The Limitations of Cosmological Collider Analyses

Abstract

Massive fields exchanged during inflation source cosmological collider features in the correlators of primordial curvature fluctuations. While their structure is fixed by the symmetries of inflation, the amplitude of such signatures is a priori unknown, though bounded by physical principles such as unitarity. We assess the observational viability of the scalar cosmological collider by combining precise numerical predictions derived using CosmoFlow with observational data from Planck bispectrum reconstructions. Working in the limit of weak quadratic mixing between the Goldstone and scalar sectors (as assumed by all previous searches), we find that perturbativity restricts the collider signals to be unobservably small, except for double- and triple-exchange configurations at low masses. Larger bispectra can be generated by increasing the mixing to non-perturbative (but still unitary) values: however, this distorts the signal, damping its amplitude and shifting the collider oscillations. Furthermore, strong mixing breaks the factorization of the bispectrum into a free amplitude and a fixed shape, and complicates both theoretical predictions and observational searches. Despite the challenges, we argue that strong mixing analyses are necessary to obtain meaningful constraints on the scalar cosmological collider from current and upcoming datasets.