Analyzing power reduction in quasifree pion-nucleon knockout reactions

Unpolarized cross sections and vector target analyzing powers for the ${}^7\mathrm{Li}\to ({\pi }^{+},{\pi }^{{+}^{\prime }}p)$ proton knockout reaction were measured using a vector polarized ${}^7\mathrm{LiH}$ target and a 240 MeV ${\pi }^{+}$ beam at the $\pi M1\mathrm{}$ channel of PSI. Typical target polarizations were >30% for ${}^7\mathrm{Li}.$ Coincident ${\pi }^{+}-p$ data are presented for three emitted pion angles (60°, 85°, and 108°). For each ${\pi }^{+}$ angle coincident data with adequate statistics were obtained for three proton angles near the quasifree ${\pi }^{+}-p$ angle. The ${\pi }^{+}$ angles were chosen to emphasize and isolate contributions to the target analyzing powers from the two-body π-nucleon interaction with a polarized nucleon whose polarization resulted from either the target polarization or from the distortion-induced effective polarization. The data are compared with factorized-amplitude distorted-wave impulse approximation (DWIA) calculations. The unpolarized cross sections are rather well described by these calculations. However, for all three angles the target analyzing powers are substantially reduced from predictions of conventional DWIA calculations. This result suggests a rather strong spin dependence in the Δ-nucleus spreading potential.