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Axion absorption and the spin temperature of primordial hydrogen

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arxiv 1808.09456 v2 pith:DCMEJH7E submitted 2018-08-28 hep-ph astro-ph.CO

classification hep-phastro-ph.CO
keywords axionratespintemperatureabsorptionaxionscosmichydrogen
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An absorption dip in the spectrum of the cosmic microwave background observed by the EDGES experiment suggests an unexplained reduction of the hydrogen spin temperature at cosmic redshift z ~ 17. The mass of dark-matter axions could correspond to the hyperfine splitting of 5.9 micro-eV, between the triplet (H1) and singlet (H0) state. We calculate the rate for a+ H0 <-> H1 in two ways, and find that it is orders of magnitude smaller than the CMB-mediated transition rate, so irrelevant. As a result, this process cannot be used to rule in or out dark matter axions of mass = hyperfine splitting. The axion rate nonetheless has interesting features, for example, on balance it heats the spin temperature, and the axion couplings to protons and electrons contribute on equal footing.

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