BMS particles

TL;DR

The paper addresses infrared issues in massless scattering by reframing asymptotic states in terms of BMS symmetry. It develops a wavefunction realization of BMS UIRs as quantum superpositions of Poincaré particles propagating on inequivalent gravity vacua, using a Lorentz-invariant hard/soft decomposition of the supermomentum: $\mathcal{P}(z,\bar z)=\partial_z^2\partial_{\bar z}^2\mathscr{N}(z,\bar z)+P(z,\bar z)$. The authors construct explicit BMS wavefunctions, connect them to Strominger's gravitational phase space, and show how BMS particles arise as genuine particles via momentum eigenstates labeled by gravity-vacuum data, with hard states recovering Poincaré behavior and soft data encoding vacuum structure. This framework provides a principled route to IR-finite observables in flat spacetime by treating the S-matrix as living over the moduli space of gravity vacua, and it suggests parallel constructions for QED, paving the way for Mandelstam-like variables and a fuller second-quantized theory of BMS particles.

Abstract

We construct wavefunctions for unitary irreducible representations (UIRs) of the Bondi-Metzner-Sachs (BMS) group, i.e. BMS particles, and show that they describe quantum superpositions of (Poincaré) particles propagating on inequivalent gravity vacua. This follows from reconsidering McCarthy's classification of BMS group UIRs through a unique, Lorentz-invariant but non-linear, decomposition of supermomenta into hard and soft pieces.