  Mixtures of polarised fermions of two different masses can form weakly-bound
clusters, such as dimers and trimers, that are universally described by the
scattering length between the heavy and light fermions. We use the resonating
group method to investigate the low-energy scattering processes involving
dimers or trimers. The method reproduces approximately the known particle-dimer
and dimer-dimer scattering lengths. We use it to estimate the trimer-trimer
scattering length, which is presently unknown, and find it to be positive.
