Axion dark radiation: Hubble tension and the Hyper-Kamiokande neutrino experiment
Yuchao Gu, Lei Wu, Bin Zhu
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 ${N}_{\mathrm{eff}}$, 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 $a+A\ensuremath{\rightarrow}\ensuremath{\gamma}+A$ in the neutrino experiments, such as Hyper-Kamiokande.