We report on the measurement of asymmetries in the single-pion photoproduction reactions γp→nπ+, γp→pπ0, and γn→pπ−, induced by linearly polarized photons of energies from 610 to 940 MeV. The experiment was carried out using the back-scattered laser beam and the 82-in. dubble chamber at SLAC. We compare the new data with predictions from a partial-wave analysis.
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Photoproduction of π + and π − on deuterium has been measured in the photon energy range from 240 to 400 MeV and for pion c.m. angles between 15° and 180°. The pions were analysed in angle and momentum by a magnetic spectrometer. From the measured π − / π + ratio, corrected for Coulomb interactions in the final state, differential cross sections of the reaction γ +n→ π − +p were calculated. Together with the π + photoproduction our data show no isotensor contribution. Comparison of our data with the recent experiments done on the inverse reaction shows no evidence of a violation of time reversal invariance. With the measured π + photoproduction on deuterium, a test of the spectator model has been made. Using the closure-approximation of Chew and Lewis our data agree within a range of ±10%.
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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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We have measured the cross sections at 90° c.m. for π± and π0 photoproduction with polarized photons. The photon energies ranged from 0.8 to 2.2 GeV. We compare the resonant "bumps" in the cross section with theoretical models. The measured asymmetry agrees with a quark-model calculation though the predicted cross sections are low.
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Previous measurements of the cross section asymmetry for single π + production on protons with linearly polarized photons of 3.4 GeV have been extended to momentum transfers within the forward peak, i.e. − = 0.0026, 0.006 and 0.01 (GeV/ c ) 2 . The results are in good agreement with pion exchange models.
Axis error includes +- 6/6 contribution.
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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 section for π + photoproduction on hydrogen have been measured over a photon energy range from 220 to 425 MeV and for pion c.m. angles between 15° and 70°. The pions have been analysed in angle and momentum by a magnetic spectrometer.
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We have studied the ratio R=[dσ(γd→π−pp)dt][dσ(γd→π+nn)dt]−1 at 8 and 16 GeV for momentum transfers |t| from about 0.001 to 1.3 GeV2. R is close to unity for |t|<mπ2, but falls very rapidly with increasing |t|, passing through ½ near |t|=0.1 GeV2 and having a minium value of about 13 near |t|=0.4 GeV2; it slowly increases at larger momentum transfers. These results are similar to those obtained in other laboratories at 3.4 and 5 GeV. This implies considerable interference between the isoscalar and isovector photon amplitudes.
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The differential cross sections for single-π+ photoproduction from hydrogen have been measured over a range of momentum transfers from -2×10−4 to -2 (GeV/c)2, and photon energies from 5 to 16 GeV. The differential cross section increases by roughly a factor of 2 as the magnitude of the square of the momentum transfer decreases from 0.02 (GeV/c)2. The cross section falls approximately as exp(−3|t|) at large momentum transfers, with a similar momentum-transfer dependence of the cross section at all photon energies studied.
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