Strong constraints on light dark matter interpretation of the EDGES signal

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Strong constraints on light dark matter interpretation of the EDGES signal

Rennan Barkana, Nadav Joseph Outmezguine, Diego Redigolo, and Tomer Volansky

Phys. Rev. D 98, 103005 – Published 6 November 2018

Abstract

Recently the EDGES collaboration reported an anomalous absorption signal in the sky-averaged 21-cm spectrum around $z = 17$ . Such a signal may be understood as an indication for an unexpected cooling of the hydrogen gas during or prior to the so-called Cosmic Dawn era. Here we explore the possibility that sub GeV dark matter cooled the gas through velocity-dependent, Rutherford-like interactions. We argue that such interactions require a light mediator that is highly constrained by 5th force experiments and limits from stellar cooling. Consequently, only a hidden or the visible photon can in principle mediate such a force. Neutral hydrogen thus plays a subleading role and the cooling occurs via the residual free electrons and protons. We find that these two scenarios are strongly constrained by the predicted dark matter self-interactions and by limits on millicharged dark matter, respectively. We conclude that the 21-cm absorption line is unlikely to be the result of gas cooling via the scattering with a dominant component of the dark matter. An order 1% subcomponent of millicharged dark matter remains a viable explanation.

Received 3 June 2018

DOI:https://doi.org/10.1103/PhysRevD.98.103005

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)

Dark matter

Particles & FieldsGravitation, Cosmology & Astrophysics

Authors & Affiliations

Rennan Barkana¹, Nadav Joseph Outmezguine^1,2, Diego Redigolo^1,2,3, and Tomer Volansky^1,2

¹Raymond and Beverly Sackler School of Physics and Astronomy, Tel-Aviv University, Tel-Aviv 69978, Israel
²School of Natural Sciences, Institute for Advanced Study, Einstein Drive, Princeton, New Jersey 08540, USA
³Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 7610001, Israel

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Issue

Vol. 98, Iss. 10 — 15 November 2018

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Images

Figure 1
The cross section defined in Eq. (9) required to fit the EDGES signal for DM-hydrogen interactions (red), DM-helium interactions (blue) and interactions with the ionized fraction assuming the interacting particle constitutes all of the DM (green) or only 1% of the DM density (brown). The solid (dashed) lines correspond to the minimal cross section needed to obtain a brightness temperature $T_{21} = - 300 mK (- 500 mK)$ assuming infinite Ly- $α$ radiation rate which couples the spin temperature to that of the gas, and assuming no heating of the gas due to X-ray radiation.
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Figure 2
Constraints on the effective couplings of light mediators to the SM as a function of the mediator mass. The gray shaded region is excluded for theories that mediate a long range Rutherford-like force which cannot be fully screened. For the astrophysical bounds we assume democratic mediator couplings between electrons and protons. The purple-shaded region holds also for a light hidden photon under which SM charges are proportional to their electric charge and can therefore be screened. The solid (dashed) lines indicated the minimal $α_{eff}$ needed to fit the EDGES signal (see Fig. 1) when cooling via the scattering of a keV (GeV) DM with hydrogen (red), helium (blue) and free electrons and protons (green) is assumed. In order to show the severeness of the constraints on $α_{eff}$ , the cross sections are obtained for the best case scenario where the coupling of the DM to the mediator, $α_{D} = 1$ , ignoring any possible limits. The gray shaded region is excluded by various 5th force experiments [15, 69] while the purple-shaded region shows various limits including those from stellar cooling constraints [20, 21].
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Figure 3
Constraints on the hidden photon parameter space for the case when the interacting particle constitutes all of the measured DM density ( $Ω_{χ} = Ω_{CDM}$ ) and for the choice of very light mediator $m_{A^{'}} = 10^{- 18} eV$ . The same parameter space is plotted on the $ε - m_{χ}$ plane (left) and ${\bar{σ}}_{e} - m_{χ}$ plane (right). In both plots we choose $α_{D} = 10^{- 10} (m_{χ} / MeV)^{3 / 2}$ which is consistent with the self-interaction limits [23]. The green line represents the minimal cross section needed to explain the EDGES measurement, while in dashed-gray lines we show contours of constant $\hat{σ}$ for better orientation. Constraints from cooling of the SN 1987A [8] (purple), direct detection limits from XENON10 [57, 64] (green), effective number of relativistic particles at CMB and at BBN [10] (blue), SLAC millicharge experiment [24] (gray), cooling of WD, HB stars and RG [10] (pink and brown), limits on DM-SM coupling at the time of CMB [25] (light green) and DM-SM momentum transfer [74] (light purple) are shown in the shaded regions.
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Figure 4
Constraints on the charge, Q, of a millicharged particle as a function of the DM mass. The red line indicates the minimal cross section needed to explain the EDGES measurement, assuming the millicharged particle constitutes only 1% of the DM density. The dashed-gray lines show contours of constant $\hat{σ}$ . Constraints from cooling of the SN 1987A [8] (purple), direct detection limits from XENON10 [57, 64] (green) and SENSEI [77], SLAC millicharge experiment [24] (gray), BBN [9] (light blue) and cooling of WD, HB stars and RG [10] (pink and brown) are shown in the shaded regions. We also add constraints from heating due to DM annihilation derived in [76] (blue). This bound only applies to fermionic DM for which the annihilation is $s$ -wave. The shaded yellow band indicates where millicharge DM might be evacuated from the galactic disk [25, 31].
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