Coupling of massless particles to scalar fields

It is investigated whether massless particles can couple to scalar fields in a special-relativistic theory with classical particles. The only possible obvious theory which is invariant under Lorentz transformationsand reparametrization of the affine parameter leads to trivial trajectories (straight lines) for the massless case, and also the investigation of the masslesslimit of the massive theory shows that there is no influence of the scalar field on the limiting trajectories. On the other hand, in contrast to this result, it is shown that massive particlesare influence by the scalar field in this theory even in the ultrarelativistic limit.