Hidden Monopole Dark Matter via Axion Portal and its Implications for Direct Detection Searches, Beam-Dump Experiments, and the $H_0$ Tension

TL;DR

This paper explores hidden monopole dark matter linked via an axion portal, analyzing its detectability in future experiments and its potential role in alleviating the H0 tension through dark radiation contributions.

Contribution

It introduces an axion portal to hidden monopole dark matter, identifying parameter regions accessible to upcoming experiments and its implications for cosmological tensions.

Findings

Future experiments like PICO-500 and SHiP can probe key parameter regions.

Hidden photon decay contributes to dark radiation with ΔN_eff ≈ 0.6.

The axion portal enables feasible detection of otherwise elusive monopole dark matter.

Abstract

Hidden monopole is a plausible dark matter candidate due to its stability, but its direct experimental search is extremely difficult due to feeble interactions with the standard model particles in the minimal form. Then, we introduce an axion, $a$, connecting the hidden monopole and the standard model particles and examine the current limits and future prospects of direct dark matter searches and beam-dump experiments. We find two parameter regions around $m_a = {\cal O}(10)$ MeV, $f_a = {\cal O}(10^{5})$ GeV and $m_a = {\cal O}(100)$ MeV, $f_a = {\cal O}(10^{4})$ GeV where monopole dark matter and the axion are respectively within the reach of the future experiments such as PICO-500 and SHiP. We also note that the hidden photons mainly produced by the axion decay contribute to dark radiation with $\Delta N_{\rm eff} \simeq 0.6$ which can relax the $H_0$ tension.