Nonanalyticity and On-Shell Factorization of Inflation Correlators at All Loop Orders

Abstract: The dynamics of quantum fields during cosmic inflation can be probed via their late-time boundary correlators. The analytic structure of these boundary correlators contains rich physical information of bulk dynamics, and is also closely related to cosmological collider observables. In this work, we study a particular type of nonanalytic behavior, called nonlocal signals, for inflation correlators with massive exchanges at arbitrary loop orders. We propose a signal-detection algorithm to identify all possible sources of nonlocal signals in an arbitrary loop graph, and prove that the algorithm is exhaustive. We then present several versions of the on-shell factorization theorem for the leading nonlocal signal in graphs with arbitrary number of loops, and provide the explicit analytical expression for the leading nonlocal signal. We also generalize the nonlocal-signal cutting rule to arbitrary loop graphs. Finally, we provide many explicit examples to demonstrate the use of our results, including an n-loop melon graph and a variety of 2-loop graphs.