  In this note, we explore the correspondence between four-dimensional flat
space S-matrix and two-dimensional CFT proposed by Pasterski et al. We
demonstrate that the factorization singularities of an n-point cubic diagram
reproduces the AdS Witten diagrams if mass conservation is imposed at each
vertex. Such configuration arises naturally if we consider the 4-dimensional
S-matrix as a compactified massless 5-dimensional theory. This identification
allows us to rewrite the massless S-matrix in the CHY formulation, where the
factorization singularities are re-interpreted as factorization limits of a
Riemann sphere. In this light, the map is recast into a form of 2d/2d
correspondence.
