Axionic Festina Lente

TL;DR

This paper extends the Festina Lente swampland conjecture to axion-like particles in string theory, deriving a new bound on axion parameters and exploring its implications for cosmology and particle physics.

Contribution

It introduces the aFL bound, relating axion decay constants and instanton actions, and applies it to string theory, black holes, wormholes, and phenomenology.

Findings

The product of instanton action and decay constant is bounded by vacuum energy.

The aFL bound constrains axion models and string inflation scenarios.

Phenomenological limits on axion-photon coupling are derived.

Abstract

The swampland conjecture known as Festina Lente (FL) imposes a lower bound on the mass of all charged particles in a quasi-de Sitter space. In this paper, we propose the aFL (axionic Festina Lente) bound, an extension of FL to axion-like particles arising from type II string theory. We find that the product of the instanton action and the axion decay constant is bounded from below by the vacuum energy. This is achieved indirectly, using dimensional reduction on Calabi-Yau threefolds, and translating the FL result for dipoles into a purely geometric bound. We discuss axionic black holes evolution, and aFL constraints on Euclidean wormholes, showing that the gravitational arguments leading to the FL bound for U$(1)$ charged particles cannot be directly applied to axions. Moreover, we discuss phenomenological implications of the aFL bound, including constraints on string inflation models and the axion-photon coupling via kinetic mixing.