Flat space amplitudes and conformal symmetry of the celestial sphere

The four-dimensional (4D) Lorentz group SL (2, C) acts as the two-dimensional (2D) global conformal group on the celestial sphere at infinity where asymptotic 4D scattering states are specified. Consequent similarities of 4D flat space amplitudes and 2D correlators on the conformal sphere are obscured by the fact that the former are usually expressed in terms of asymptotic wave functions which transform simply under spacetime translations rather than the Lorentz SL (2, C). In this paper we construct on-shell massive scalar wave functions in 4D Minkowski space that transform as SL (2, C) conformal primaries. Scattering amplitudes of these wave functions are SL (2, C) covariant by construction. For certain mass relations, we show explicitly that their three-point amplitude reduces to the known unique form of a 2D CFT primary three-point function and compute the coefficient. The computation proceeds naturally via Witten-like diagrams on a hyperbolic slicing of Minkowski space and has a holographic flavor.