  We consider a field theory describing interacting nonrelativistic particles
of two types, which map to each other under time reversal, with point-like
interaction. We identify a new type of interaction which depends on the
relative velocity between the particles. We compute the renormalization group
running of the coupling constants and find a fixed point and a fixed line. We
show that the scattering amplitudes can be expressed in terms of three
parameters. The result matches with a quantum mechanical analysis and
represents the most general point-like interaction consistent with unitarity
and time reversal invariance.
