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

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 π0 and ~pn →{pp}s πreactions over the full angular range. Here {pp}s denotes adiproton, i.e.,a two-proton system in a 1S 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 Ax,x and Ay,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. 1 The physics case for near-threshold single pion production 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]. The symbol {pp}s here denotes a diproton, that is, an unbound proton pair with a very low excitation energy, Epp < 3 MeV. The selection of a low excitation energy ensures the dominance of the 1S 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 → deν 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 π0 and np → {pp}s π reactions have the big advantage for COSY that both the pion and diproton have spin-zero, which means that the only spin degrees of freea e-mail: [email protected] Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20123701020 EPJ Web of Conferences dom are connected with the initial state. There are therefore no non-trivial spin-transfer observables, which means that rescattering experiments are not required. Four types of experiments are possible for both π0 and π production. 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 then 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. These data will provide two determinations of the LEC d. 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. 2 Measurement of cross section and analysing power As first steps in the programme, measurements with a polarised proton beam incident on unpolarised hydrogen and deuterium cluster targets were performed at a beam energy of Tp = 353 MeV [2,3]. 2.1 The ~pp → {pp}s π0 process The polarised proton COSY beam and the ANKE hydrogen cluster target were used in this measurement. The final proton pair was recorded in the ANKE forward detector and identified with the use of the time-of-flight difference between the two protons. After the selection of the proton pairs, the ~pp → {pp}s π0events were identified by the missing-mass criterion. The beam polarisation and luminosity were determined by detecting simultaneously the ~pp → dπ and ~pp → pp processes, for which the cross sections and analysing powers are known with high precision at this energy. cm π θ cos -1 -0.5 0 0.5 1 [n b/ sr ]