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Nongaussianity from Tachyonic Preheating in Hybrid Inflation

Neil Barnaby, James M. Cline

TL;DR

This work analyzes the generation of non-Gaussianity from tachyonic preheating at the end of hybrid inflation, identifying two distinct regimes: a scale-invariant regime where large $f_{NL}$ can arise, and a nonscale-invariant regime with a dominant $n=4$ spectral contamination. By deriving the second-order tachyon contribution to the curvature perturbation from first-order tachyon fluctuations and solving the tachyon mode equations with both WKB and adiabatic approximations, the authors map constraints onto the parameter spaces of hybrid inflation, brane-antibrane inflation, and P-term inflation. They show that in brane inflation the scale-noninvariant constraints largely vanish in realistic warped scenarios, while scale-invariant constraints can still bound the models; in D-term and F-term inflation, spectral distortions impose meaningful exclusions in small-$g$ regimes. Extending the analysis to complex tachyons (O(M) multiplets) preserves the qualitative conclusions, with only order-unity changes to the spectra and non-Gaussianity. Overall, tachyonic preheating offers a mechanism for observable non-Gaussianities or spectrum distortions in specific hybrid-inflation realizations, with implications for brane-inflation models and supergravity constructions.

Abstract

In a previous work we showed that large nongaussianities and nonscale-invariant distortions in the CMB power spectrum can be generated in hybrid inflation models, due to the contributions of the tachyon (waterfall) field to the second order curvature perturbation. Here we clarify, correct, and extend those results. We show that large nongaussianity occurs only when the tachyon remains light throughout inflation, whereas n=4 contamination to the spectrum is the dominant effect when the tachyon is heavy. We find constraints on the parameters of warped-throat brane-antibrane inflation from nongaussianity. For F-term and D-term inflation models from supergravity, we obtain nontrivial constraints from the spectral distortion effect. We also establish that our analysis applies to complex tachyon fields.

Nongaussianity from Tachyonic Preheating in Hybrid Inflation

TL;DR

This work analyzes the generation of non-Gaussianity from tachyonic preheating at the end of hybrid inflation, identifying two distinct regimes: a scale-invariant regime where large can arise, and a nonscale-invariant regime with a dominant spectral contamination. By deriving the second-order tachyon contribution to the curvature perturbation from first-order tachyon fluctuations and solving the tachyon mode equations with both WKB and adiabatic approximations, the authors map constraints onto the parameter spaces of hybrid inflation, brane-antibrane inflation, and P-term inflation. They show that in brane inflation the scale-noninvariant constraints largely vanish in realistic warped scenarios, while scale-invariant constraints can still bound the models; in D-term and F-term inflation, spectral distortions impose meaningful exclusions in small- regimes. Extending the analysis to complex tachyons (O(M) multiplets) preserves the qualitative conclusions, with only order-unity changes to the spectra and non-Gaussianity. Overall, tachyonic preheating offers a mechanism for observable non-Gaussianities or spectrum distortions in specific hybrid-inflation realizations, with implications for brane-inflation models and supergravity constructions.

Abstract

In a previous work we showed that large nongaussianities and nonscale-invariant distortions in the CMB power spectrum can be generated in hybrid inflation models, due to the contributions of the tachyon (waterfall) field to the second order curvature perturbation. Here we clarify, correct, and extend those results. We show that large nongaussianity occurs only when the tachyon remains light throughout inflation, whereas n=4 contamination to the spectrum is the dominant effect when the tachyon is heavy. We find constraints on the parameters of warped-throat brane-antibrane inflation from nongaussianity. For F-term and D-term inflation models from supergravity, we obtain nontrivial constraints from the spectral distortion effect. We also establish that our analysis applies to complex tachyon fields.

Paper Structure

This paper contains 20 sections, 82 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Review of previous results
  3. Hybrid inflation
  4. Second order curvature perturbation
  5. Tachyon mode functions
  6. Integrated results
  7. Constraints on hybrid inflation parameter space
  8. The issue of scale invariance
  9. Nonscale-invariant case
  10. Scale-invariant case; the adiabatic approximation
  11. Implications for Brane-Antibrane Inflation
  12. Implications for P-term Inflation
  13. D-term Inflation
  14. F-term Inflation
  15. The Case of a Complex Tachyon
  16. ...and 5 more sections

Figures (2)

  • Figure 1: Excluded regions of the hybrid inflation parameter space, for $\log_{10}v/M_p = -1,-3,-5,-7,-9$.
  • Figure 2: Excluded regions of the KKLMMT brane-antibrane inflation parameter space, in the plane of $\log_{10} a_i$ versus $\log_{10} g_s$ for $\log_{10}a_i M_s /M_p =-13,-11,\dots,-5$ from nongaussianity.