  The field of few-body physics has originally been motivated by understanding
nuclear matter. New model systems to experimentally explore few-body quantum
systems can now be realized in ultracold gases with tunable interactions.
Albeit the vastly different energy regimes of ultracold and nuclear matter (peV
as compared to MeV), few-body phenomena are universal for near-resonant
two-body interactions. Efimov states represent a paradigm for universal
three-body states, and evidence for their existence has been obtained in
measurements of three-body recombination in an ultracold gas of caesium atoms.
Interacting samples of halo dimers can provide further information on universal
few-body phenomena. Here we study interactions in an optically trapped mixture
of such halo dimers with atoms, realized in a caesium gas at nanokelvin
temperatures. We observe an atom-dimer scattering resonance, which we interpret
as being due to a trimer state hitting the atom-dimer threshold. We discuss the
close relation of this observation to Efimov's scenario, and in particular to
atom-dimer Efimov resonances.
