where the reaction is dominated by the exchange of the muon between two identical nuclei. There is also the possibility, though much smaller, of spin flip in asymmetric collisions:

Obviously, unlike the symmetric case, this cannot be achieved by muon exchange, and a relativistic interaction is required to flip the spin so the cross sections are much lower. According to Cohen [21], who calculated these processes in the Improved Adiabatic approach (Section 2.1.3), non-symmetric spin flip cross sections are several orders of magnitude smaller than symmetric spin flip cross sections.

In addition, there is a prediction that spin flip in the pure deuterium
system occurs via the back decay of the muonic molecular complex (resonant
spin flip reaction) [34],

but the experimental data become rather inconsistent with theoretical predictions, if the resonant spin flip is included [12,35].