Combining Universal and Odd RR Axions for Aligned Natural Inflation

TL;DR

This work embeds the Kim-Nilles-Peloso (KNP) axion-alignment mechanism for natural inflation into the Large Volume Scenario (LVS) of Type IIB orientifolds, using the plane spanned by the universal axion $C_0$ and an involutively odd axion $C_2$. A racetrack non-perturbative superpotential stabilizes the moduli so that a two-field KNP-type potential emerges, and after stabilizing the heavy axion, the light combination drives inflation with an effectively trans-Planckian decay constant $f_{\rm eff}$ (e.g., $f_{\rm eff} \gtrsim 7$) in benchmark realizations. Detailed inflationary analysis shows compatibility with PLANCK and (preliminary) BICEP2 data for $4 < f_{\rm eff} < 20$, with heavy fields decoupled and the dynamics well approximated by a single-field trajectory; the study also discusses open challenges, including flux choices, tadpole constraints, and the need for large gauge groups to realize the required hierarchies while staying in the perturbative regime. Overall, the paper provides a concrete UV-complete route to aligned natural inflation in string theory with controlled EFT and explicit moduli-stabilization dynamics.

Abstract

We successfully embed the Kim-Nilles-Peloso (KNP) alignment mechanism for enhancing the axion decay constant in the context of large volume type IIB orientifolds. The flat direction is generated in the plane of ($C_0$-$C_2$) axions corresponding to the involutively even universal axion $C_0$ and odd axion $C_2$, respectively. The moduli stabilization with large volume scheme has been established as well.

Figures (7)

• Figure 1: The two field potentials $V(\phi_1, \phi_2)$ and $V(\psi_1, \psi_2)$ (multiplied by $10^8$) respectively given in eq.(\ref{['eq:Vaxion2']}) and eq.(\ref{['eq:Vaxion3']}) are plotted for the sampling eq.(\ref{['eq:samp1']}). The second figure shows the enhanced decay constant for $\psi_2$ direction as compared to the sub-Planckian ones shown in the first figure.
• Figure 2:
• Figure 3: Evolution of heavier ($\psi_1$) and lighter ($\psi_2$) axionic combinations during inflationary process reflecting its single field nature..
• Figure 4: A closer look at the evolution of heavier field $\psi_1$ showing a negligible shift from their respective minima in each of the five inflationary trajectories. The shift in heavier field $\psi_1$ is less than 0.01 within the inflationary regime.
• Figure 5: Number of e-foldings $N_e$ versus inflaton field. Here, the largest possible values of $N_e$ are shown and the fast enhancement at the end is due to the field values approaching towards the maxima of the potential ($\phi_{\rm max} = \pi \, f$). Here $f$ varies from $1$ to 3 in the upward direction.