  We propose a new program for computing a certain integrand of scattering
amplitudes of four-dimensional gauge theories which we call the \textit{form
factor integrand}, starting from 6d holomorphic theories on twistor space. We
show that the form factor integrands can be expressed as sums of products of
1.) correlators of a 2d chiral algebra, related to the algebra of asymptotic
symmetries uncovered recently in the celestial holography program, and 2.) OPE
coefficients of a 4d non-unitary CFT. We prove that conformal blocks of the
chiral algebras are in one-to-one correspondence with local operators in 4d. We
use this bijection to recover the Parke-Taylor formula, the CSW formula, and
certain one-loop scattering amplitudes. Along the way, we explain and derive
various aspects of celestial holography, incorporating techniques from the
twisted holography program such as Koszul duality. This perspective allows us
to easily and efficiently recover the infinite-dimensional chiral algebras of
asymptotic symmetries recently extracted from scattering amplitudes of massless
gluons and gravitons in the celestial basis. We also compute some simple
one-loop corrections to the chiral algebras and derive the three-dimensional
bulk theories for which these 2d algebras furnish an algebra of boundary local
operators.
