The differential cross section for the reaction γ+p→π+n was measured at 19 photon energies between 300 and 750 MeV in the laboratory frame, for pion angles between 0° and 130° in the c.m. system. The pions were analyzed in angle and momentum with a magnetic spectrometer and detected by a counter telescope. The 0° measurements could be achieved, in spite of the excessive positron rate, owing to a mass-spectrometer arrangement. No direct indication for the electromagnetic excitation of the P11 resonance (1466 MeV) was found. Comparison is made with theoretical calculations of π+ photoproduction.
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We have performed an experiment to study the reaction π−+p→η+n near threshold, preliminary to a forthcoming measurement of charge asymmetry in η-meson decay. The η was identified by the velocity of the associated neutron. We detected neutrons produced in the forward hemisphere in the center-of-mass system corresponding to the most energetic neutrons in the laboratory. Data were taken at π− momenta between 670 and 805 MeVc. The four neutron detectors made it possible to detect neutrons at angles of 0° to 21° from the incident pion beam. We present backward differential cross sections for both pion charge exchange and η production calculated from the data. We looked for η′ at pion momenta of 1.5 BeVc and observed none. We obtained σ(π−p→nη′)≤60 μb.
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The π0 photoproduction cross section has been measured at 180° for photon energies from 220 to 380 MeV, in steps of 20 MeV, by detecting the recoil proton at 0°. The statistical accuracy of the measurements varies between 3 and 7%, depending on the energy. Absolute cross sections have been deduced from a comparison of the measurements with electron-proton scattering. The experimental data are compared with theoretical results calculated from fixed-momentum-transfer dispersion relations. Special attention is paid to the prediction of the multipoles at the first resonance, namely, E1+32, M1+32, and E0+π0 to obtain agreement with experiment.
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Differential cross sections for the reaction π−p→π0n were measured at nine incident-pion kinetic energies in the interval from 500 to 1300 MeV. The negative pion beam from the bevatron was focused on a liquidhydrogen target completely surrounded by a cubic array of six steel-plate spark chambers. The spark chambers were triggered on events with neutral final states. Charge-exchange events were identified from the one-shower and two-shower events in the spark-chamber pictures. By the Monte Carlo technique, the π0 distributions were calculated from the bisector distributions of the two-shower π0 events together with the observed γ-ray distributions of the one-shower π0 events. These π0 distributions were fitted with both Legendre-polynomial expansions and power-series expansions by the method of least squares. The extrapolated forward differential cross sections are in good agreement with the dispersion calculations. The Legendre coefficients for the differential cross sections in isospin state T=12 were obtained by combining our results with available data on π±p elastic scattering. In the light of existing phase-shift solutions, the behavior of these coefficients is discussed. The D5F5 interference term that peaks near 900 MeV is verified to be in isospin state T=12 only. We report here also the total neutral cross sections and the cross sections for the production of neutral multipion final states 2π0n and 3π0n. The 4π solid angle and the calibrated energy response of the spark chambers contribute to the accuracy of the results.
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The differential cross sections for the photoproduction reactions γ+p→π++n, γ+p→K++Λ0, and γ+p→K++Σ0 have ben measured for incident laboratory photon energies between 3.4 and 4.0 GeV and for meson center-of-mass angles from about 25° to 45°. The reactions were studied by observing only the charged mesons. The momenta, velocities, and angles of the mesons were measured with a magnetic spectrometer, and the equivalent of nearly monochromatic gamma rays was obtained by performing bremsstrahlung subtractions. The cross sections agree with the inequality predicted from unbroken SU(3). The measured behavior of dσdt as a function of t shows similarities to that observed in studies of mesonnucleon scattering.
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The reaction γ+p→π++n has been investigated for photon energies between 1.2 and 3 GeV and pion c.m. angles from 2.5 to 15°. The cross section is strongly peaked in the forward direction and shows resonance structure in the region of the N32*(1920) and N12*(2190).
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Differential cross sections for the reaction γ+p→n+π+ are presented for incident photon energies between 1.2 and 3 GeV and pion center-of-mass production angles of 15 to 50 deg.
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Measurements of the angular distribution of the cross section for the photoproduction of the K+Λ system from hydrogen have been made in the c.m. angular interval from 15° to 85° at a photon energy of 1200 MeV. The reaction was identified by detecting the K+ mesons with a magnet spectrometer and a velocity selection system consisting of two Čerenkov counters. The angular distribution at this energy is very similar to that at lower energies in that it is peaked forward and is easily fit with a quadratic in cosθc.m.. Special emphasis was placed on the forward direction in an attempt to find evidence for the one-K-exchange pole. A Taylor-Moravcsik analysis of the data is presented, but the results are inconclusive.
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Results of a spark chamber experiment on elastic scattering of pions on protons are presented and analyzed. The processes studied were π+p at 2.92 GeV/c, and π−p at 3.15, 4.13, and 4.95 GeV/c. The data are fitted to an exponential function of the four-momentum transfer, t, in several different ways in attempts to explore systematic energy and angular dependences. No shrinkage of the diffraction peak is seen in comparing the coefficients of a linear exponential fit for |t|<0.4 (GeV/c)2; at larger |t|, however, the cross section falls off with increasing energy. The large-angle differential cross section is examined for structure and is compared with all other large angle scattering data. The results are compared with proton-proton scattering data over the same energy range and substantial differences between the two processes are evident.
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