  We study the annihilation of topological solitons in the simplest setting: a
one-dimensional ferromagnet with an easy axis. We develop an effective theory
of the annihilation process in terms of four collective coordinates: two zero
modes of the translational and rotational symmetries $Z$ and $\Phi$,
representing the average position and azimuthal angle of the two solitons, and
two conserved momenta $\zeta$ and $\varphi$, representing the relative distance
and twist. Comparison with micromagnetic simulations shows that our approach
captures well the essential physics of the process.
