Axion dark matter from frictional misalignment

The main mechanism responsible for Axion-Like-Particle (ALP) production in the early universe is the so-called misalignment mechanism. Three regimes have been investigated in this context: standard misalignment, large misalignment and kinetic misalignment. The latter applies if the axion inherits a large initial velocity in the early universe, such that the field rolls through many wiggles during its evolution, before it gets trapped in one minimum. This largely opens the region of parameter space for ALP dark matter towards higher values for the axion-photon coupling, which can be probed by the whole set of next decade's upcoming experiments. In fact, almost the entire parameter space in the [mass, decay constant] plane can now accommodate dark matter. In this paper, we show that in kinetic misalignment, the axion field is almost always entirely fragmented, meaning that the energy density of the homogeneous field is redistributed over higher-mode axions. We present a general model-independent analytical description of kinetic fragmentation, including discussion of the modified initial conditions for the mode functions due to the axion's initial velocity, and how they impact the growth of the adiabatic fluctuations. We calculate precisely the parameter regions corresponding respectively to standard misalignment, kinetic misalignment with weak fragmentation, fragmentation after trapping and fragmentation before trapping. While axion fragmentation can impact the precise determination of the relic abundance, another main observational implication is the formation of much denser compact axion halos, that is described in a companion paper. We also point out a new gravitational-wave signature that arises in the large misalignment regime with complete fragmentation and could be seen in measurements of μ distortions in the Cosmic Microwave Background.

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“…Another recent proposal to expand the ALP DM parameter space to larger couplings is the frictional misalignment[11].…”

mentioning

confidence: 99%

ALP dark matter from kinetic fragmentation: opening up the parameter window

Eröncel

Sato

Servant

et al. 2022

J. Cosmol. Astropart. Phys.

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“…The latter causes the ALP field to probe the non-quadratic parts of its potential which causes instabilities, while the former ensures that the field amplitude decays slowly so that the fragmentation becomes efficient. We expect that the effects studied in this work are relevant for any ALP model which predict a delay in the onset of oscillations, such as the recently proposed frictional misalignment [68].…”

Section: Jcap01(2023)009mentioning

confidence: 84%

ALP dark matter mini-clusters from kinetic fragmentation

Eröncel

Servant²

2023

J. Cosmol. Astropart. Phys.

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We show that very compact axion mini-clusters can form in models where axion-like-particle (ALP) dark matter is produced via the kinetic misalignment mechanism, which is well-motivated in pre-inflationary U(1) symmetry breaking scenarios. This is due to ALP fragmentation. We predict denser halos than what has been obtained so far in the literature from standard misalignment in post-inflationary U(1) breaking scenarios or from large misalignment. The main reason is that adiabatic fluctuations are significant at early times, therefore, even if amplification from parametric resonance effects is moderate, the final size of ALP fluctuations is larger in kinetic misalignment. We compare halo mass functions and halo spectra obtained in kinetic misalignment, large misalignment and standard misalignment respectively. Our analysis does not depend on the specific model realization of the kinetic misalignment mechanism. We present our results generally as a function of the ALP mass and the ALP decay constant only. We show that a sizable region of this ALP parameter space can be tested by future experiments that probe small-scale structures.

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“…Ref. [51] also noticed a possibility that axion dark matter abundance is enhanced by thermal friction compared to the conventional scenario due to spontaneous breaking of the hidden gauge symmetry right before the decay phase, which is not addressed in this work. On the other hand, our work includes a study of the effect of thermal friction on density perturbation of axion dark matter, which is not covered in ref.…”

Section: Acknowledgmentsmentioning

confidence: 90%

“…Our discussion of the axion dark matter abundance largely overlaps with ref. [51], whose first version was put forward on the arXiv a few days before ours. Ref.…”

Section: Acknowledgmentsmentioning

confidence: 99%

Axion dark matter with thermal friction

Choi

Kim

et al. 2023

J. High Energ. Phys.

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Cosmological evolution of axion field in the early universe might be significantly affected by a thermal friction induced by the axion coupling to thermalized hidden sector. We examine the effects of such a thermal friction on axion dark matter density and its perturbation when the thermal friction dominates over the Hubble friction until when the axion field begins to oscillate around the potential minimum. We show that in the presence of sizable thermal friction there can be an exponential decay phase of the axion field before the oscillation phase, during which the axion energy density is efficiently dissipated into hidden thermal bath. Consequently, the previously excluded parameter region due to overclosing relic axion density becomes cosmologically viable with thermal friction. In particular, a QCD axion much lighter than μeV is viable without tuning the initial misalignment angle. We also find that thermal friction can affect the density perturbation of axion dark matter in various ways. For instance, it can alleviate the large-scale isocurvature bound on axion dark matter in the pre-inflationary PQ breaking scenario, which would make the pre-inflationary axion dark matter compatible with high scale inflation over a wide range of model parameters. In the post-inflationary PQ breaking scenario, thermal friction can also significantly change the scaling behavior of axionic strings, and therefore the typical size of the resultant axion miniclusters.

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Axion dark matter from frictional misalignment

Cited by 4 publications

References 121 publications

ALP dark matter from kinetic fragmentation: opening up the parameter window

ALP dark matter from kinetic fragmentation: opening up the parameter window

ALP dark matter mini-clusters from kinetic fragmentation

Axion dark matter with thermal friction

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