Towards large r from [p,q]-inflation

TL;DR

This work investigates axion-monodromy-inspired large-field inflation within type IIB string theory by replacing NS5-branes with general [p,q] 7-branes to induce a robust inflaton potential. Depending on the brane setup, the inflaton potential for either B-axions or RR-axions can be linear or quadratic, with the RR-axion scenario yielding chaotic-inflation-like dynamics that predict $r\approx0.14$ and $n_s\approx0.96$ for $N\approx60$ e-folds. The authors emphasize that supersymmetry must be explicitly broken during inflation to generate the potential and discuss critical challenges such as backreaction, moduli stabilization, and eta-problems, which complicate a fully controlled string-theory embedding. They also outline potential high-scale SUSY-breaking after inflation and possible refinements via warping or non-Abelian brane configurations, suggesting several directions for future work in realizing observable tensor modes from string theory.

Abstract

The recent discovery of B-mode polarizations in the CMB by the BICEP2 collaboration motivates the study of large-field inflation models which can naturally lead to significant tensor-to-scalar ratios. A class of such models in string theory are axion monodromy models, where the shift symmetry of an axion is broken by some branes. In type IIB string theory such models so far utilized NS5 branes which lead to a linear potential with an induced tensor-to-scalar ratio of $r \sim 0.07$. In this short note we study a modification of the scenario to include [p,q] 7-branes and show that this leads to an enhanced tensor-to-scalar ratio $r \sim 0.14$. Unlike 5-branes, 7-branes are in-principle compatible with supersymmetry, however we find that an implementation of the inflationary scenario requires an explicit breaking of supersymmetry by the 7-branes during inflation. This leads to similar challenges as in 5-brane models. We discuss the relation to high-scale supersymmetry breaking after inflation.