Status Report for COSY Proposal # 213 Measurement of the spin correlation parameter A x , z of the quasi-free ~ p ~ n → { pp } s π − reaction at ANKE

The measurement of the spin-correlation coefficient Ax,z of the ~ p~n → {pp}sπ − reaction for excitation energies of the final diproton less than 3 MeV represents the final part of the pion production programme at ANKE. It aims to resolve the ambiguities in the partial wave analysis of the existing data and the results will be of relevance for Chiral Perturbation Theory. The experiment requires the use of a longitudinally polarised proton beam of 353 MeV energy and a transversally polarised deuterium target at ANKE, equipped with an openable cell, and Silicon Tracking Telescopes (STT). As a first stage, a preparatory measurement has been carried out to test the feasibility of the experiment and estimate the achievable luminosity. In the current document, we give the status report on the proposal and present the data collected for the first time with the openable cell at ANKE in November 2013. An important result of the beam time was to understand the background conditions when measuring the ~p~n → {pp}sπ − reaction with an openable cell at 353 MeV, and the optimization of the cell dimensions. The measurement resulted in a more robust estimation of the total beam time required for the main experiment, which is still in a stage of waiting the Siberian snake commissioning at COSY. The physics case of this proposal was accepted positively by the PAC session 40 and 41, but the final decision was postponed until the installation of the Siberian snake at COSY. The present document contains an update of the proposed plans and an estimation of the beam time required. The preliminary time line of Snake delivery in Jülich is March/April 2014.