Non-Gaussianity as a Particle Detector

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.

• The paper classifies couplings of massive spin particles to the Goldstone boson and graviton, extending the effective field theory of inflation.
• It employs the in-in formalism to separate local analytic from non-local non-analytic contributions in scalar and tensor bispectra.
• The distinctive angular signatures in the bispectra offer a potential observational probe into high-energy physics during inflation.

Non-Gaussianity as a Particle Detector: An Expert Review

The paper "Non-Gaussianity as a Particle Detector" by Hayden Lee, Daniel Baumann, and Guilherme L. Pimentel explores the imprints of massive particles with spin on cosmological correlators in the context of the effective field theory (EFT) of inflation. The authors aim to understand the effects of these particles on the scalar and tensor bispectra by considering their interactions with the Goldstone boson of broken time translations and the graviton.

Summary of Key Contributions

1. Classification of Couplings: The paper effectively classifies the couplings of massive particles to both the Goldstone boson and the graviton. This classification extends the EFT of inflation, providing a robust framework to model the interaction of additional particles during inflation.
2. Analytical Techniques: Using in-in formalism, the authors compute the contributions of these particles to the bispectra of scalar and tensor perturbations. They differentiate between local (analytic) and non-local (non-analytic) processes, with a focus on the distinctive non-local contributions that cannot be captured by local operators added to the effective theory of the light fields alone.
3. Signatures of Massive Particles: The research demonstrates that massive particles with spin imprint specific non-Gaussian features into the cosmological correlators. The presence of these particles is indicated by the angular dependencies in the resulting bispectra, which are characterized by Legendre polynomials reflecting the particles' spin and mass.
4. Effects of Conformal Symmetry Breaking: The study explores how breaking the approximate conformal symmetry of the inflationary background, via a non-trivial sound speed $c_\pi$ for the Goldstone boson, influences the detectable range of particle masses. For $c_\pi < \mu_s^{-1}$, the sensitivity to massive particles is enhanced, reducing the exponential suppression typically expected in their detection.
5. Tensor-Scalar-Scalar Bispectrum: A notable extension in this paper is the investigation of the $\langle \gamma \zeta \zeta \rangle$ correlator's squeezed limit, showing how massive particles with spin generate deviations from the consistency conditions characteristic of single-field inflation models.

Implications and Future Directions

The results suggest that detecting such non-Gaussian signals could offer a unique portal into the high-energy particle spectrum of the early universe, potentially probing physics beyond the reach of current colliders. The distinctive angular patterns in the bispectra serve as a compelling spectroscopic signature of these massive particles' existence during inflation.

Furthermore, the paper identifies future observational strategies, such as refined measurements of the CMB and large-scale structure (LSS), necessary to constrain or detect these non-Gaussian signals. The improved sensitivity of upcoming experiments may allow us to explore this window into the inflationary period, offering insights into the symmetry-breaking scale and details of the initial conditions of our universe.

Concluding Remarks

This paper provides a comprehensive analysis of the interaction between massive spin particles and primordial cosmological perturbations. By systematically enhancing the EFT of inflation, it offers solid theoretical predictions that can guide future empirical investigations. The insights gained here could significantly advance our understanding of particle physics at the highest energy scales and the inflationary epoch's intricate dynamics.