at the Bonn 2.5 GeV electron synchrotron in the energy range from 0.4 to 2.2 GeV for a c.m. angle of 150 degrees. The protons were detected in a magnetic spectrometer system. The excitation curve shows a distinct resonance structure. The total corrections to the counting rate are about 3%. The contribution of the process γ +p→p+2 π was separated. The uncertainty of this separation leads to an error of about 4% in the cross section.
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The differential cross section for the reaction γp → π 0 p at forward angles has been measured in the energy region between 350 MeV and 1175 MeV. A phenomenological multiple analysis was carried out on the present data together with other data.
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The asymmetry ratioA = (σ⊥ -σ∥)/(σ⊥ +σ∥) has been measured by means of linearly polarized γ-rays for π0 and π+ photoproduction. For the reaction γ + p → n + π+, measurements were taken for θc.m. = 135° at γ-ray energies ranging from 390 to 909 MeV. For the reaction γ + p → p + π0, measurements were taken for θc.m. = 60° and 90° at γ-ray energies ranging from 426 to 918 Mev.
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Cross sections for the photoproduction of neutral pions have been measured at the 1.1-GeV Frascati electron synchrotron for bombarding photon energies k between 400 and 800 MeV and for π0 c.m. angles of θπ*=90∘, 120∘, and 135∘. The main feature of the experiment is good resolution in incident photon energy. The results are in good agreement with the existing theories in the energy range of 450 to 550 MeV. The cross sections exhibit a smooth behavior as a function of energy for k=400−600 MeV. No immediate evidence is found of a contribution of the P11 resonance. An anomaly at the limit of statistical significance appears for k≃700−740 MeV, indicating a possible structure of the so-called second resonance. We attempt to interpret the observed anomaly as a reflection of the sharp opening of the η production channel (η cusp effect).
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We report measurements of the differential cross section for photoproduction of π0 mesons from hydrogen, with the pion emerging near 0 deg, in the photon energy range 290 to 700 MeV. The results show no unusual behavior of the cross section in the forward direction. They are consistent with the angular distribution characteristic of a magnetic-dipole transition to a P32 state. The results agree reasonably well with theoretical predictions of Gourdin and Salin, but disagree with a prediction of DeTollis and Verganelakis. Least-squares fits in powers of cosθ have been made to the available angular distributions.
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The bremsstrahlung beam of the Cornell Bev electron synchrotron has been used to study the reaction γ+p→π0+p over the photon energy range 250 Mev to 1 Bev, and for center-of-mass pion angles between 20° and 70°. The recoil protons, of energies between 10 and 60 Mev, were identified and their energies determined using a range telescope of eight thin plastic scintillators enclosed in a vacuum chamber with the thin liquid hydrogen target. Correlated pulse-height information was obtained by photographing an oscilloscope display and was used to sort out the protons from mesons and electrons. Corrections were made for the background of photoprotons from the Mylar target cup, the energy loss of the protons in the liquid hydrogen, absorption and scattering in the counter telescope, and the variation of beam intensity profile with energy. Compared with previous experiments and extrapolations the results show a somewhat smaller forward differential cross section above 400 Mev. The angular distributions obtained from a least-squares fit to all existing data indicate a d32 assignment for the 760-Mev resonance level. Other implications of the data are also discussed.
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