Measurement of spin observables in the quasifree np→{pp}sπ- reaction at 353 MeV

The transverse spin correlations A_x,x and A_y,y have been measured in the d⃗p⃗→p_spec{pp}_sπ^- reaction at COSY-ANKE at 353 MeV per nucleon. Here {pp}_s denotes a proton-proton pair with low excitation energy, which is dominantly in the ¹S₀ state. By measuring three protons in the final state it was possible to extract events where there was a spectator proton p_spec so that the reaction could be interpreted in terms of quasifree n⃗p⃗→{pp}_sπ^-. The proton and neutron analyzing powers in this reaction were also deduced from this data set by averaging over the polarizations of the deuteron beam and hydrogen target, respectively. The values of A_y were shown to be consistent with a refined analysis of our earlier results obtained with a polarized proton incident on a deuterium target. Taking these data in combination with our previous measurements of the differential cross sections and analyzing powers in the p⃗p→{pp}_sπ⁰ reaction, a more robust partial wave decomposition was achieved. Three different acceptable solutions were found, and the only way of resolving this ambiguity without further theoretical input would be through a measurement of the mixed spin-correlation parameter A_x,z.