Natural inflation with multiple sub-Planckian axions

Kiwoon Choi; Hyungjin Kim; Seokhoon Yun

doi:10.1103/PhysRevD.90.023545

paper

Natural inflation with multiple sub-Planckian axions

arXiv:1404.6209 · doi:10.1103/PhysRevD.90.023545

Abstract

We extend the Kim-Nilles-Peloso (KNP) alignment mechanism for natural inflation to models with $N > 2$ axions, which obtains a super-Planckian effective axion decay constant $f_{eff} ≫ M_{P l}$ through an alignment of the anomaly coefficients of multiple axions having sub-Planckian fundamental decay constants $f_{0} ≪ M_{P l}$ . The original version of the KNP mechanism realized with two axions requires that some of the anomaly coefficients should be of the order of $f_{eff} / f_{0}$ , which would be uncomfortably large if $f_{eff} / f_{0} ≳ O (100)$ as suggested by the recent BICEP2 results. We note that the KNP mechanism can be realized with the anomaly coefficients of $O (1)$ if the number of axions $N$ is large as $N ln N ≳ 2 ln (f_{eff} / f_{0})$ , in which case the effective decay constant can be enhanced as $f_{eff} / f_{0} \sim N! n^{N - 1}$ for $n$ denoting the typical size of the integer-valued anomaly coefficients. Comparing to the other multiple axion scenario, the N-flation scenario which requires $N \sim f_{eff}^{2} / f_{0}^{2}$ , the KNP mechanism has a virtue of not invoking to a too large number of axions, although it requires a specific alignment of the anomaly coefficients, which can be achieved with a probability of $O (f_{0} / f_{eff})$ under a random choice of the anomaly coefficients. We also present a simple model realizing a multiple axion monodromy along the inflaton direction.

19 pages, 2 figures. v2: typos corrected, references added, some arguments improved. v3: references added, examples added. v4: references added, matches published version

Natural inflation with multiple sub-Planckian axions

Abstract

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