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

Abstract

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 χPT expansions of these processes. This will establish links between pion production and other low energy phenomena within the χPT approach. The first step in the programme was to measure the differential cross-section and the proton analysing power in the pp → {pp} s π 0 and pn → {pp} s π − reactions over the full angular range. Here {pp} s denotes a diproton, i.e.,a two-proton system in a 1 S 0 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 n p → {pp} s π − 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. The physics case for near-threshold single pion productionThe ANKE [1] experimental programme on the near-threshold pion production aims to measure the cross sections and spin observables in the pp → {pp} s π 0 and np → {pp} s π − reactions [2,3]. The symbol {pp} s here denotes a diproton, that is, an unbound proton pair with a very low excitation energy, E pp < 3 MeV. The selection of a low excitation energy ensures the dominance of the 1 S 0 state of the diproton, which simplifies significantly the theoretical analysis. A full data set of all observables at low beam energies would allow us to determine the partial wave amplitudes which, in turn, would provide a non-trivial test of chiral perturbation theory [4] and also lead to the determination of the value of the parameter d, which represents the important contact term that affects the pion p-wave amplitudes.The short range physics in chiral effective field theories, which provide a model-independent understanding of Nature, is encoded in the so-called low energy constants (LEC). These LECs, once determined from one process, can be applied to predict many others. For example, the LECs c1-c4 extracted from πN analysis on the basis of chiral perturbation theory are now widely used to parameterise the short range physics in the NN-interaction, few-nucleon systems, single (and multi-) pion production in NN collisions etc. Analogously, by studying the p-wave pion production amplitudes we get access to the 4Nπ contact operator, the strength of which is controlled by the low energy constant d. This LEC enters also in electroweak processes, such as pp → de + ν and triton β decay, in few-body operators (e.g. in pd → pd), pion photoproduction γd → nnπ + and its inverse πd → γNN. It therefore plays a very important role in connecting different low-energy reactions.On the practical side, the pp → {pp} s π 0 and np → {pp} s π − reactions have the big advantage f...

Internal Experiments at COSY-Juelich

“…As the first part of this larger programme, we report in these conference proceedings on a measurements of the cross section and proton analysing power in the pp → {pp} s π • and np → {pp} s π − reactions [16].…”

“…By means of a Siberian snake, the spin closed orbit at the location of the polarized target can be aligned along the direction of motion of the stored particles. The required field strengths that can be obtained with a solenoid together for the various experimental investigations [16,22,23,24,25] are listed in table 1. The snake is ordered and will be delivered during the next year.…”