Exclusive electroproduction of pi0 mesons on protons in the backward hemisphere has been studied at Q**2 = 1.0 GeV**2 by detecting protons in the forward direction in coincidence with scattered electrons from the 4 GeV electron beam in Jefferson Lab's Hall A. The data span the range of the total (gamma* p) center-of-mass energy W from the pion production threshold to W = 2.0 GeV. The differential cross sections sigma_T+epsilon*sigma_L, sigma_TL, and sigma_TT were separated from the azimuthal distribution and are presented together with the MAID and SAID parametrizations.
Cross section SIG(T) + EPSILON*SIG(L) for COS(THETA*) = -0.975.
Cross section SIG(T) + EPSILON*SIG(L) for COS(THETA*) = -0.925.
Cross section SIG(T) + EPSILON*SIG(L) for COS(THETA*) = -0.875.
The angular dependence of the asymmetry for negative-pion photoproduction on neutrons by linearly polarized photons has been measured for photon energies 260, 300, 350, 400, 450, and 500 MeV at center-of-mass angles 60°, 75°, 90°, 150°, and 120°. The results are compared with theoretical models of low-energy single-pion photoproduction. The observed asymmetry below 400 MeV shows good agreement with predictions of dispersion-theoretical models by Berends, Donnachie, and Weaver and by Schwela. The asymmetry values in the 400-500 MeV energy region suggest that smaller M1− amplitude is more favorable.
No description provided.
No description provided.
The differential cross sections of the proton Compton scattering around the second resonance have been measured at a c.m. angle of 90° for incident photon energies between 450 MeV and 950 MeV in steps of 50 MeV, and at an angle of 60° for energies between 600 MeV and 800 MeV. The results show that the peak of the 2nd resonance agrees with that of the pion photoproduction process. We also calculated the proton Compton scattering based on unitarity and fixed- t dispersion relations. The calculation describes well the data of the cross section and the recoil proton polarization.
No description provided.
The polarized target asymmetry in the reaction γ p → π 0 p has been measured at c.m. angles of 30°, 80°, 105° and 120° for incident photon energies below 1 GeV. Two decay photons from π 0 were detected in coincidence at 30°, and at the other angles recoil protons and single photons from π 0 were detected. The results are compared with recent phenomenological analyses.
No description provided.
The polarized target asymmetry for the process γ p → π + n has been measured for incident photon energies below 1.02 GeV over a range of c.m. angles from 40° to 160°. π + mesons from a polarized butanol target were detected by a magnetic spectrometer. The results are compared with predictions given by existing analyses. A tentative interpretation of the data is performed, and a larger contribution of S-wave resonances is suggested. The photocouplings of dominant resonances were hardly changed by the inclusion of new data and they seem to be almost uniquely determined.
No description provided.
The polarized target asymmetry in the reaction γp→π°p has been measured at c.m. angles around 100° for photon energies between 0.4 and 1.0 GeV by detecting both the recoil proton and the π°. The result is compared with recent analyses.
No description provided.
The reaction γ V p → p π + π − was studied in the W , Q 2 region 1.3–2.8 GeV, 0.3–1.4 GeV 2 using the streamer chamber at DESY. A detailed analysis of rho production via γ V p→ ϱ 0 p is presented. Near threshold rho production has peripheral and non-peripheral contributions of comparable magnitude. At higher energies ( W > 2 GeV) the peripheral component is dominant. The Q 2 dependence of σ ( γ V p→ ϱ 0 p) follows that of the rho propagator as predicted by VDM. The slope of d σ /d t at 〈 Q 2 〉 = 0.4 and 0.8 GeV 2 is within errors equal to its value at Q 2 = 0. The overall shape of the ϱ 0 is t dependent as in photoproduction, but is independent of Q 2 . The decay angular distribution shows that longitudinal rhos dominate in the threshold region. At higher energies transverse rhos are dominant. Rho production by transverse photons proceeds almost exclusively by natural parity exchange, σ T N ⩾ (0.83 ± 0.06) σ T for 2.2 < W < 2.8 GeV. The s -channel helicity-flip amplitudes are small compared to non-flip amplitudes. The ratio R = σ L / σ T was determined assuming s -channel helicity conservation. We find R = ξ 2 Q 2 / M ϱ 2 with ξ 2 ≈ 0.4 for 〈 W 〉 = 2.45 GeV. Interference between rho production amplitudes from longitudinal and transverse photons is observed. With increasing energy the phase between the two amplitudes decreases. The observed features of rho electroproduction are consistent with a dominantly diffractive production mechanism for W > 2 GeV.
DIPION CHANNEL CROSS SECTION.
The polarized target asymmetry for γ + p → π + + n was measured at c.m. angles around 130° for the energy range between 0.3 and 1.0 GeV. A magnetic spectrometer system was used to detect π + mesons from the polarized butanol target. The data show two prominent positive peaks at 0.4 and 0.8 GeV and a deep minimum at 0.6 GeV. These features are well reproduced by the phenomenological analysis made by us.
No description provided.
We present new high statistics data on hadron production in photon-photon reactions. The data are analyzed in terms of an electron-photon scattering formalism. The dependence of the total cross section of Q 2 , the four-momentum transfer squared of the scattered electron, and on the mass W of the hadronic system is investigated. The data are compared to predictions from Vector-Meson Dominance and the quark model.
DEPENDENCE ON VISIBLE HADRONIC INVARIANT MASS.
The ratio of π - to π + electroproduction cross sections from deuterium has been measured in the resonance region, at a four-momentum transfer squared close to −1.0 (GeV/ c ) 2 . Results in the forward direction are presented and a comparison is made with predictions based on SU(6) W and the Melosh transformation.
No description provided.