Natural inflation with multiple sub-Planckian axions

We extend the Kim-Nilles-Peloso (KNP) alignment mechanism for natural inflation to models with N>2 axions, which obtains super-Planckian effective axion decay constant f₄₅₅M₏₋ through an alignment of the anomaly coefficients of multiple axions having sub-Planckian fundamental decay constants f₀M₏₋. The original version of the KNP mechanism realized with two axions requires that some of the anomaly coefficients should be of the order of f₄₅₅/f₀, which would be uncomfortably large if f₄₅₅/f₀ (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 NlnN2ln (f₄₅₅/f₀), in which case the effective decay constant can be enhanced as f₄₅₅/f₀!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 Nf₄₅₅^2/f₀^2, 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 (f₀/f₄₅₅) under a random choice of the anomaly coefficients. We also present a simple model realizing a multiple axion monodromy along the inflaton direction.