Quasielastic Pion Scattering Near the (3,3) Resonance. [255 MeV, Differential Cross Section Ratio]. PDF Download

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The quasielastic pion scattering process (.pi., .pi.p), in which an energetic pion scatters off a target nucleus, knocking-out a bound proton, was studied to determine the role of recoil nucleon charge exchange in the mechanism of the process near the (3,3) free particle pion-nucleon resonance. Calculations, which incorporate the hypothesis of final state charge exchange of the outgoing nucleon, were performed to predict expectations for observing the process. Experimental measurements were made on 27Al and 2°8Pb, using 255-MeV .pi. and .pi.− beams. The outgoing protons were observed in a counter telescope in singles and coincidence modes. Singles spectra were measured at proton angles theta/sub rho/ = 45, 55, 64, and 90° and cross sections were calculated as a function of the energy of the detected proton, for each of the targets. Values of the ratio of .pi. to .pi.− cross sections were calculated for each of the angles of observation. The results obtained indicate that the singles spectra contain events from processes other than quasielastic scattering and that the quasielastic events cannot be easily disentangled from the large background due to such events. The study has thus established the inadequacy of observing quasielastic pion scattering in a single arm measurement. Coincidence measurements were made by observing the recoil protons in coincidence with the scattered pions, which were detected in a scintillator counter telescope. The ratio of .pi.+ to .pi.− cross sections were obtained for each target for the angular settings (theta/sub rho/, theta/sub .pi./) = (55, 50°) and (64, 37.5°). The measured values of 7.0 +- 0.7 for 27Al and 4.5 +- 0.5 for 2°8Pb are substantially below the impulse approximation no-charge-exchange limit of 9. The observed A dependence of this cross section ratio is in agreement with the predictions of the semiclassical charge exchange model, showing the influence of charge exchange. 66 references.