Carrollian conformal fields and flat holography

Abstract: The null conformal boundary $\mathscr{I}$ of Minkowski spacetime $\mathbb{M}$ plays a special role in scattering theory, as it is the locus where massless particle states are most naturally defined. We construct quantum fields on $\mathscr{I}$ which create these massless states from the vacuum and transform covariantly under Poincar\'e symmetries. Since the latter symmetries act as Carrollian conformal isometries of $\mathscr{I}$, these quantum fields are Carrollian conformal fields. This group theoretic construction is intrinsic to $\mathscr{I}$ by contrast to existing treatments in the literature. However we also show that the standard relativistic massless quantum fields in $\mathbb{M}$, when pulled back to $\mathscr{I}$, provide a realisation of these Carrollian conformal fields. This correspondence between bulk and boundary fields should constitute a basic entry in the dictionary of flat holography. Finally we show that $\mathscr{I}$ provides a natural parametrisation of the massless particles as described by irreducible representations of the Poincar\'e group, and that in an appropriate conjugate basis they indeed transform like Carrollian conformal fields.