Moduli-Induced Axion Problem

TL;DR

The paper addresses the persistence of the cosmological moduli problem when moduli stabilized by SUSY breaking decay predominantly into ultralight axions, producing dark radiation constrained by Planck. It derives general decay channels and branching ratios for moduli, and analyzes two string-motivated realizations—LVS and KKLT—to illustrate when axion production is unavoidable and when it can be suppressed. The key finding is that Ba is typically of order 0.1, with LVS predicting inevitable axion radiation while KKLT offers potential suppression through geometric tuning or enhanced SM decays; these results impose robust constraints on moduli stabilization and reheating scenarios. The work has significant implications for string cosmology and early-universe phenomenology, highlighting a universal constraint on heavy scalar decays and their cosmological impact.

Abstract

We point out that the cosmological moduli problem is not necessarily resolved even if the modulus mass is heavier than O(10)TeV, contrary to the common wisdom. The point is that, in many scenarios where the lightest moduli fields are stabilized by supersymmetry breaking effects, those moduli fields tend to mainly decay into almost massless axions, whose abundance is tightly constrained by the recent Planck results. We study the moduli-induced axion problem in concrete examples, and discuss possible solutions. The problem and its solutions are widely applicable to decays of heavy scalar fields which dominate the energy density of the Universe, for instance, the reheating of the inflaton.

Figures (2)

• Figure 1: The cosmological bounds on the partial decay rates, $\Gamma_a$ and $\Gamma_{SM}$, are shown. In the upper left shaded (pink) region, the axionic dark radiation is overproduced, leading to $\Delta N_{\rm eff} > 0.84$. In the lower left shaded (green) region, the modulus decay temperature is lower than $6$ MeV, and the $^4$He abundance is too large to be consistent with observations Kawasaki:1999na. The dotted (blue) lines are contours of the decay temperature, $T_d = 4, 10, 30, 100$ MeV from left to right. In this figure, we have not considered the LSP overproduction through the decay.
• Figure 2: Contours of the saxion branching fraction into the axion pair $B_a$ on ($\kappa_2/\kappa_1, n$) plane. In this plot we have taken $z=0$ and $N_g=12$. The shaded region is excluded from the axion overproduction.