The ΔS = 0 hadronic weak interaction

Abstract

Our present understanding of the ΔS=0 hadronic weak interaction is based on a series of high precision experiments in the two-nucleon system and light nuclei which isolate the weak interaction via its parity violating signature. At low energy, the link between parity nonconserving (PNC) observables and the standard electroweak theory is via a set of weak meson-nucleon coupling constants which are predicted using a quark model of the physical hadrons. One striking feature that emerges is that the isovector pion coupling, which is almost entirely carried by neutral currents at the quark level, is strongly suppressed relative to theoretical predictions. At higher energy, a surprisingly large PNC asymmetry in p-p scattering has been found at 6 GeV/c. Direct QCD-based calculations which explain the data have been the subject of recent theoretical debate. New measurements at energies of 5 GeV and higher are needed to confirm the effect and guide theoretical efforts. With the availability of polarized proton beams at higher energy, as recently achieved at Brookhaven and planned for the KAON facility in Canada, the prospect for performing such experiments is very good.