Near-threshold pion production in diproton reactions with polarised beams and target at ANKE-COSY

¹ Physikalisches Institut II, Universität Erlangen-Nürnberg, 91058 Erlangen, Germany
² Laboratory of Nuclear Problems, Joint Institute for Nuclear Research, 141980 Dubna, Russia

^a e-mail: s.dymov@fz-juelich.de

Abstract

An extensive experimental study of near-threshold pion production in diproton reactions is underway at the ANKE-COSY spectrometer (Jülich). The programme is aimed at isolating the four-nucleon-pion contact interaction term appearing in the _XPT expansions of these processes. This will establish links between pion production and other low energy phenomena within the _XPT approach. The first step in the programme was to measure the differential cross-section and the proton analysing power in the $\overrightarrow{p}p\to \left\{pp\right\}{}_s\pi {}^0$ and $\overrightarrow{p}n\to \left\{pp\right\}{}_s\pi {}^{-}$ reactions over the full angular range. Here {pp}_s denotes a diproton, i.e., a two-proton system in a ¹S₀ state. These data allow a partial wave analysis to be carried out provided that simplifying assumptions are made when applying the Watson theorem. To make the analysis more robust, and independent of the uncertainties in the relative normalization, the spin-correlation coefficients A_x,x and A_y,y in the $\overrightarrow{n}\overrightarrow{p}\to \left\{pp\right\}{}_s\pi {}^{-}$ reaction were measured in a follow-up experiment. The first results of the data analysis are presented and future developments of the programme are outlined.