Axion-philic cosmological moduli

TL;DR

This paper demonstrates that string moduli naturally decay into axions and Nambu-Goldstone bosons without mass suppression, making them significant sources of dark radiation and potential dark matter candidates in string cosmology.

Contribution

It reveals the axion-philic decay properties of string moduli, highlighting their role in producing dark radiation and dark matter in string theory frameworks.

Findings

Moduli decay into axions without mass suppression.

String moduli can produce observable dark radiation.

Potential contribution to dark matter from superpartners.

Abstract

We show that string moduli have axion-philic nature owing to the model-insensitive derivative interactions arising from the Kaehler potential. The decay of a modulus into stringy axions occurs without suppression by the mass of final states. Interestingly, it turns out to hold in general not only for the scalar partner of the stringy axion but also for any other moduli. The decay into (pseudo) Nambu-Goldstone bosons (NGBs) also avoids such mass suppression if the modulus is lighter than or similar in mass to the scalar partner of the NGB. Such axion-philic nature makes string moduli a natural source of an observable amount of dark radiation in string compactifications involving ultralight stringy axions, and possibly in extensions of the Standard Model that include a cosmologically stable NGB such as the QCD axion. In the latter case, the fermionic superpartner of the NGB can also contribute to the dark matter as a feebly interacting massive particle.