  In this work, we revisit unitary irreducible representations of the
Bondi-Metzner-Sachs (BMS) group discovered by McCarthy. Representations are
labelled by an infinite number of super-momenta in addition to four-momentum.
Tensor products of these irreducible representations lead to particle-like
states dressed by soft gravitational modes. Conservation of 4-momentum and
supermomentum in the scattering of such states leads to a memory effect encoded
in the outgoing soft modes. We note there exist irreducible representations
corresponding to soft states with strictly vanishing four-momentum, which may
nevertheless be produced by scattering of particle-like states. This fact has
interesting implications for the S-matrix in gravitational theories.

High Energy Physics - Theory