Scalars at the Cosmological Collider: Full Shapes of Tree Diagrams and Bispectrum Searches using Planck Data

TL;DR

This paper derives full bispectrum shapes from tree-level diagrams involving massive particles during inflation, and searches for these signals in Planck data, finding a slight hint of non-Gaussianity.

Contribution

It provides a unified evaluation of all three classes of tree-level diagrams contributing to primordial non-Gaussianity and extends the search to include scalar chemical potential effects.

Findings

Derived explicit shape functions for the bispectrum across all kinematic space.

Performed a Planck data search for these signals, finding no significant evidence.

Found a 1.5σ hint of non-zero NG in extended models with scalar chemical potential.

Abstract

The Cosmological Collider (CC) provides a unique opportunity to probe the particle spectrum and fundamental interactions at extremely high energies. Massive particles, via their decay into inflaton quanta, can induce a non-analytic, oscillatory, primordial non-Gaussianity (NG), including the bispectrum. At tree level, three classes of such processes contribute to the bispectrum: 'single exchange', 'double exchange', and 'triple exchange', depending on the number of massive particle propagators. We provide a unified evaluation of all three diagrams and derive the explicit shape functions for the bispectrum, valid across the entire kinematic space. We perform a search for these three processes with the Planck data, finding no evidence for NG. We also consider simple extensions of the minimal scenario that can counter the exponential suppression of the non-analytic signature, and produce on-shell particles with masses $M\gg H$, the Hubble scale during inflation. In particular, we focus on the 'scalar chemical potential' mechanism and extend our previous search to a wider range of chemical potential ($\omega$) and $M$, finding global 1.5$\sigma$ evidence for non-zero NG for the parameter space $\omega - M \simeq 3H$.