Inflation Correlators at the One-Loop Order: Nonanalyticity, Factorization, Cutting Rule, and OPE

Abstract: Inflation correlators with one-loop massive exchange encode rich information about the dynamics of the massive loop particles. Their nonanalytic behavior in certain soft limits leads to characteristic oscillatory pattern, which is the leading signal of many particle models of cosmological collider physics. In this work, we investigate systematically such nonanalyticity for arbitrary one-particle-irreducible (1PI) one-loop correlators in various soft limits. With the partial Mellin-Barnes representation, we present and prove a factorization theorem and a cutting rule for arbitrary 1PI one-loop inflation correlators, which is reminiscent of the on-shell cutting rule for flat-space scattering amplitudes. We also show how to understand this factorization theorem from the viewpoint of operator product expansion on the future boundary. As an application of the one-loop factorization theorem, we derive new analytic and exact formulae for nonlocal cosmological collider signals for massive one-loop four-point inflation correlators of all possible 1PI topologies, including the bubble, the triangle, and the box graphs. Finally, we show how to push the computation of nonlocal signals to higher orders in the momentum ratio.