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 ma = mathcal{O} (10) MeV, fa = mathcal{O} (105) GeV and ma = mathcal{O} (100) MeV, fa = mathcal{O} (104) 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 ∆Neff ≃ 0.6 which can relax the H0 tension.
Highlights
The monopole is a promising candidate for DM, but it is formidable to search directly by experiments because its interactions to the standard model (SM) particles are feeble
The monopole can interact with the nucleon via the Higgs portal, but the expected cross section is much smaller than any direct DM search experiments in the foreseeable future
As we shall see later, we find two parameter regions with the axion mass and decay constant given by ma = O(10) MeV, fa = O(105) GeV and ma = O(100) MeV, fa = O(104) GeV, where both monopole DM and the axion are within the reach of the direct DM searches and SHiP
Summary
’t Hooft and Polyakov showed that a magnetic monopole can arise when a non-Abelian gauge symmetry is spontaneously broken via the Higgs mechanism [4, 5]. Expanding the Lagrangian density around the vacuum state, we obtain a massless hidden photon γH, two massive hidden gauge bosons WH±, and a massive neutral Higgs field φ. This theory allows a static field configuration with finite energy due to the nontrivial π2(SU(2)H/U(1)H) = Z. Such a topological soliton with nonzero winding number possesses a magnetic charge, and is identified with the magnetic monopole M. The electric charge of the hidden monopole due to the Witten effect will be explained in the subsection
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