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

The transverse spin correlations Ax,x and Ay,y have been measured in the ~d~p ! pspec{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 pspec so that the reaction could be interpreted in terms of quasi-free ~n ~p ! {pp}sπ−. The proton analyzing power in this reaction was also deduced from this data set by averaging over the polarization of the deuteron beam. The values of Apy 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 earlier measurements of the differential cross sections and analyzing powers in the ~pp ! {pp}s π0 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 Ax,z.