  The angular distributions of the unpolarised differential cross section and
tensor analysing power $A_{xx}$ of the $\vec{d}d\to\alpha \eta$ 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 $\eta
\alpha$ system. The results do not, however, unambiguously prove the existence
of a quasi-bound $\eta \alpha$ state.

Nuclear Experiment