The differential cross sections \sigma_0=\sigma_T+\epsilon \sigma_L, \sigma_{LT}, and \sigma_{TT} of \pi^0 electroproduction from the proton were measured from threshold up to an additional center of mass energy of 40 MeV, at a value of the photon four-momentum transfer of Q^2= 0.05 GeV^2/c^2 and a center of mass angle of \theta=90^\circ. By an additional out-of-plane measurement with polarized electrons \sigma_{LT'} was determined. This showed for the first time the cusp effect above the \pi^+ threshold in the imaginary part of the s-wave. The predictions of Heavy Baryon Chiral Perturbation Theory are in disagreement with these data. On the other hand, the data are somewhat better predicted by the MAID phenomenological model and are in good agreement with the dynamical model DMT.
The separated cross section SIG(0), SIG(LT) and SIG(TT).
Beam helicity asymmetry.
At the electron accelerator ELSA a linearly polarised tagged photon beam is produced by coherent bremsstrahlung off a diamond crystal. Orientation and energy range of the linear polarisation can be deliberately chosen by accurate positioning of the crystal with a goniometer. The degree of polarisation is determined by the form of the scattered electron spectrum. Good agreement between experiment and expectations on basis of the experimental conditions is obtained. Polarisation degrees of P = 40% are typically achieved at half of the primary electron energy. The determination of P is confirmed by measuring the beam asymmetry, \Sigma, in pi^0 photoproduction and a comparison of the results to independent measurements using laser backscattering.
Beam asymmetry as a function of the PI0 centre of mass scattering angle.
Beam asymmetry as a function of the PI0 centre of mass scattering angle.
Beam asymmetry as a function of the PI0 centre of mass scattering angle.
Large-angle cross sections for γd→π0d are systematically measured in the photon energy range between 500 and 1000 MeV. A good fit is obtained by use of a Glauber-model calculation which includes the dibaryon resonances F33(2.26) and G41(2.51), but the fit has an unusual nature in the role of resonance and nonresonance contributions.
Liquid hydrogen target for final calibration.
LIQUID DEUTERIUM TARGET.
Cross sections for π0 photoproduction from hydrogen in the backward direction have been measured for photon energies of 6, 8, 12, and 18 GeV. The range of momentum transfer covered in these measurements is −1 (GeVc)2<u<0(GeVc)2, dσdu is found to have a backward peak and decreases with energy as s−3.0±0.2. Photoproduction of η0 and ρ0 mesons has also been observed.
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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.
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Differential cross sections for π0 and ρ0 photoproduction from protons have been measured at photon energies 6, 12, and 18 GeV and momentum transfers 0.5 to 3 (GeVc)2.
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The production of π∘ mesons in the reaction γ+p→π∘+p is investigated as a function of the incident γ-ray energy in the region from 200 Mev to 300 Mev. For the π∘ emitted at approximately 90° laboratory angle, the differential cross section can be represented by (dσπ∘dΩ)π2=C(K−145)1.9±0.4, where K= energy of incident γ-ray in Mev. The approximate threshold for the reaction is 145 Mev. The ratio of the cross section at 60° laboratory angle to that at 90° laboratory angle, for γ-rays between 250 Mev and 300 Mev, is 1.45±0.25.
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The polarization of the recoil proton in neutral single-pion photoproduction from hydrogen, γ+p→p+π0, has been measured for pion center-of-mass angles near 90° at 7 photon energies from 450 to 900 MeV. The polarization rises to a maximum of 0.58 near 600 MeV and is still 0.42 at 900 MeV. The sign of the polarization is negative in the sense of k×q, where k is the photon momentum and q is the pion momentum. The measured values are given as functions of laboratory photon energy and c.m. pion angle as follows: 450 MeV, 109°, -0.16±0.14; 525 MeV, 84°, -0.36±0.19; 585 MeV, 86°, -0.58±0.15; 660 MeV, 77°, -0.51±0.17; 755 MeV, 76°, -0.55±0.15; 810 MeV, 89°, -0.45±0.17; 895 MeV, 90°, -0.42±0.16. The recoil protons were momentum-analyzed with a magnetic spectrometer. Nuclear emulsion was used as scatterer and detector. The emulsion technique is discussed in detail. The number of individual scatterings in emulsion used for each measurement varied between 750 and 1000.
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Measurements have been made of the double polarisation parameters G and H in the photoproduction of neutral pions from protons, for incident photon energies between 1300 MeV and 2300 MeV and for pion c.m. angles between 50δ and 80δ. The results are compared with predictions from a recent comprehensive analysis of earlier photoproduction data.
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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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