The authors introduce a Lorentz-invariant hard/soft decomposition of supermomenta and use it to give explicit realizations, branching rules, and a memory-carrying example for generic unitary irreducible representations of the 4D BMS group.
BMS symmetry, soft particles and memory
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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.
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BMS representations for generic supermomentum
The authors introduce a Lorentz-invariant hard/soft decomposition of supermomenta and use it to give explicit realizations, branching rules, and a memory-carrying example for generic unitary irreducible representations of the 4D BMS group.