A key process in the muon-catalysed fusion cycle is a low-energy colli
sion of t mu with a D-2 molecule which leads, at appropriate incident
energies, to the formation of a resonant complex containing dt mu. In
this paper the result is described of a formal derivation of the parti
al wave cross section for resonant dt mu formation, which makes use of
elements of Feshbach's treatment of resonances. The expression obtain
ed is similar to the Breit-Wigner formula. Full details of the calcula
tion will be published elsewhere.