Conformal basis for flat space amplitudes

We study solutions of the Klein-Gordon, Maxwell, and linearized Einstein equations in R1,d+1 that transform as d-dimensional conformal primaries under the Lorentz group SO(1,d+1). Such solutions, called conformal primary wavefunctions, are labeled by a conformal dimension Δ and a point in Rd, rather than an on-shell (d+2)-dimensional momentum. We show that the continuum of scalar conformal primary wavefunctions on the principal continuous series Δ∈d2+iR of SO(1,d+1) spans a complete set of normalizable solutions to the wave equation. In the massless case, with or without spin, the transition from momentum space to conformal primary wavefunctions is implemented by a Mellin transform. As a consequence of this construction, scattering amplitudes in this basis transform covariantly under SO(1,d+1) as d-dimensional conformal correlators.