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Neutrino Signatures in Primordial Non-Gaussianities

Xingang Chen, Yi Wang, Zhong-Zhi Xianyu

TL;DR

This work develops a comprehensive EFT framework to study neutrino signatures in primordial non-Gaussianities, treating neutrinos as heavy fermionic fields that couple to the inflaton and Higgs during inflation. By combining exact de Sitter propagators with slow-roll corrections, the authors show that a nonzero inflaton background induces a chemical potential that can significantly enhance oscillatory signals in the squeezed bispectrum for heavy neutrinos, even at one loop. They analyze multiple scenarios, including unbroken and broken electroweak symmetry and Higgs-portal couplings, deriving explicit mass spectra corrections, loop-induced masses, and clock signals characterized by oscillatory momentum dependence, and they identify parameter regimes where observable non-Gaussianity can be generated in a natural way. The results highlight a robust mechanism by which slow-roll dynamics augment cosmological collider signals, with potential applicability to broader classes of heavy fermions beyond the Standard Model neutrinos, and point to practical implications for probing high-scale physics through precision measurements of primordial non-Gaussianities.

Abstract

We study the cosmological collider phenomenology of neutrinos in an effective field theory. The mass spectrum of neutrinos and their characteristic oscillatory signatures in the squeezed limit bispectrum are computed. Both dS-covariant and slow-roll corrections are considered, so is the scenario of electroweak symmetry breaking during inflation. Interestingly, we show that the slow-roll background of the inflaton provides a chemical potential for the neutrino production. The chemical potential greatly amplifies the oscillatory signal and makes the signal observably large for heavy neutrinos without the need of fine tuning.

Neutrino Signatures in Primordial Non-Gaussianities

TL;DR

This work develops a comprehensive EFT framework to study neutrino signatures in primordial non-Gaussianities, treating neutrinos as heavy fermionic fields that couple to the inflaton and Higgs during inflation. By combining exact de Sitter propagators with slow-roll corrections, the authors show that a nonzero inflaton background induces a chemical potential that can significantly enhance oscillatory signals in the squeezed bispectrum for heavy neutrinos, even at one loop. They analyze multiple scenarios, including unbroken and broken electroweak symmetry and Higgs-portal couplings, deriving explicit mass spectra corrections, loop-induced masses, and clock signals characterized by oscillatory momentum dependence, and they identify parameter regimes where observable non-Gaussianity can be generated in a natural way. The results highlight a robust mechanism by which slow-roll dynamics augment cosmological collider signals, with potential applicability to broader classes of heavy fermions beyond the Standard Model neutrinos, and point to practical implications for probing high-scale physics through precision measurements of primordial non-Gaussianities.

Abstract

We study the cosmological collider phenomenology of neutrinos in an effective field theory. The mass spectrum of neutrinos and their characteristic oscillatory signatures in the squeezed limit bispectrum are computed. Both dS-covariant and slow-roll corrections are considered, so is the scenario of electroweak symmetry breaking during inflation. Interestingly, we show that the slow-roll background of the inflaton provides a chemical potential for the neutrino production. The chemical potential greatly amplifies the oscillatory signal and makes the signal observably large for heavy neutrinos without the need of fine tuning.

Paper Structure

This paper contains 20 sections, 100 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Neutrinos in an Inflationary Background
  3. Effective Couplings between Neutrinos and the Inflaton
  4. Weyl Spinors on dS Background
  5. Weyl Spinors on Slow-Roll Background
  6. dS Covariant Signatures of Neutrinos
  7. dS Covariant Mass Correction to Neutrinos from the Higgs Loop
  8. dS Covariant Signatures in Bispectrum
  9. Neutrino Signatures on a Slow-Roll Background
  10. Slow-Roll Correction to Neutrino Spectrum
  11. A special case with inflaton $\mathbb{Z}_2$ symmetry.
  12. Neutrino Signatures with Electroweak Symmetry Breaking
  13. Neutrino Signatures with Higgs-Portal Coupling.
  14. Signatures in the Bispectrum
  15. Neutrino Signatures from Higgs-Portal Coupling.
  16. ...and 5 more sections

Figures (3)

  • Figure 1: 1-loop corrections to the mass of the left-handed neutrino $\Delta m_\nu$ as functions of right-handed neutrino mass $m_{N0}$. The blue solid curve shows the correction from Yukawa interaction derived in (\ref{['Dmnu']}) with zero-mode approximation, while the dashed orange curve shows the correction from the effective Weinberg operator valid in large $m_{N0}$ limit.
  • Figure 2: The dimensionless strength of the clock signals, as functions of the fermion mass $\mkern 2mu \widetilde{\mkern -2mu m \mkern -2mu}\mkern 2mu=m/H$. The 4 solid curves from bottom to top correspond to choosing $\Lambda = (5,4,3,2)\dot\phi_0^{1/2}$, respectively, and the 4 dashed curves are corresponding approximate results of (\ref{['fNLapp']}).
  • Figure 3: The clock signals $S$ in (\ref{['NloopShape']}) as functions of the momentum ratio $k_1/k_3$. In this plot $\Lambda=3\dot\phi_0^{1/2}$ and the blue, orange, and green curves correspond to $m = (1,3,5)H$, respectively.