Non-Gaussianity as a Particle Detector

TL;DR

The paper develops an EFT framework for inflation that includes massive spinning particles coupled to the Goldstone mode $\pi$ and the graviton. It shows that observable non-Gaussian signatures can arise when particle masses are near the Hubble scale and conformal symmetry is sufficiently broken, and provides explicit scalar and tensor bispectrum templates arising from single, double, and triple exchange topologies. A key finding is that lowering the Goldstone sound speed $c_π$ can substantially enhance non-analytic (particle-production) signals, improving detectability, while odd-spin signals face additional angular suppressions. The work delivers detailed predictions for the momentum and angular dependences of $\langle\zeta\zeta\zeta\rangle$ and $\langle\gamma\zeta\zeta\rangle$, including observationally relevant scalings in the squeezed limit and practical templates for future surveys.

Abstract

We study the imprints of massive particles with spin on cosmological correlators. Using the framework of the effective field theory of inflation, we classify the couplings of these particles to the Goldstone boson of broken time translations and the graviton. We show that it is possible to generate observable non-Gaussianity within the regime of validity of the effective theory, as long as the masses of the particles are close to the Hubble scale and their interactions break the approximate conformal symmetry of the inflationary background. We derive explicit shape functions for the scalar and tensor bispectra that can serve as templates for future observational searches.

Figures (10)

• Figure 1: Diagrams contributing to $\langle\zeta\zeta\zeta\rangle$ and $\langle\gamma\zeta\zeta\rangle$. The solid, dashed, and wavy lines represent the curvature perturbation $\zeta$, a massive spin-$s$ field $\sigma_{i_1\cdots i_s}$, and the graviton $\gamma_{ij}$, respectively.
• Figure 2: Tree-level diagram contributing to the two-point function $\langle\zeta\zeta\rangle$. The solid and dashed lines represent the curvature perturbation $\zeta$ and a massive spin-$s$ field $\sigma_{i_1\cdots i_s}$, respectively.
• Figure 3: Pictorial representations of the horizon crossing scale of the Goldstone boson (solid) and the scale associated with the turning point in the dynamics of a massive particle (dashed), with the left (right) diagram corresponding to $c_\pi =1$ ($c_\pi <\mu^{-1}$). The Hubble radius is denoted by $r_H\equiv H^{-1}$. We see that for $c_\pi <\mu^{-1}$ the horizon crossing of the Goldstone boson occurs before the turning point of the massive particles, while for $c_\pi =1$ it occurs after.
• Figure 4: Wick-rotated integrand of the integral in (\ref{['C1']}) as a function of $x=|k\eta|$ and for $\mu=5$. The vertical dotted lines indicate the times of sound horizon crossing of $\pi$, i.e. $x = c_\pi^{-1}$, for each value of $c_\pi$. The solid vertical line marks the turning point of $\sigma$, i.e. $x=\mu$.
• Figure 5: ${\cal C}_1$ and ${\cal C}_2$ as functions of $c_\pi$ for $\mu=1$ (black) and $\mu=3$ (red). The solid and dotted lines denote ${\cal C}_1$ and ${\cal C}_2$, respectively.