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Register a new userHidden Monopole Dark Matter via Axion Portal and its Implications for Direct Detection Searches, Beam-Dump Experiments, and the $H_0$ Tension
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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.
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A number of proposed and ongoing experiments search for axion dark matter with a mass nearing the limit set by small scale structure (${cal O} ( 10 ^{ - 21 } {rm eV} ) $). We consider the late universe cosmology of these models, showing that requiring the axion to have a matter-power spectrum that matches that of cold dark matter constrains the magnitude of the axion couplings to the visible sector. Comparing these limits to current and future experimental efforts, we find that many searches require axions with an abnormally large coupling to Standard Model fields, independently of how the axion was populated in the early universe. We survey mechanisms that can alleviate the bounds, namely, the introduction of large charges, various forms of kinetic mixing, a clockwork structure, and imposing a discrete symmetry. We provide an explicit model for each case and explore their phenomenology and viability to produce detectable ultralight axion dark matter.
Searches for invisible Higgs decays at the Large Hadron Collider constrain dark matter Higgs-portal models, where dark matter interacts with the Standard Model fields via the Higgs boson. While these searches complement dark matter direct-detection experiments, a comparison of the two limits depends on the coupling of the Higgs boson to the nucleons forming the direct-detection nuclear target, typically parameterized in a single quantity $f_N$. We evaluate $f_N$ using recent phenomenological and lattice-QCD calculations, and include for the first time the coupling of the Higgs boson to two nucleons via pion-exchange currents. We observe a partial cancellation for Higgs-portal models that makes the two-nucleon contribution anomalously small. Our results, summarized as $f_N=0.308(18)$, show that the uncertainty of the Higgs-nucleon coupling has been vastly overestimated in the past. The improved limits highlight that state-of-the-art nuclear physics input is key to fully exploiting experimental searches.
We investigate a scenario where the dark matter of the Universe is made from very light hidden photons transforming under a $Z_{2}$-symmetry. In contrast to the usual situation, kinetic mixing is forbidden by the symmetry and the dark photon interacts with the Standard Model photon only via an axion-like particle acting as a messenger. Focusing on signatures involving the ordinary photon, our survey of the phenomenology includes limits from cosmological stability, CMB distortions, astrophysical energy loss, light-shining-through-walls experiments, helioscopes and solar X-ray observations.
Hidden photons are dark matter candidates motivated by theories beyond the standard model of particle physics. They mix with conventional photons, and they can be detected through the very weak electromagnetic radiation they induce at the interface between a metal and the air. SHUKET (SearcH for U(1) darK matter with an Electromagnetic Telescope) is a dedicated experiment sensitive to the hidden photon-induced signal. The results from a hidden photon search campaign are reported, with no significant detection of a dark matter signal. Exclusion limits are derived from the observed noise fluctuations in a 5 GHz to 6.8 GHz frequency range, corresponding to a hidden photon mass region ranging from 20.8 micro-eV to 28.3 micro-eV. SHUKET is currently the most sensitive instrument in this mass range and the obtained limits on the kinetic mixing term constrain significantly dark matter models inspired from string theory.
The recent measurements of the cosmological parameter $H_0$ from the direct local observations and the inferred value from the Cosmic Microwave Background show $sim 4 sigma$ discrepancy. This may indicate new physics beyond the standard $Lambda$CDM. We investigate the keV gravitino dark matter that has a small fraction of non-thermal component (e.g. from the late decay of NLSP bino) under various cosmological constraints. We find such a scenario is highly predictive and can be tested by searching for the dilepton plus missing energy events at the LHC. Besides, we also discuss its implication for Hubble tension, however, which can be reduced to $3sigma$ level marginally.
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