The variation of the differential cross section for π+ photoproduction from hydrogen, with γ-ray energy, has been examined at a laboratory angle of 58° to the γ-ray beam. A thin hydrogen target, and a counter system designed to eliminate random events, have been employed. Mean values for the differential cross section dσdΩ at γ-ray energies of 162, 168, 175, and 192 Mev are 5.42±0.38, 5.77±0.41, 6.74±0.47, and 8.22±0.58 μb/sr, respectively, where the error limits refer to relative values. The results substantiate the rising trend of the interaction quantity {(dσdΩ)(μ2pε)(1+ωM)2} near threshold, in accord with dispersion theory; and the absolute cross sections are compatible with a threshold value for a0+ near 20 μb/ steradian, consistent with findings in related pion work.
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Report on the investigation of interactions in π−p collisions at a pion momentum of 1.59 GeV/c, by means of the 50 cm Saclay liquid hydrogen bubble chamber, operating in a magnetic field of 17.5 kG. The results obtained concern essentially the elastic scattering and the inelastic scattering accompanied by the production of either a single pion in π−p→ pπ−π0 and nπ−π+ interactions, or by more than one pion in four-prong events. The observed angular distribution for the elastic scattering in the diffraction region, can be approximated by an exponential law. From the extrapolated value, thus obtained for the forward scattering, one gets σel= (9.65±0.30) mb. Effective mass spectra of π−π0 and π−π+ dipions are given in case of one-pion production. Each of them exhibits the corresponding ρ− or ρ0 resonances in the region of ∼ 29μ2 (μ = mass of the charged pion). The ρ peaks are particularly conspicuous for low momentum transfer (Δ2) events. The ρ0 distribution presents a secondary peak at ∼31μ2 due probably to the ω0 → π−π+ process. The branching ratio (ω0→ π+π−)/(ω0→ π+π− 0) is estimated to be ∼ 7%. The results are fairly well interpreted in the frame of the peripheral interaction according to the one-pion exchange (OPE) model, Up to values of Δ2/μ2∼10. In particular, the ratio ρ−/ρ0 is of the order of 0.5, as predicted by this model. Furthermore, the distribution of the Treiman-Yang angle is compatible with an isotropic one inside the ρ. peak. The distribution of\(\sigma _{\pi ^ + \pi ^ - } \), as calculated by the use of the Chew-Low formula assumed to be valid in the physical region of Δ2, gives a maximum which is appreciably lower than the value of\(12\pi \tilde \lambda ^2 = 120 mb\) expected for a resonant elastic ππ scattering in a J=1 state at the peak of the ρ. However, a correcting factor to the Chew-Low formula, introduced by Selleri, gives a fairly good agreement with the expected value. Another distribution, namely the Δ2 distribution, at least for Δ2 < 10 μ2, agrees quite well with the peripheral character of the interaction involving the ρ resonance. π− angular distributions in the rest frame of the ρ exhibit a different behaviour for the ρ− and for the ρ0. Whereas the first one is symmetrical, as was already reported in a previous paper, the latter shows a clear forward π− asymmetry. The main features of the four-prong results are: 1) the occurrence of the 3/2 3/2 (ρπ+) isobar in π−p → pπ+π−π− events and 2) the possible production of the ω0→ π+π−π0 resonance in π−p→ pπ−π+π−π0 events. No ρ’s were observed in four-prong events.
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The differential cross-section for π+ photoproduction from hydrogen by γ-rays of laboratory energy 187 MeV has been measured at four angles. Two identical counter systems, designed to detect low energy pions unambiguosly in intense electron and γ-ray backgrounds, were used in conjunction with a cylindrical liquid hydrogen target, of very low boil-off rate. The cross-sections at laboratory angles of 39.2°, 66.7°, 111.6°, and 134° are 7.49±0.47, 8.10±0.57, 8.36±0.61 and 9.54±0.61, ·10−30cm2/sr, respectively, where the assigned errors refer only to the relative values. The absolute cross-sections are in substantial agreement with the dispersion theory and confirm the front to back asymmetry.
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Approximately 60 000 events have been collected in a spark chamber experiment at the CERN Proton Synchrotron which studied elastic diffraction scattering of π--p and p-p at incident momenta of 8.5, 12.4 and 18.4 GeV/c and of π+-p at 8.5 and 12.4 GeV/c. Magnetic analysis of the incoming and diffraction scattered particle, together with measurement of all angles, permitted each event to be determined as elastic subject to three constraints, so that the inelastic background was rejected with. high efficiency, even at the larger momentum, transfers. Much of the data have been processed by the CERN Automatic Flying-Spot DigitizerHPD. A detailed description of the experimental technique and of the methods of analysis is given. The results, together with data from lower energies, confirm the remarkable energy-independence of the shape of the pion-proton diffraction scattering peak up to |t| = 1.5 (GeV/c)2, wheret is the square of the four-momentum transfer, over a range of pion energies from 2 to 18 GeV. Proton-proton scattering does however appear to show a shrinking diffraction peak. In general, the data agree with other experiments using both counter and bubble chamber techniques, but some differences do appear. During the experiment, data were taken which set an upper limit of 2·102 μb/(GeV/c)2 on the differential elastic cross-section dσ/dt over a range of |t| from 20.9 to 23.4 (GeV/c)2 at 13.4 GeV/c incident pion momentum.
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A determination of the electric and magnetic form factors of the proton has been made by studying the elastic scattering of electrons from a polyethylene target by observation of the recoiling proton at 0° and 30° for values of q2 between 1 and 1.8 F−2. From these measurements we have deduced the charge radius Rc and the magnetic radius Rm of the proton and find equality within the experimental errors (Rc=0.800±0.025 F; Rm=0.810±0.029 F).
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Cross sections for the reaction γ+p→π0+p for incident gamma-ray energies of 2.0 to 5.0 GeV and for baryon four-momentum transfers squared of 0.5 to 4.0 (GeV/c)2 are presented. The results are compared with theoretical predictions based on Reggeized vector-meson exchange.
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