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 a super-Planckian
effective axion decay constant $f_{\textrm{eff}}\gg M_{Pl}$ through an
alignment of the anomaly coefficients of multiple axions having sub-Planckian
fundamental decay constants $f_0\ll M_{Pl}$. The original version of the KNP
mechanism realized with two axions requires that some of the anomaly
coefficients should be of the order of $f_{\textrm{eff}}/f_0$, which would be
uncomfortably large if $f_{\rm eff}/f_0 \gtrsim {\cal O}(100)$ as suggested by
the recent BICEP2 results. We note that the KNP mechanism can be realized with
the anomaly coefficients of $\mathcal{O}(1)$ if the number of axions $N$ is
large as $N\ln N\gtrsim 2\ln (f_{\textrm{eff}}/f_0)$, in which case the
effective decay constant can be enhanced as $f_{\rm eff}/f_0 \sim \sqrt{N
!}\,n^{N-1}$ 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 $N \sim f_{\textrm{eff}}^2/f_0^2$, 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 ${\cal O}(f_0/f_{\rm eff})$ under a random
choice of the anomaly coefficients. We also present a simple model realizing a
multiple axion monodromy along the inflaton direction.
