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Differential cross sections of proton Compton scattering have been measured at the Bonn 2.5 GeV synchrotron. The experiment covers photon laboratory energies between 1.2 GeV and 1.7 GeV and the square of the four-momentum transfer ranges fromt=−0.17 GeV2 to −0.98GeV2 corresponding to c.m. scattering angles between 35° and 80°. The cross sections exhibit a forward peak followed by a monotone fall-off up to the largest measured |t|-values. Fits of the formdσ/dt=A·exp(Bt) to the data points with |t|≦0.5 GeV2 yield forward cross sectionsA, which are consistent with the 0° cross sections calculated from the measured total photon-proton cross section. The average slope isB=5.6±0.14 GeV2.
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Cross-section values for Compton scattering on the proton were measured at 25 kinematic settings over the range s = 5-11 and -t = 2-7 GeV2 with statistical accuracy of a few percent. The scaling power for the s-dependence of the cross section at fixed center of mass angle was found to be 8.0 +/ 0.2, strongly inconsistent with the prediction of perturbative QCD. The observed cross-section values are in fair agreement with the calculations using the handbag mechanism, in which the external photons couple to a single quark.
Cross section of proton Compton Scattering at centre of mass energy squared of 4.82 GeV.
Cross section of proton Compton Scattering at centre of mass energy squared of 6.79 GeV.
Cross section of proton Compton Scattering at centre of mass energy squared of 8.90 GeV.
Compton scattering from the proton was investigated at s=6.9 (GeV/c)**2 and \t=-4.0 (GeV/c)**2 via polarization transfer from circularly polarized incident photons. The longitudinal and transverse components of the recoil proton polarization were measured. The results are in excellent agreement with a prediction based on a reaction mechanism in which the photon interacts with a single quark carrying the spin of the proton and in disagreement with a prediction of pQCD based on a two-gluon exchange mechanism.
Polarization transfer parameters.
Differential cross sections of proton Compton scattering have been measured at the Bonn 2.5 GeV synchrotron. 78 data points are presented as angular distributions at photon lab energies of 700, 750, 800, 850, 900, and 950MeV. The c.m. scattering angle ranges from 40°–130°, corresponding to a variation of the four momentum transfer squared betweent=−0.10 tot=−0.96 GeV2 at 700 and 950 MeV, respectively. Two additional differential cross sections have been measured at 1000MeV, 35.6° and 47.4°. The angular distributions show forward peaks whose extrapolations to 0° are consistent with calculated forward cross sections taken from literature. The small angle data (|t| ≲0.2 GeV2) together with the calculated cross sections at 0° are also consistent with the assumption of a slope parameterB of 5 GeV−2. For the first time a rerise of the angular distributions towards backward angles has been observed. It becomes less steep with increasing energy. The most interesting feature of the angular distributions is a sharp structure which appears betweent=−0.55 GeV2 at 700MeV andt=−0.72 GeV2 at 950 MeV. Such a rapid varation of the differential cross section witht has never been ovserved in elastic hadron-hadron scattering or photoproduction processes. It indicates the existence of a dynamical mechanism which could be a peculiarity of Compton scattering.
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