The reaction γ+p→π0+p has been studied in three adjacent 100-Mev energy intervals between 900 and 1200 Mev and at pion center-of-mass angles of 47°, 90°, and 125°. The reaction was observed as a coincidence between the recoil proton and one of the photons from the meson's decay. The kinematics were determined by the energy of the incident photon and the angle of the recoil proton. The differential cross sections at the forward and backward angles show pronounced maxima near 1050 Mev, while the 90° cross sections decrease slowly with energy. The estimated total cross sections suggest a narrow maximum near 1050 Mev. These features are consistent with the previously proposed existence of a resonant state in the pion-nucleon system of total angular momentum 52.

The ratio of the cross sections for photoproduction of neutral pions from neutrons to that from protons has been obtained at average photon energies of 750, 875, and 1050 mev at a pion CM angle of 60° and at average photon energies of 875 and 1050 mev at a pion CM angle of 90°. The experimental technique required simultaneous detection of both the pions and the nucleons. Pions were detected by three scintillation counters. Lead plates of 2.4 radiation lengths and 1.2 radiation lengths were placed in front of the second and third counters. Neutral pions were identified by the absence of output in the first counter and the large outputs in the second and third counters. Nucleons were detected in two scintillation counters. The second of the two counters is 11” thick and has approximately 20% efficiency of detecting neutrons. Neutrons were identified by the absence of output in the first counter. The energy of the incident photons was determined by synchrotron subtraction. Since the statistical accuracy of synchrotron subtraction is poor, a system of three fast coincidence circuits was used as a time-of-flight instrument to reduce the number of events initiated by low energy photons. The statistical errors assigned to the ratio range between 15-30%. The results of this experiment agree with the results of Bingham within statistical errors, but show a general tendency for the σ^(no)/ σ^o ratio to lower. The ratio of σ^(no)/ σ^o obtained in this experiment ranges between 0.4 and 0.8. The cross sections for neutral pion photoproduction from neutrons are derived from the σ^(no)/ σ^o ratio and the Caltech data on neutral pion photoproduction from hydrogen.

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The cross section for photoproduction of neutral pions from protons has been measured at energies near 750, 915 and 1150 Mev and over most of the forward-going [...] C.M. hemisphere. The experimental technique consisted of detecting both of the [...] decay photons with lead glass total absorption counters and, when convenient, the recoil proton with a single scintillation counter. The method is subject to rather large systematic errors but, within these, our results are consistent with other experiments wherever there are overlapping points. Our data has the striking feature that the cross section is very small at [...] in the region of the second and third pion nucleon resonances. Also, although the data is not inconsistent with a simple first, second and third resonance model, it appears likely that above the third resonance the pole process consisting of the exchange of a single vector meson is becoming important or even dominant. The evidence at this time mildly suggests that this behaviour is largely due to [...] mesons and under that hypothesis we are able to estimate some [...] meson coupling constants. For example, using a prescription of Gell-Mann and Zachariasen, we estimate the partial width for the decay [...] to be 240 Kev.

Measurements of the differential cross section for the process γ+p→π0+p have been made at three pion center-of-mass angles: 60°, 90°, and 120°. Values were obtained at intervals of 0.05 BeV (incident laboratory photon energy, k) from approximately 0.6 to 1.2 BeV. Most of the data were obtained by detecting only the recoil protons with a large, wedge-shaped, single-focusing magnetic spectrometer and associated equipment. For θ′π0=60° and k≤0.94 BeV the π0 decays were also required, the decay photons being detected by a lead glass total absorption counter. Although the experimental resolution was considerably narrower than that of most of the previous experiments, its averaging effect was still appreciable in certain regions. Using a six-parameter fit, the data at each angle were unfolded in an effort to eliminate the effects of resolution and to obtain the true cross sections as a function of energy. The results compare reasonably well with those of previous experiments once differences in resolutions and systematic errors are taken into account. The results did not agree with the predictions of a simple resonance model with the resonance quantum numbers suggested by Peierls. The positions and widths of the two cross-section peaks in this energy region are quite similar to those observed in π−p scattering.

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Measurements of the differential cross section for the process γ+p→π0+p have been made at eight pion center-of-mass angles in the range 51-135° and for incident photon energies from approximately 600-1200 MeV. The bremsstrahlung photon beam used was obtained from the California Institute of Technology electron synchrotron. Both the recoil proton and one γ ray from the decay of the π0 were detected. The incident photon energy was determined by measuring the laboratory angle and time of flight of the recoil proton. The angular distributions obtained indicate that the third pion-nucleon resonance is predominantly a D(52) resonance excited by a magnetic quadrupole transition. It can also be concluded that any contribution to the π0 photoproduction cross section from a virtual vector-meson exchange process is probably negligible in the region of the second and third pion-nucleon resonances.

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We measured the π0 photoproduction differential cross section at 180° for a range of incident photon energies between 650 and 1750 MeV. The cross sections are dominated by the D13(1525), D15(1688), and F37(1920) resonances.

π ° photoproduction cross sections on proton have been measured at π° c.m. angles between 115° and 180° and between 700 MeV and 1150 MeV photon energies by detecting only the recoil proton. Our results show clearly a peak at 135° around 1000 MeV energies as predicted by Walker's partial wave analysis.

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