Dark matter targets for axionlike particle searches

Open Access

Dark matter targets for axionlike particle searches

Nikita Blinov, Matthew J. Dolan, Patrick Draper, and Jonathan Kozaczuk

Phys. Rev. D 100, 015049 – Published 29 July 2019

Abstract

Many existing and proposed experiments targeting QCD axion dark matter (DM) can also search for a broad class of axionlike 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 $O (1)$ 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.

Received 2 June 2019

DOI:https://doi.org/10.1103/PhysRevD.100.015049

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Axions Dark matter Particle dark matter

Particles & FieldsGravitation, Cosmology & Astrophysics

Authors & Affiliations

Nikita Blinov¹, Matthew J. Dolan², Patrick Draper³, and Jonathan Kozaczuk^3,4

¹Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
²ARC Centre of Excellence for Particle Physics at the Terascale, School of Physics, University of Melbourne, 3010, Australia
³Department of Physics, University of Illinois, Urbana, Illinois 61801, USA
⁴Amherst Center for Fundamental Interactions, Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003, USA

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Issue

Vol. 100, Iss. 1 — 1 July 2019

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Images

Figure 1
Schematic evolution of the ALP energy density relative to the total energy as a function of the scale factor $R$ for different cosmologies. The scale factor is normalized to unity at the start of ALP oscillations. The lettuce, mustard and tomato lines correspond to a universe with early matter (EMD), radiation, or kination domination before primordial nucleosynthesis, respectively. The transition from EMD or kination to radiation domination is denoted by the vertical dashed line. The initial ALP density is fixed by requiring that ALP-radiation equality occurs at the same value of $R / R_{osc}$ for all three cases, such that these models have equal DM densities at late times. Since the initial value of the ALP energy density depends on $f_{a} θ_{0}$ , cosmologies with an early period of early matter (kination) domination, require larger (smaller) values of $f_{a} θ_{0}$ to saturate the observed dark matter relic density than in standard radiation domination, for fixed $m_{a}$ .
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Figure 2
Preferred regions in the ALP parameter space allowing for a temperature-dependent ALP mass given by Eq. (27) with $b = 4$ . The left (right) panel corresponds to a hidden sector with temperature ratio $ξ_{osc} = 1$ ( $ξ_{osc} = 0.1$ ) relative to the SM. The pastel shaded regions feature an ALP that saturates the observed dark matter relic density with $θ_{0} \in [0.1, 2]$ for radiation domination (gold) and early matter domination with $T_{RH} = 10 MeV$ (green) and 500 MeV (purple), obtained via numerical solution of the evolution equations. The dotted contours show the analytic predictions given in the text, which we find are a good match to the full numerical solutions. For reference, we also indicate the preferred region for the RD scenario with a $T$ -independent ALP mass between the yellow dotted contours. In both the RD and EMD cosmologies, the relic density can be substantially increased for a fixed $m_{a}$ and $f_{a}$ if the ALP mass is temperature-dependent during the onset of oscillations. Note that $T$ -dependence in the EMD cosmology interpolates between the $T$ -independent EMD and $T$ -dependent RD scenarios. In the left panel, the gray shaded region is excluded for the $T$ -dependent case by the value of $N_{eff}$ during BBN. These constraints are avoided in the right panel due to the lower hidden sector temperature, at the price of a smaller enhancement of the relic abundance. Note that the scales of the vertical axes are different in the left and right panels.
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Figure 3
Theoretical targets (colored bands) and current experimental constraints (filled regions) on the ALP-photon coupling $g_{a γ γ}$ as a function of the ALP mass $m_{a}$ . The shaded bands show regions where the ALP saturates the observed DM relic abundance for the standard (yellow), early matter-dominated (green) and kination (red) cosmologies for initial misalignment angles of $θ_{0} \in [0.1, 2]$ . For the latter cosmologies, we take the reheating/kination temperature to be 10 MeV. We also show the QCD axion band, which does not have a relic density requirement imposed, in blue. The gray dotted diagonal lines correspond to ALPs which get their mass from dimension 8, 10 and 12 Planck-suppressed operators. Further discussion can be found in Sec. 4.
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Figure 4
Theoretical targets (colored bands) and projected experimental reach (colored lines) in the ALP-photon coupling $g_{a γ γ}$ as a function of the ALP mass $m_{a}$ . The shaded bands show regions where the ALP saturates the observed DM relic abundance for the standard, early matter-dominated, and kination cosmologies for initial misalignment angles of $θ_{0} \in [0.1, 2]$ . For the latter cosmologies, we take the reheating temperature to be 10 MeV. We also show the standard QCD axion target in blue and existing experimental constraints in solid gray. The gray dotted diagonal lines correspond to ALPs which obtain mass from dimension 8, 10 or 12 Planck scale suppressed operators. Further discussion can be found in Sec. 4.
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Figure 5
As in Fig. 4, but for a $T$ -dependent ALP mass. In the left panel, the hidden sector responsible for generating the ALP potential is assumed to be in thermal equilibrium with the SM, while in the right panel we assume the hidden sector is decoupled with temperature given by $0.1 \times T_{SM}$ . In both cases $b = 4$ was assumed in Eq. (27). Note that there may be additional important constraints on the hidden sector, as discussed further in the text. On the left, the EMD band assumes $T_{RH} = 10 MeV$ , while on the right $T_{RH} = 500 MeV$ . In both cases $T_{kin} = 10 MeV$ . The parameter space on the left is constrained by $Δ N_{eff}$ at BBN, since there are necessarily new HS states in equilibrium with the SM bath at $T_{Λ}$ . The ALP target regions assume that $g_{* HS} = 52$ above $T_{Λ}$ , corresponding to the expected value for a $S U (3)$ hidden sector with 3 light flavors.
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