BMS representations for generic supermomentum

TL;DR

This paper classifies and constructs explicit representations of the BMS group, revealing their differences from Poincaré particles and their capacity to encode gravitational memory effects, with implications for infrared physics.

Contribution

It introduces a Lorentz-invariant decomposition of supermomenta and details how generic BMS representations relate to gravitational vacua and memory effects.

Findings

Explicit realisations of BMS representations are provided.

Different gravity vacua lead to distinct interpretations of BMS states.

Generic BMS particles can encode gravitational memory effects.

Abstract

We revisit the classification, and give explicit realisations, of unitary irreducible representations of the BMS group. As compared to McCarthy's seminal work, we make use of a unique, Lorentz-invariant, decomposition of supermomenta into a hard and a soft piece, that we introduce and properly define, to investigate the extent to which generic representations depart from usual Poincar\'e particles and highlight their relations to gravitational infrared physics. We insist on making wavefunctions as explicit as possible. Similarly, we explain how branching to a Poincar\'e subgroup works in practice: this is physically relevant because this amounts to reading off the field content of a given BMS state in terms of a choice of gravity vacuum. In particular, we emphasise how different gravity vacua differ in their interpretation of the same BMS state, here again providing concrete examples as well as the general procedure. Finally, we demonstrate on an example that generic BMS particles are flexible enough to encode memory, as opposed to usual Poincar\'e particles.