Measurement of spin observables in the quasifreenp→{pp}sπ−reaction at 353 MeV

Abstract

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 1 S0 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 A p 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 pp → {pp}s π 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 Ax,z.

The legacy of the experimental hadron physics programme at COSY

“…A simple estimate of the total cross section for E pp < 3 MeV from the pp effective mass distributions of Ref. [124] would suggest a somewhat lower value than the ANKE result [125] discussed in sect. 4.3.2, but such an evaluation is very crude.…”

“…38. It should be noted that the angle is here defined with respect to the direction of the incident (virtual) neutron and not the proton that was used for the ANKE data [125] shown in Fig. 42, where only events with the final pp excitation energy E pp < 3 MeV were considered.…”

“…Such experiments are only possible at ANKE close to threshold where the spectrometer acceptance covers a large fraction of the solid angle. The programme involved the measurement of the differential cross section and analyzing power in pp → {pp} s π 0 [120], the differential cross section and analyzing power in quasi-free pn → {pp} s π − with a deuterium target [137], and the analyzing powers and spincorrelations in quasi-free n p → {pp} s π − with a polarized deuteron beam and a polarized hydrogen cell target [125].…”

“…The full kinematics were determined by detecting either the spectator proton in an STT or the produced π − in the negative detector, in coincidence with the diproton pair. In the subsequent experiment, a polarized deuteron beam collided with a polarized hydrogen cell [125]. In this case the fast spectator proton was measured in the forward detector of ANKE and the diproton pair in the positive detector.…”

“…[deg] The full, long-dashed, and short-dashed lines correspond to solutions 1, 2, and 3, respectively. The ANKE experimental data with statistical errors correspond to a) the differential cross-section [137], b) A p y [125], and c) Ax,x [125]. There are no experimental data to compare with the predictions for Ax,z shown in panel d).…”