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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The ratio of π− to π+ off deuterium was measured as a function of incident photon energy from 600 to 1700 MeV in the forward direction. The ratio shows a broad dip around a center-of-mass energy of 1700 MeV, resulting presumably from the collective effect of several isospin-½ resonances in this energy region. Such a change in the ratio is reflected in the rapid variation of the isoscalar photoproduction amplitude since we found the isovector photoproduction amplitude to be a relatively smooth function decreasing slowly with increasing incident photon energy.
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Elastic electron-proton scattering cross sections have been measured using the internal beam of the 6-BeV Cambridge Electron Accelerator at laboratory scattering angles between 31° and 90° for values of the four-momentum transfer squared ranging from q2=0.389 to 6.81 (BeV/c)2 (q2=10 to 175F−2). Incident electron energies ranged from 1.0 to 6.0 BeV. Scattered electrons from an internal liquid-hydrogen target were momentum-analyzed using a single quadrupole spectrometer capable of momentum analysis up to 3.0 BeV/c. Čerenkov and shower counters were used to help reject pion and low-energy background. The cross sections presented are absolute cross sections with experimental errors ranging from 6.8% to 20%. Separation of proton electromagnetic form factors have been made for all but the two highest momentum transfer points, using the Rosenbluth formula. Both form factors, GEp and GMp, were observed to continue to decrease as the momentum transfer increases. An upper limit to the possible asymptotic values of the proton electromagnetic form factors has been established.
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Using the internal beam of DESY elastic electron-proton cross-sections were measured at various angles between 32° and 130°, and with momentum transfers ofq 2=39, 60, 80 and 110 fm−2. Two single-quadrupole spectrometers, movable around a common liquid-hydrogen target, were used for analysing the momentum of the scattered electrons. Čerenkov and shower counters discriminated against pion and low-energy background. As a cross-section reference, recoil protons from elastic scattering atq 2=10 fm−2 were used, with a quantameter serving as an intermediate monitor. The data are consistent with the Rosenbluth formula, giving real form factorsG E andG M . Both continue to decrease with increasing momentum transfer, but somewhat faster than indicated by measurements performed so far.
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Total and differential elastic cross-section data are presented at eight incident π+ momenta: 1.28, 1.34, 1.40, 1.43, 1.55, 1.68, 1.77, and 1.84 GeVc. These data were obtained from a hydrogen-bubble-chamber exposure at the Bevatron, and contain more than 65 000 events. This represents more than 1½ times the world's data hitherto available in this energy region.
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We measured elastic-scattering angular distributions for π++p scattering at 1.5, 2.0, and 2.5 BeV/c using spark chambers to detect scattered pions and protons. A bump that decreases in amplitude with increasing momentum is observed in the backward hemisphere in the 1.5- and 2.0-BeV/c distributions, but is not observed in the 2.5-BeV/c distributions. It appears reasonable to attribute this phenomenon to the 1.45-BeV/c resonance observed in the π++p total cross section. The data are compared with π−+p data and are found to support the theoretical prediction that the scattering cross sections for both charge states should become equal at high energies. We fit the angular distributions with a power series in cosθ*, and compare the extrapolated values for the scattering cross section in the backward direction with the calculation of the neutron-exchange pole contribution to the cross section. The "elementary" neutron-pole term contribution is calculated to be 90 mb/sr at 2.0 BeV/c, in violent disagreement with the extrapolated value, ≈0.5 mb/sr.
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Differential cross sections for the elastic scattering of positive pi mesons by protons were measured at the Berkeley Bevatron at pion laboratory kinetic energies between 500 and 1600 MeV. Fifty scintillation counters and a matrix coincidence system were used to identify incoming pions and detect the recoil proton and pion companions. Results were fitted with a power series in the cosine of the center-of-mass scattering angle, and total elastic cross sections were obtained by integrating under the fitted curves. The coefficients of the cosine series are displayed, plotted versus the laboratory kinetic energy of the pion. The most striking features of these curves are the large positive value of the coefficient of cos6θ*, and the large negative value of the coefficient of cos4θ*, both of which maximize in the vicinity of the 1350-MeV peak in the total cross section. These results indicate that the most predominant state contributing to the scattering at the 1350-MeV peak has total angular momentum J=72, since the coefficients for terms above cos6θ* are negligible at this energy. One possible explanation is that the 1350-MeV peak is the result of an F72 resonance lying on the same Regge-pole trajectory as the (32, 32) resonance near 195 MeV.
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