Precision measurement of pion-proton absolute differential cross sections at energies spanning the delta resonance

• Main Author: Pavan, Marcello Maurizio
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/8694

A measurement of the absolute differential cross section in the π±p elastic reaction for incident pion laboratory kinetic energies of Tx = 141.2, 168.8, 193.2, 218.1, 240.9, and 267.3 MeV is described. Measurements where the scattered pion and recoil proton were detected in coincidence with the incident pion beam were taken at all energies. These measurements obtained data at six laboratory pion angles from 600 to 1550. Single arm measurements, where only the scattered pions were detected at six near— forward angles from 200 to 700, were performed at the lowest four energies. Both the single arm and coincidence measurements employed a flat—walled supercooled liquid hydrogen target. The uncertainty in the proton density of this target was 0.5%. Solid CH₂ targets were also employed for coincidence measurements at the lowest four en ergies. The proton density uncertainties of these targets was 1%. Numerous mea surements were performed to test the reproducibility of the cross sections under a variety of experimental conditions, and to elucidate potential sources of systematic uncertainty. Excellent consistency was found amongst all these data sets. In partic ular, the results obtained with the LH₂ and CH₂ targets were consistent at all energies. The final π⁺p data have typically 1-1.5% statistical and 1% normalization uncertain ties, whereas these uncertainties are typically 1.5-2% and 1.5%, respectively, for the π⁻p data. At incident pion energies above 190 MeV, there is reasonable agreement between the results from these measurements and those of Bussey et aL. The agreement with the data of Sadler et al. at 263 MeV is excellent. Below 190 MeV, the data presented here are systematically below the data of Bussey et al. , well outside the stated nor malization uncertainties. The data are consistent within the stated uncertainties with the Brack et a). results near 140 MeV, although the latter results are systematically a few percent lower. At all energies and angles, the results from this work are in agreement within one standard deviation with the predictions of the Virginia Polytechnic Institute SP95 par tial wave analysis (PWA) solution, although the data were not included in the analysis. Consequently, the work presented here adds support to this PWA which concludes that the pion nucleon coupling constant g²/4π = 13.7, and that the mass of the A res onance is 1233.7 MeV, in disagreement with earlier results of partial wave analyses from other groups.