Parity nonconservation in elastic pαscattering

Parity nonconservation in the hadronic interaction was studied by elastic scattering of 46 MeV longitudinally polarized protons on ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. The scattering chamber consisted of a 110 bar helium gas target, surrounded by a cylindrical ionization chamber in an axially symmetric arrangement, and a Faraday cup. The relative cross sections, ${\sigma }^{+}$ and ${\sigma }^{\mathrm{-}}$, for positive and negative helicity of the incident beam were measured by integrating the currents from the Faraday cup and from the ionization chamber during 20 ms intervals, and digitizing the charges. The angular acceptance function of the ionization chamber extends from ${\mathrm{theta}}_{\mathrm{lab}=31\mathrm{}}$° to approximately 80°, with maximum weight at 41°. The parity nonconserving longitudinal analyzing power $A_z$=(${\sigma }^{+}$-${\sigma }^{\mathrm{-}}$)/(${\sigma }^{+}$+σ ${}_{}{}^{\mathrm{-}}{}_{}{}^{}$) was deduced from 2.3×${10}^6$ individual measurements of $p_z$ $A_z$. The magnitude of the beam polarization was ‖$p_z$‖≃0.83. The emphasis is on quantitative treatments of instrumental effects, the most important of which is caused by the nonuniform distribution of residual transverse polarization components of the beam. The final result for the longitudinal analyzing power at a mean laboratory energy of 46 MeV is $A_z$=-(3.3±0.9)×${10}^{\mathrm{-}7}$. The uncertainty includes the statistical error as well as the systematic uncertainties from transverse polarization, modulation of intensity, position, emittance, and energy of the beam associated with helicity reversal, double scattering, asymmetry from β decay, and electronic cross talk. The result is compared to theoretical predictions and to related experiments.