New results are presented on the differential cross-section for the reaction α+p→π0+p, at energies between 600 and 1000 MeV, and c.m. pion angles Θ*π=40° and Θ*π=60°. The present data, together with that at Θ*π=40° already published (11), show an angle-independent position of the second resonance at about 750 MeV. Rather flat angular distributions in the forward c.m. hemisphere are also favoured by these data. On comparing the cross-sections obtained when detecting both the neutral pion and the recoil proton, and when detecting only the latter, estimates of the background of «ghost protons» are obtained, in agreement with the empirical curve proposed in ref. (11).
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The elastic scattering of photons by protons has been measured for 100 MeV to 290 MeV photons at 90° c.m.s. and 139° c.m.s. scattering angles. The expected large increase in cross-section is observed at energies approaching that of (3/2, 3/2) pion-nucleon resonance. The scattering can be qualitatively explained by the ordinary Thomson amplitude combined with that of the (3/2, 3/2) resonance. A more detailed examination of the cross-section in the region just above the photo-meson threshold has shown that it is sensitive to the π0 photon coupling. From the experimental data, one may conclude that the π0 mean life should be between 10−16 and 10−18 s.
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Differential cross sections for the reaction $\gamma p \to p \pi^0$ have been measured with the CEBAF Large Acceptance Spectrometer (CLAS) and a tagged photon beam with energies from 0.675 to 2.875 GeV. The results reported here possess greater accuracy in the absolute normalization than previous measurements. They disagree with recent CB-ELSA measurements for the process at forward scattering angles. Agreement with the SAID and MAID fits is found below 1 GeV. The present set of cross sections has been incorporated into the SAID database, and exploratory fits have been extended to 3 GeV. Resonance couplings have been extracted and compared to previous determinations.
Differential cross section for indicent photon energy 675 MeV.
Differential cross section for indicent photon energy 725 MeV.
Differential cross section for indicent photon energy 775 MeV.
Single pi0 photoproduction has been studied with the CB-ELSA experiment at Bonn using tagged photon energies between 0.3 and 3.0 GeV. The experimental setup covers a very large solid angle of about 98% of 4 pi. Differential cross sections (d sigma)/(d Omega) have been measured. Complicated structures in the angular distributions indicate a variety of different resonances being produced in the s channel intermediate state gamma p --> N* (Delta*) --> p pi0. A combined analysis including the data presented in this letter along with other data sets reveals contributions from known resonances and evidence for a new resonance N(2070)D15.
Total cross section for GAMMA P --> P PI0 obtained by integration of the angular distributions and extrapolation into the forward and backward regions using the PWA result.
Differential cross section as a function of c.m. angle for the photon energy range 300 to 425 GeV.
Differential cross section as a function of c.m. angle for the photon energy range 425 to 550 GeV.
At the Bonn 500 MeV synchrotron the differential cross sections for the photoproduction of neutral pions on protons and neutrons have been measured in a single experiment using a target of liquid deuterium. The final state has been completely determined by measuring the outgoing pion and one nucleon in coincidence. Measurements of the ratio R = π 0 n/ π 0 p have been done at different angles and energies. The possible existence of an isotensor contribution has been limited to (−2 ± 3)% of the isovector amplitude.
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The differential cross section for photoproduction of π° on hydrogen has been measured in a photon energy range of 560-690 MeV and for production angles in the interval 90°-105° in the centre of mass system. The experiment detects the recoil proton and a π°-decay photon in coincidence, using optical spark chambers and a lead glass Cerenkov counter. Presented cross sections, based on 35 000 events recorded on film, are in good agreement with recent phase shift analysis.
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Measurements of differential cross sections for pi-zero photoproduction from protons have been made at angles between 60° and 140° c.m. in the photon energy range 0.7 GeV to 1.7 GeV. The data are compared with the rits provided by three recent partial-wave analyses of pion photoproduction and some significant discrepancies observed.
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The ratio R = (d σ /d t )( γ d → ( π 0 n)p)/(d σ /d t )( γ d → ( π 0 p)n), was measured at the Deutsches Elektronen-Synchrotron DESY, Hamburg at a mean photon energy of 4.0 GeV in the four-momentum transfer range between t = − 0.2 (GeV/ c ) 2 and t = − 1.2 (GeV/ c ) 2 in steps of approximately 0.08 (GeV/ c ) 2 . The ratio R is less than 1 up to t = − 0.9 (GeV/ c ) 2 and shows a broad minimum around t = − 0.6 (GeV/ c ) 2 . Corrections for nuclear effects in the deuterium were not applied but are shown to be small.
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Differential cross sections for neutral-pion photoproduction on hydrogen in the region of the first resonance have been measured by two independent experiments detecting the recoil protons. The results of both measurements have been combined into one set of cross sections covering the photon energy range from 200 to 440 MeV at pion c.m. angles between 50 and 160 degrees.
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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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