Cross section and tensor analysing power of the dd→ηαdd→ηα reaction near threshold

The angular distributions of the unpolarised differential cross section and tensor analysing power AxxAxx of the d→d→αη reaction have been measured at an excess energy of 16.6 MeV. The ambiguities in the partial-wave description of these data are made explicit by using the invariant amplitude decomposition. This allows the magnitude of the s-wave amplitude to be extracted and compared with results published at lower energies. In this way, firmer bounds could be obtained on the scattering length of the ηα system. The results do not, however, unambiguously prove the existence of a quasi-bound ηα state.