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In this paper, we propose a generalized natural inflation (GNI) model to study axion-like particle (ALP) inflation and dark matter (DM). GNI contains two additional parameters $(n_1, n_2)$ in comparison with the natural inflation, that make GNI more general. The $n_1$ build the connection between GNI and other ALP inflation model, $n_2$ controls the inflaton mass. After considering the cosmic microwave background and other cosmological observation limits, the model can realize small-field inflation with a wide mass range, and the ALP inflaton considering here can serve as the DM candidate for certain parameter spaces.
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Many existing and proposed experiments targeting QCD axion dark matter (DM) can also search for a broad class of axion-like particles (ALPs). We analyze the experimental sensitivities to electromagnetically-coupled ALP DM in different cosmological scenarios with the relic abundance set by the misalignment mechanism. We obtain benchmark DM targets for the standard thermal cosmology, a pre-nucleosynthesis period of early matter domination, and a period of kination. These targets are theoretically simple and assume $mathcal{O}(1)$ misalignment angles, avoiding fine-tuning of the initial conditions. We find that some experiments will have sensitivity to these ALP DM targets before they are sensitive to the QCD axion, and others can potentially reach interesting targets below the QCD band. The ALP DM abundance also depends on the origin of the ALP mass. Temperature-dependent masses that are generated by strong dynamics (as for the QCD axion) correspond to DM candidates with smaller decay constants, resulting in even better detection prospects.
In this paper, we investigate the Axion-like Particle inflation by applying the multi-nature inflation model, where the end of inflation is achieved through the phase transition (PT). The events of PT should not be less than $200$, which results in the free parameter $ngeq404$. Under the latest CMB restrictions, we found that the inflation energy is fixed at $10^{15} rm{GeV}$. Then, we deeply discussed the corresponding stochastic background of the primordial gravitational wave (GW) during inflation. We study the two kinds of $n$ cases, i.e., $n=404, 2000$. We observe that the magnitude of $n$ is negligible for the physical observations, such as $n_s$, $r$, $Lambda$, and $Omega_{rm{GW}}h^2$. In the low-frequency regions, the GW is dominated by the quantum fluctuations, and this GW can be detected by Decigo at $10^{-1}~rm{Hz}$. However, GW generated by PT dominates the high-frequency regions, which is expected to be detected by future 3DSR detector.
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 axion has emerged in recent years as a leading particle candidate to provide the mysterious dark matter in the cosmos, as we review here for a general scientific audience. We describe first the historical roots of the axion in the Standard Model of particle physics and the problem of charge-parity invariance of the strong nuclear force. We then discuss how the axion emerges as a dark matter candidate, and how it is produced in the early Universe. The symmetry properties of the axion dictate the form of its interactions with ordinary matter. Astrophysical considerations restrict the particle mass and interaction strengths to a limited range, which facilitates the planning of experiments to detect the axion. A companion review discusses the exciting prospect that the axion could indeed be detected in the near term in the laboratory.
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