Measurement of the spin-correlation coefficients Ax,x and Ay,y in the quasi-free np → {pp}sπ− reaction near threshold at ANKE-COSY

A kinematically complete double-polarization measurement of the quasi-free np → { pp } s π − reaction near threshold has been performed using the ANKE spectrometer at COSY-Julich. Here { pp } s represents a two-proton system with an excitation energy less than 3 MeV. The transversely vector polarized deuteron beam and the internal polarized ANKE hydrogen target were used to determine the spin-correlation coefficients A x,x and A y,y . Events from the quasi-free np → dπ 0 reaction were recorded simultaneously and used for both beam and target polarimetry. In addition, the product of the beam and target polarizations could be estimated from the A y,y coefficient. The storage cell within the polarized target was the main source of background events. Dedicated runs, with no gas in the cell and with the cell filled with N 2 gas, were taken to study the backgrounds. The data analysis procedure and the initial results of the experiment are presented.