0.9@percent A large enhancement in the excitation function of the pp → ppη reaction observed close to the kinematical threshold indicates a strong attractive interaction within the ppη system [1,2]. The effect can be described assuming, that the proton-proton pair is produced from a large object of a 4 fm radius [3]. It is however still not established whether the low energy ppη system can form a Borromean or resonant state. Recently published high statistics data for the pp → ppη reaction measured by the COSY-11 collaboration will be used to elucidate this question. This data are presently evaluated using the well known intensity interferometry method, commonly referred to as the HBT effect [4]. This technique permits the size of the source from which protons are emitted to be determined. It is based on the correlation function of two protons relative momenta. A comparison of the experimental results with theoretical predictions will be presented and the achieved accuracy of the determination of the source size will be discussed. In particular it will be shown how the shape of two-proton correlation function calculated including Coulomb interaction and Pauli exclusion principle depends on the spatial size of the source.

Supported by FZ-Jülich and DAAD.