Celestial amplitudes from conformal correlators with bulk-point kinematics

Abstract

We show that two- and three-point celestial (C)CFT$_{d-1}$ amplitudes can be directly obtained from correlation functions in a unitary Lorentzian CFT$_d$ on $\mathbb{R}\times S^{d-1}$. The recipe involves a rescaling of the operators, followed by an expansion around a bulk point configuration and a transformation to an $S^{d-1}$ conformal primary basis. The first two steps project the CFT$_d$ correlators onto distributions on $S^{d-1}$. The final step implements a dimensional reduction yielding CCFT$_{d-1}$ amplitudes that are manifestly vanishing for all in/out configurations and Poincaré invariant. The dimensional reduction may be implemented either by evaluating certain time integral transforms around the bulk-point limit, or by analytically continuing the CFT$_d$ operator dimensions and restricting the operators to $S^{d-1}$ time slices separated by $π$ in global time. The latter prescription generates the correct normalization for both two- and three-point functions. On the other hand, the celestial three-point amplitudes obtained via the former prescription are found to only agree after evaluating a residue at an integer linear combination of the CFT$_d$ conformal dimensions. The correct normalization may also be obtained by considering a different integration path in the uplift of the complexified time plane to its universal cover.