Measurements on the 3He+\eta System at ANKE

Abstract

The differential and total cross-sections for the dp \to 3He \eta reaction have been measured in a high precision high statistics COSY-ANKE experiment near threshold using a continuous beam energy ramp up to an excess energy Q of 11.3 MeV with essentially 100% acceptance. The kinematics allowed the mean value of Q to be determined to about 9 keV. Evidence is found for the effects of higher partial waves for Q > 4 MeV. The very rapid rise of the total cross-section to its maximum value within 0.5 MeV of threshold implies a very large \eta 3He scattering length and hence the presence of a quasi-bound state extremely close to threshold. In addition, differential and total cross-sections have been measured at excess energies of 19.5, 39.4, and 59.4 MeV over the full angular range. While at 19.5 MeV the results can be described in terms of s- and p-wave production, by 59.4 MeV higher partial waves are required. Including the 19.5 MeV point together with the near-threshold data in a global s- and p-wave fit gives a poorer overall description of the data though the position of the pole in the \eta 3He scattering amplitude, corresponding to the quasi-bound or virtual state, is hardly changed.