“…In the context of high-energy scattering, it is particularly convenient to switch to light-front components defined as a ± = (a 0 ± a 3 )/ √ 2, where the z-direction is the collision axis. In this formulation of relativistic dynamics, the little group is Galilean and the longitudinal light-front momentum plays in the (x, y)-plane the same role as mass does in the non-relativistic context [16,46,76,77]. Finally, coming back to the fact that mass can be seen as the rest-frame energy and noting that light-front boosts are kinematical transformations, one may consider alternatively the light-front version of the energy decomposition discussed e.g.…”