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Aligned Natural Inflation in String Theory

Cody Long, Liam McAllister, Paul McGuirk

TL;DR

This work addresses the challenge of achieving large-field inflation with trans-Planckian axion excursions in string theory by embedding the Kim–Nilles–Peloso decay-constant alignment mechanism into type IIB Calabi–Yau orientifold compactifications. The authors show that gaugino condensation on magnetized or multiply-wrapped D7-branes can generate two non-perturbative axion couplings whose near-alignment yields an enhanced effective decay constant f_{eff} ~ f/δ when the alignment condition f_{A1}/f_{A2} = f_{B1}/f_{B2} (or its near variant) is satisfied, with the potential written in the standard KNP form. They provide explicit expressions for the non-perturbative superpotential and gauge-kinetic functions, derive the resulting aligned-axion potentials, and present toy examples demonstrating large f_{eff} in controlled regimes, while noting substantial hurdles in full moduli stabilization and matching the scalar power spectrum. The results offer a concrete string-theoretic route to large-field natural inflation and suggest that combining multiple alignment mechanisms or kinetic-alignment ideas could improve robustness and allow parametric control within stabilized compactifications.

Abstract

We propose a scenario for realizing super-Planckian axion decay constants in Calabi-Yau orientifolds of type IIB string theory, leading to large-field inflation. Our construction is a simple embedding in string theory of the mechanism of Kim, Nilles, and Peloso, in which a large effective decay constant arises from alignment of two smaller decay constants. The key ingredient is gaugino condensation on magnetized or multiply-wound D7-branes. We argue that, under very mild assumptions about the topology of the Calabi-Yau, there are controllable points in moduli space with large effective decay constants.

Aligned Natural Inflation in String Theory

TL;DR

This work addresses the challenge of achieving large-field inflation with trans-Planckian axion excursions in string theory by embedding the Kim–Nilles–Peloso decay-constant alignment mechanism into type IIB Calabi–Yau orientifold compactifications. The authors show that gaugino condensation on magnetized or multiply-wrapped D7-branes can generate two non-perturbative axion couplings whose near-alignment yields an enhanced effective decay constant f_{eff} ~ f/δ when the alignment condition f_{A1}/f_{A2} = f_{B1}/f_{B2} (or its near variant) is satisfied, with the potential written in the standard KNP form. They provide explicit expressions for the non-perturbative superpotential and gauge-kinetic functions, derive the resulting aligned-axion potentials, and present toy examples demonstrating large f_{eff} in controlled regimes, while noting substantial hurdles in full moduli stabilization and matching the scalar power spectrum. The results offer a concrete string-theoretic route to large-field natural inflation and suggest that combining multiple alignment mechanisms or kinetic-alignment ideas could improve robustness and allow parametric control within stabilized compactifications.

Abstract

We propose a scenario for realizing super-Planckian axion decay constants in Calabi-Yau orientifolds of type IIB string theory, leading to large-field inflation. Our construction is a simple embedding in string theory of the mechanism of Kim, Nilles, and Peloso, in which a large effective decay constant arises from alignment of two smaller decay constants. The key ingredient is gaugino condensation on magnetized or multiply-wound D7-branes. We argue that, under very mild assumptions about the topology of the Calabi-Yau, there are controllable points in moduli space with large effective decay constants.

Paper Structure

This paper contains 9 sections, 60 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Natural Inflation and Decay Constant Alignment
  3. Axions in Type IIB String Theory
  4. Large Decay Constants from Magnetized Branes
  5. Large Decay Constants from Multiple Windings
  6. Examples
  7. Alignment from multiply wrapped branes
  8. Alignment from magnetized branes
  9. Conclusions

Figures (3)

  • Figure 1: Three branes wrapping locally volume minimizing representatives of the same homology class.
  • Figure 2: Two branes multiply wrapping the same cycle the same number of times.
  • Figure 3: The 2-axion potential \ref{['eq:example_pot']}. Darker regions correspond to higher points in the potential. Inflation can be accommodated along the super-Planckian trajectory indicated by the arrow. The very slight tilt of the trajectory is a consequence of a very slight shift in the position of the minimum for $\psi^{1}$ as $\psi^{2}$ evolves.