Parity nonconservation in proton-nucleus scattering at 6 GeV/c

Abstract

A parity-nonconserving asymmetry in the total cross section for 6-GeV/c polarized protons on a water target has been measured. The experiment used two independent detector systems to measure the helicity dependence of the transmission through the target. Special attention was paid to possible sources of systematic errors. The result is ${A}_{L}=(2.65\ifmmode\pm\else\textpm\fi{}0.60\ifmmode\pm\else\textpm\fi{}0.36)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ where ${A}_{L}=\frac{({\ensuremath{\sigma}}_{+}\ensuremath{-}{\ensuremath{\sigma}}_{\ensuremath{-}})}{({\ensuremath{\sigma}}_{+}+{\ensuremath{\sigma}}_{\ensuremath{-}})}$ is the fractional difference of the total cross sections for positive- and negative-helicity protons on an unpolarized target. The first error is statistical and the second is an estimate of systematic uncertainties. This result is much larger than predictions based on meson-exchange calculations but consistent with a recent QCD calculation.