The excitation functions for positive pion production from hydrogen have been obtained in the energy region from 230 Mev to 450 Mev and at laboratory pion angles of 24°, 38°, 53°, 73°, 93°, 115°, 140°, and 160°. The pions are detected and identified by measuring their range and ionization in a scintillation counter telescope. The above data are analyzed to give the angular distributions in the center-of-momentum system, and a least-squares analysis made to determine coefficients in σ(θ)=A+Bcosθ+Ccos2θ. The total cross section shows a peak at 300 Mev of magnitude 2.20×10−28 cm2. The coefficient B passes through a maximum negative value at 250 Mev and then passes through zero at 325 Mev and remains positive up to the highest energy measured.
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Positive pions produced in a cold, high-pressure hydrogen gas target by the 500-Mev bremsstrahlung of the CalTech synchrotron, have been analyzed by a large magnetic spectrometer. The photoproduction cross section has been measured as a function of photon energy at laboratory angles of 12.5°, 30°, 51°, 73°, 104°, 140°, and 180°. The energy region covered depends somewhat on the angle, but is typically from 200 to 470 Mev. From these excitation curves the angular distribution of the photopions in the center of momentum system is obtained for various photon energies, and these angular distributions are analyzed in the form A+Bcosθ+Ccos2θ. The angular distribution has a backward maximum at low energies and a forward maximum at high energies, the coefficient B changing sign at about 340 Mev. The total cross section shows a striking maximum near 290 Mev, of magnitude 205×10−30 cm2, and falls off above the maximum faster than λ2.
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Angular distributions for positive-pion photoproduction from liquid hydrogen have been measured at photon energies near 225, 250, 275, 300, and 350 MeV. These have been normalized to an absolute cross section near the peak of the first resonance, measured by means of a polyethylene-carbon subtraction using solid targets. The results are compared with results of previous experiments as well as several dispersion-theoretic predictions of the cross sections. The data can be fitted within the experimental and theoretical uncertainties by a theoretical calculation containing only the pion pole term and the transition to the P3,3 state due to the first resonance.
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Measurements have been made on the ratio of pion-production cross sections at right angles to and along the photon electric-field vector. The positive and negative pions were first momentum-analyzed and counted by means of a counter telescope. Data have been taken at 45, 90, and 135° in the c.m. system, and at proton energies of 225, 330, and 450 MeV. A comparison of the data is made with the dispersion-relation calculation of McKinley.
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Absolute measurements of the elastic electron-proton cross section have been made with a precision of about 4% for values of the square of the four-momentum transfer, q2, in the range 6.0 to 30.0 F−2 and for electron scattering angles in the range 45° to 145°. To within the experimental errors, it is found that the charge and magnetic form factors of the proton have a common dependence on q2 when normalized to unity at q2=0, and that an accurate representation of the behavior of the form factor and that of the cross sections themselves can be given in terms of a three-pole approximation to the dispersion theory of nucleon form factors.
Axis error includes +- 2./2. contribution (RANDOM ERROR).
Axis error includes +- 2./2. contribution (RANDOM ERROR).
Axis error includes +- 2./2. contribution (RANDOM ERROR).
Measurements of π0 photoproduction have been made at 235, 285, 335, and 435 MeV, using a beam of polarized x rays. Using a calculated value of polarization, an analysis is made which indicates a possible need for γ, ρ, π, or γ, ω, π coupling. The polarization calculations are checked by measurements made as a function of photon production angle at 335 MeV.
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We report measurements of the differential cross section for photoproduction of π0 mesons from hydrogen, with the pion emerging near 0 deg, in the photon energy range 290 to 700 MeV. The results show no unusual behavior of the cross section in the forward direction. They are consistent with the angular distribution characteristic of a magnetic-dipole transition to a P32 state. The results agree reasonably well with theoretical predictions of Gourdin and Salin, but disagree with a prediction of DeTollis and Verganelakis. Least-squares fits in powers of cosθ have been made to the available angular distributions.
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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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