Measurement of spin observables in the quasi-free np-> {pp}_s pi- reaction at 353 MeV

The transverse spin correlations A(x,x) and A(y,y) have been measured in the (d) over right arrow(p) over right arrowp -> p(spec){pp}(s)pi(-) 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 S-1(0) 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 quasi-free (n) over right arrow(p) over right arrow -> {pp}(s)pi(-). 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 (p) over right arrowp -> {pp}(s)pi(0) 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).