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Register a new userDark Matter in Axion Landscape
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If there are a plethora of axions in nature, they may have a complicated potential and create an axion landscape. We study a possibility that one of the axions is so light that it is cosmologically stable, explaining the observed dark matter density. In particular we focus on a case in which two (or more) shift-symmetry breaking terms conspire to make the axion sufficiently light at the potential minimum. In this case the axion has a flat-bottomed potential. In contrast to the case in which a single cosine term dominates the potential, the axion abundance as well as its isocurvature perturbations are significantly suppressed. This allows an axion with a rather large mass to serve as dark matter without fine-tuning of the initial misalignment, and further makes higher-scale inflation to be consistent with the scenario.
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We present an interesting Higgs portal model where an axion-like particle (ALP) couples to the Standard Model sector only via the Higgs field. The ALP becomes stable due to CP invariance and turns out to be a natural candidate for freeze-in dark matter because its properties are controlled by the perturbative ALP shift symmetry. The portal coupling can be generated non-perturbatively by a hidden confining gauge sector, or radiatively by new leptons charged under the ALP shift symmetry. Such UV completions generally involve a CP violating phase, which makes the ALP unstable and decay through mixing with the Higgs boson, but can be sufficiently suppressed in a natural way by invoking additional symmetries.
Extending the Standard Model with three right-handed neutrinos and a simple QCD axion sector can account for neutrino oscillations, dark matter and baryon asymmetry; at the same time, it solves the strong CP problem, stabilizes the electroweak vacuum and can implement critical Higgs inflation (satisfying all current observational bounds). We perform here a general analysis of dark matter (DM) in such a model, which we call the $a u$MSM. Although critical Higgs inflation features a (quasi) inflection point of the inflaton potential we show that DM cannot receive a contribution from primordial black holes in the $a u$MSM. This leads to a multicomponent axion-sterile-neutrino DM and allows us to relate the axion parameters, such as the axion decay constant, to the neutrino parameters. We include several DM production mechanisms: the axion production via misalignment and decay of topological defects as well as the sterile-neutrino production through the resonant and non-resonant mechanisms and in the recently proposed CPT-symmetric universe.
The cosmological scenario where the Peccei-Quinn symmetry is broken after inflation is investigated. In this scenario, topological defects such as strings and domain walls produce a large number of axions, which contribute to the cold dark matter of the universe. The previous estimations of the cold dark matter abundance are updated and refined based on the field-theoretic simulations with improved grid sizes. The possible uncertainties originated in the numerical calculations are also discussed. It is found that axions can be responsible for the cold dark matter in the mass range $m_a=(0.8-1.3)times 10^{-4}mathrm{eV}$ for the models with the domain wall number $N_{rm DW}=1$, and $m_aapproxmathcal{O}(10^{-4}-10^{-2})mathrm{eV}$ with a mild tuning of parameters for the models with $N_{rm DW}>1$. Such higher mass ranges can be probed in future experimental studies.
This Letter reports results from a haloscope search for dark matter axions with masses between 2.66 and 2.81 $mu$eV. The search excludes the range of axion-photon couplings predicted by plausible models of the invisible axion. This unprecedented sensitivity is achieved by operating a large-volume haloscope at sub-kelvin temperatures, thereby reducing thermal noise as well as the excess noise from the ultra-low-noise SQUID amplifier used for the signal power readout. Ongoing searches will provide nearly definitive tests of the invisible axion model over a wide range of axion masses.
Recently there has been interest in the physical properties of dark matter axion condensates. Due to gravitational attraction and self-interactions, they can organize into spatial localized clumps, whose properties were examined by us in Refs. [1, 2]. Since the axion condensate is coherently oscillating, it can conceivably lead to parametric resonance of photons, leading to exponential growth in photon occupancy number and subsequent radio wave emission. We show that while resonance always exists for spatially homogeneous condensates, its existence for a spatially localized clump condensate depends sensitively on the size of clump, strength of axion-photon coupling, and field amplitude. By decomposing the electromagnetic field into vector spherical harmonics, we are able to numerically compute the resonance from clumps for arbitrary parameters. We find that for spherically symmetric clumps, which are the true BEC ground states, the resonance is absent for conventional values of the QCD axion-photon coupling, but it is present for axions with moderately large couplings, or into hidden sector photons, or from scalar dark matter with repulsive interactions. We extend these results to non-spherically symmetric clumps, organized by finite angular momentum, and find that even QCD axion clumps with conventional couplings can undergo resonant decay for sufficiently large angular momentum. We discuss possible astrophysical consequences of these results, including the idea of a pile-up of clump masses and rapid electromagnetic emission in the sky from mergers.
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