U.S. Department of Energy

Pacific Northwest National Laboratory

Direct Detection of Dark Matter

A wealth of evidence supports the existence of a large amount of dark matter in the Universe that indicates the dark matter is cold, collision-less, and non-baryonic. By combining optical, x-ray, and gravitational lensing, we have gathered evidence that modifying fundamental theories of gravity will not explain dark matter. We are, therefore, looking for dark matter particles.

Most matter in the universe is not understood, and two good candidates may comprise the unknown part. Axions are theoretically well-motivated candidates for the cold, dark matter known to make up about 25% of the total mass-energy budget of the universe. The other candidate is weakly interacting massive particles (WIMPs), and neither candidate is more likely than the other. Physics beyond the Standard Model makes a compelling case for both, so it is plausible they both exist and contribute to the cold dark matter of the universe. Our plan to search for both WIMPs and axions is a strategic hedge against the unknown content of the universe – it could be either.

Why does matter exist after the Big Bang?

Even more profound than the question of why so much of the universe’s matter is dark, is why there is so much matter of any kind at all. Given the known laws of physics, we would expect that nearly all of the matter would have annihilated with antimatter in the first moments after the Big Bang. That we exist to wonder why that did not happen is evidence that our laws are incomplete.

A theoretical explanation for at least part of the matter-antimatter imbalance is a process called leptogenesis, where leptons (electrons, muons, and taus) were preferentially created over antileptons in the hot early universe. Leptogenesis requires that neutrinos are their own antiparticles, a hypothesis to be tested by the nEXO neutrinoless double beta decay experiment.

The existence of axion dark matter can be naturally linked to the process of leptogenesis, while leptogenesis can be in tension with models leading to WIMP dark matter. Thus, leptogenesis could severely constrain the nature of WIMPs, or vice versa, with WIMPs constraining the nature of the neutrino, depending on the order of discovery.

Dark Matter Axion Phase Space
The figure shows the dark matter axion phase space. The vertical axis is the strength of the axion coupling to photons; the horizontal axis is the axion mass. Hatched regions are ruled out by astrophysical observations inconsistent with axion dark matter. Shaded regions are those probed by G2 ADMX. The goal of G3 ADMX is to cover all remaining possibilities for axion dark matter, bounded on the plot by the purple lines. G3 ADMX will either discover axions and identify them as a component of dark matter, or definitely exclude axions as a candidate for dark matter. Either outcome is a major advance in our understanding of the universe.

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