  The 4D 4-point scattering amplitude of massless scalars via a massive
exchange is expressed in a basis of conformal primary particle wavefunctions.
This celestial amplitude is expanded in a basis of 2D conformal partial waves
on the unitary principal series, and then rewritten as a sum over 2D conformal
blocks via contour deformation. The conformal blocks include intermediate
exchanges of spinning light-ray states, as well as scalar states with positive
integer conformal weights. The conformal block prefactors are found as expected
to be quadratic in the celestial OPE coefficients.
