Parity violation in proton scattering: low energy region

Abstract

Measurements of the parity-violating longitudinal analyzing power, A,, in the scattering of low-energy protons (Ep < 50 MeV) are reviewed. The experiments are based on the determination of the relative scattering cross sections a+ and a-for incident protons of positive and hegative helicity, respect~vely. The first results were reported from Los Alamos, where a significant analyzing power [ A z = (-1.7 ? 0.8) X was found for p-p scattering at 15 MeV. All other results are for a proton energy near 45 MeV, primarily by the group working at Schweizerisches Instrtut fiir Nuklearforschung, which recently reported a new, very accurate result for p-p scattering of A, = (-1.50 -C 0.22) X A review is presented of the methods that have been developed by this group over the last 10 years to reduce systematic errors in the determination of A, to a few times lop9.On passe en revue les mesures du pouvoir d'analyse longitudinal A, qui ne respectent pas la paritt dans la diffusion de protons i basses tnergies (Ep < 50 MeV). Les expkriences sont bastes sur la dttermination des sections efficaces relatives de diffusion a + et a-pour des protons incidents d'helicitt positive et ntgative. Les premiers rtsultats furent rapportts par Los Alamos, o t~ un pouvoir d'analyse significatif [ A , = (-1.7 2 0,8) X fut trouve pour la diffusion p-p a 15 MeV. Tous les autres rksultats sont ii une energie des protons de prks de 45 MeV, obtenus prlncipalement par le groupe travaillant a Schweizerisches Institut fiir Nuklearforschung, qui a rkcemment rapport6 un nouveau resultat trks prkcis pour la diffusion p-p de A, = (-1,50 2 0,22) X lo-'. On prtsente une revue des mithodes qui ont CtC dtvelopptes par ce groupe durant les 10 dernikres anntes pour rtduire les erreurs systtmatiques dans la determination de A: it quelques-parties dans [Traduit par la revue] Can 1. Phys 66. 485 (1988)

Absolute measurement of thep+panalyzing power at 183 MeV

“…Such a difIerence could arise from a displacement of the beam position on target (+1.5 mm is the positioning accuracy of the beam on the target), from a nonzero angle of the beam with respect to the optical reference line, or from the breaking of the symmetry of the setup by the target, inclined by 45 to the left or the right. In principle, a spin-dependent beam shift is also conceivable but such an effect has been shown to be small [8] for an atomic beam source, as was used for the present experiment, where RF transitions are switched for spin reversal. The difference between 2 I ('H) and 2 z('H) is shown in Fig.…”

Parity Violation and the Nucleon-Nucleon System

“…For reasons of symmetry, one would expect the false effect from coherent beam motion to vanish if the beam is exactly on the effective center of the scattering chamber. However, a very large sensitivity to vertical beam motion (false parity signal of 27 × 10 −7 per µm motion) was observed even when the beam was on the geometric axis of the chamber [31]. The effect was traced to temperature gradients in the high pressure gas target caused by beam heating.…”