Interaction properties of complex solitons are studied for the two U (1)-invariant integrable
generalizations of the modified Korteweg–de Vries (mKdV) equation, given by the Hirota
equation and the Sasa–Satsuma equation, which share the same traveling wave (single-
soliton) solution having a sech profile characterized by a constant speed and a constant
phase angle. For both equations, nonlinear interactions in which a fast soliton collides with a
slow soliton are shown to be described by 2-soliton solutions that can have three different …