D6-branes and axion monodromy inflation

TL;DR

The paper presents a string-theoretic construction of large-field inflation in type IIA via D6-branes that generate a bilinear inflaton superpotential $W_{\rm inf}=a\,\Phi\,T$, enabling axion monodromy for both a B-field axion and a D6 Wilson line. By combining 4d supergravity analysis with the D6-brane DBI action, it derives an effective inflaton potential that remains consistent with moduli stabilisation, while DBI corrections flatten the potential at trans-Planckian field values. The work identifies two viable scenarios (B-field and Wilson line inflatons), demonstrates how warping can establish a mass hierarchy between inflaton and heavy moduli, and shows that the B-field scenario yields a tensor-to-scalar ratio in the observationally allowed window $r\in[0.08,0.12]$, with linear-to-quadratic interpolation depending on model parameters. It further provides a Kaloper-Sorbo–type multi-branch structure for the potential and discusses theoretical challenges and future directions, including explicit geometric realizations and full $\alpha'$-corrected potentials. Overall, the approach offers a UV-complete avenue for large-field inflation with controlled backreaction and testable cosmological predictions.

Abstract

We develop new scenarios of large field inflation in type IIA string compactifications in which the key ingredient is a D6-brane that creates a potential for a B-field axion. The potential has the multi-branched structure typical of F-term axion monodromy models and, near its supersymmetric minima, it is described by a 4d supergravity model of chaotic inflation with a stabiliser field. The same statement applies to the D6-brane Wilson line, which can also be considered as an inflaton candidate. We analyse both cases in the context of type IIA moduli stabilisation, finding an effective potential for the inflaton system and a simple mechanism to lower the inflaton mass with respect to closed string moduli stabilised by fluxes. Finally, we compute the B-field potential for trans-Planckian field values by means of the DBI action. The effect of Planck suppressed corrections is a flattened potential which, in terms of the compactification parameters, interpolates between linear and quadratic inflation. This renders the cosmological parameters of these models compatible with current experimental bounds, with the tensor-to-scalar ratio ranging as 0.08 < r < 0.12

Figures (2)

• Figure 1: Tensor-to-scalar ratio (left) and spectral index (right) in terms of ${\delta}$.
• Figure 2: Primordial tilt $n_{s}$ vs tensor-to-scalar ratio $r$ superimposed by the plot given by Planck Collaboration (2015) Ade:2015tva. The yellow area shows the region of parameters covered by the potential (\ref{['sqrtpotb']}) for the parameter range ${\delta} \sim 10^{-1} - 10^{-3}$.