Null-plane formalism and polarization

A systematic study is made of the properties of spin in the context of null-plane (or "infinite-momentum") formalism, where the spin projection is invariant under the action of a Galilean subgroup of the Poincar\'e group. We give explicitly its relations with the helicity and the canonical formalism. The crossing relations and the structure of kinematic singularities of the corresponding amplitudes are studied. It is shown that these amplitudes can be kinematically regularized in a fairly simple fashion. Reduction of direct products and partial-wave expansions are also discussed and some particular simple properties are pointed out.