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