Abstract
An extensive experimental study of near-threshold pion production in dipro- ton reactions is underway at the ANKE-COSY spectrometer (JThe programme is aimed at isolating the four-nucleon-pion contact interaction term appearin g in thePT ex- pansions of these processes. This will establish links between pion production and other low energy phenomena within thePT approach. The first step in the programme was to measure the differential cross-section and the proton analysing power in the~!f ppg s� 0 and~n!f ppg s� − reactions over the full angular range. Heref ppg 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 coeffi cients Ax; x and Ay㭹 in the~ n~! f ppg s� − reac- tion were measured in a follow-up experiment. The first results of the data analysis are presented and future developments of the programme are outlined.
Highlights
The ANKE [1] experimental programme on the near-threshold pion production aims to measure the cross sections and spin observables in the pp → {pp}s π0 and np → {pp}s π− reactions [2, 3]
An extensive experimental study of near-threshold pion production in diproton reactions is underway at the ANKE-COSY spectrometer (Julich)
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)
Summary
The ANKE [1] experimental programme on the near-threshold pion production aims to measure the cross sections and spin observables in the pp → {pp}s π0 and np → {pp}s π− reactions [2, 3]. These are the measurement of the unpolarised differential cross section dσ/dΩ, the beam or target analysing power Ay, the in-plane spin-correlation Ax,x, and the mixed correlation parameter Ax,z Knowing these one can determine the magnitudes and the relative phase of the two scalar amplitudes as functions of the pion production angle for either the pp or pn experiment. At low energies it is reasonable to assume that data can be analysed by truncating the partial wave expansion at orbital angular momentum l = 2 It is shown in [5] that the magnitude of one of the pwave amplitudes is fixed completely by the measurement of (1 − Ax,x) · dσ/dΩ for np → {pp}s π− and that the magnitude of the other p-wave amplitude and its relative phase can be deduced from a combined analysis of this with our cross section and analysing power data for pp → {pp}s π0and np → {pp}s π−. Measurements of the mixed spin-correlation parameters Az,x are not required for the extraction of the p-wave amplitudes, though such information is vital in order to identify the d-wave terms
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