We have measured the differential cross section for the gamma n --> pi- p and gamma p --> pi+ n reactions at center of mass angle of 90 degree in the photon energy range from 1.1 to 5.5 GeV at Jefferson Lab (JLab). The data at photon energies greater than 3.3 GeV exhibit a global scaling behavior for both pi- and pi+ photoproduction, consistent with the constituent counting rule and the existing pi+ photoproduction data. Possible oscillations around the scaling value are suggested by these new data The data show enhancement in the scaled cross section at a center-of-mass energy near 2.2 GeV. The cross section ratio of exclusive pi- to pi+ photoproduction at high energy is consistent with the prediction based on one-hard-gluon-exchange diagrams.
Differential cross section at THETA(CM) = 90 degrees.
The cross section for photoproduction of single π+ from hydrogen has been measured at laboratory angles of 110°, 127.5° and 152°, between 0.9- and 3.2-GeV incident photon energy. Measurements have been made with approximately 15% statistical accuracy at about 40 photon energies at each angle. The results agree well with the previous Caltech data of Thiessen. The cross section shows a rapid drop with increasing energy with superimposed bumps or shoulders corresponding to the N(1688), Δ(1920), and Δ(2420). A shallow minimum is observed at the N(2190) resonance.
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Differential cross sections for the reaction γ+p→n+π+ are presented for incident photon energies between 1.2 and 3 GeV and pion center-of-mass production angles of 15 to 50 deg.
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The reaction γ+p→π++n has been investigated for photon energies between 1.2 and 3 GeV and pion c.m. angles from 2.5 to 15°. The cross section is strongly peaked in the forward direction and shows resonance structure in the region of the N32*(1920) and N12*(2190).
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The differential cross section for the photoproduction of a π− meson from the neutron bound in the deuteron was measured for pion laboratory angles of 76°, 96°, and 118° at incident gamma-ray energies in the region of 275 MeV. The π− meson and the high-energy proton were detected. The pion momentum and angle were measured by sets of spark chambers situated in front of and behind a magnetic field. The proton angle and range were also measured with spark chambers. To calculate "free" neutron cross sections from our data, we used a modified version of the extrapolation method suggested by Chew and Low. By observing the π+ only, the differential cross section for π+ photoproduction from hydrogen also was measured. As determined by this experiment, the differential cross section for photoproduction of a π− meson from a "free" neutron and the differential cross section for photoproduction of a π+ meson from hydrogen are as follows: Eγlab≃275 MeV These results disagree with the dispersion theory predictions of Chew, Goldberger, Low, and Nambu. They also disagree with McKinley's dispersion theory calculations which include a bipion or ρ-meson term in the production amplitudes.
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The differential cross section for the gamma +n --> pi- + p and the gamma + p --> pi+ n processes were measured at Jefferson Lab. The photon energies ranged from 1.1 to 5.5 GeV, corresponding to center-of-mass energies from 1.7 to 3.4 GeV. The pion center-of-mass angles varied from 50 degree to 110 degree. The pi- and pi+ photoproduction data both exhibit a global scaling behavior at high energies and high transverse momenta, consistent with the constituent counting rule prediction and the existing pi+ data. The data suggest possible substructure of the scaling behavior, which might be oscillations around the scaling value. The data show an enhancement in the scaled cross section at center-of-mass energy near 2.2 GeV. The differential cross section ratios at high energies and high transverse momenta can be described by calculations based on one-hard-gluon-exchange diagrams.
Differential cross section for the process GAMMA N --> PI- P for an incident electron energy of 5.614 GeV.
Differential cross section for the process GAMMA N --> PI- P for an incident electron energy of 4.236 GeV.
Differential cross section for the process GAMMA N --> PI- P for an incident electron energy of 3.400 GeV.
Pions from the reaction γ + p → π + + n were analysed in the backward direction by a magnetic spectrometer. The photon energy region of 0.394 GeV to 1.397 GeV was covered by 19 different momentum settings. Data reduction resulted in 74 measured differential cross sections with statistical uncertainties typically from 4% to 8%. The systematic uncertainty was estimated to be ±5%. The data are compared to other recent experiments and predictions of phenomenological analyses.
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perimental analysis of the process is presented. Theσ(−)/σ(+) ratio has been measured in the photon energy interval (170÷230) MeV and Lab. angles 45°, 75°, 105°, 150°. The results are interpreted on the base of the impulse approximation with the aim of getting information on the processhv+n →π −+p.
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