Axion Dark Radiation: Hubble Tension and Hyper-kamiokande Neutrino Experiment

TL;DR

This paper explores a supersymmetric axion model where dark radiation and late-decaying dark matter can address the Hubble tension and be detectable via neutrino experiments like Hyper-Kamiokande.

Contribution

It introduces a novel scenario linking axion dark radiation and late-decaying dark matter to cosmological tensions and experimental detection prospects.

Findings

Dark radiation from saxion decay can increase N_eff, easing Hubble tension.

Late-decaying gravitino/axino produce energetic axions detectable in neutrino experiments.

Entropy production from saxion decay helps avoid overclosure and relax reheating constraints.

Abstract

In this work, we investigate the dark sector of a supersymmetric axion model, consisting of the late-decaying gravitino/axino dark matter and axion dark radiation. In the early universe, the decay of the scalar superpartner of the axion (saxion) will produce a large amount of entropy. The additional entropy can not only dilute the relic density of the gravitino/axino dark matter to avoid overclosing the universe but also relax the constraint on the reheating temperature $T_{R}$ after inflation. Meanwhile, the axion dark radiation from the saxion decay will increase the effective number of neutrino species $N_{\rm eff}$, which can help to reduce the cosmological Hubble tension. In the late universe, the decay of long-lived gravitino/axino dark matter produces the axions with MeV-scale kinetic energy. We study the potential of searching for such energetic axions through the inverse Primakoff process $a+A \to \gamma+ A$ in the neutrino experiments, such as Hyper-Kamiokande.