BMS symmetry, soft particles and memory

TL;DR

This paper explores the representations of the BMS group, linking soft gravitational modes with memory effects and scattering processes, revealing new insights into gravitational symmetries and soft states.

Contribution

It revisits BMS group representations, connecting soft modes with memory effects and analyzing their implications for the gravitational S-matrix.

Findings

Representations are labeled by super-momenta and four-momentum.

Particle-like states are dressed by soft gravitational modes.

Memory effects are encoded in outgoing soft modes.

Abstract

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.