One might conclude that the global hunt for dark matter is entering desperate days, at least when it comes to the mystery material's most-often theorized form—the weakly interacting massive particle (WIMP). Results out today from the Stanford University-led LUX experiment, among the premiere dark matter detection projects, once again offer no detections, while, at the same time, offering better data representing finer sensitivities on particle-mass ranges where slight dark matter hints had been previously registered. Those ranges can now be safely ruled out.
The new LUX results are set to be published in the Physical Review Letters, while an open-access preprint version of the report was posted to the arXiv server last week.
To recap, dark matter is the general term given to the vast pools of matter in the universe that we know of only because of their gravitational effects on galaxies. It's what allows galaxies to form in the first place, in fact, but we've never directly detected it. Scientists can say what it does but not really what it is. Dark matter is thought to account for around 85 percent of all matter in the universe, which means we're pretty much surrounded by astrophysical ghosts.
There are a lot of of dark matter explanations out there. Some of them are less plausible than others—theories of modified gravity, for example—but WIMPS have always been the favorite. These are very, very heavy particles that don't experience the electromagnetic force and, thus, don't interact with light and all that that entails. There are no WIMP atoms or compounds or chemical reactions, just clumps of deeply, profoundly inert mass.
So, where did the whole WIMP theory come from in the first place then? Great question. There's a proposed principle in particle physics called supersymmetry, a unifying idea of how all of this fundamental physics junk might come together. It takes our current incomplete Standard Model of Physics—sort of a periodic table of fundamental forces and matter—and all of its member particles and it gives each of those particles a partner superparticle. Superparticles solve a few problems in fundamental physics, but the big idea is that it brings together the fundamental forces of the universe with its fundamental units of matter in a very perfect-seeming way.
Supersymmetry is elegant, but also very useful. It might explain why the Higgs boson exists in the first place; why gravity is such a weak fundamental force compared to its peers, like electromagnetism and the strong nuclear force; and, finally, what's up with dark matter.
In some supersymmetric models, there is a theorized superparticle with properties that might perfectly account for the abundance and properties of the dark matter we indirectly observe in the universe. This coincidence is known as the "WIMP miracle."
To some degree, WIMPs have seemed foretold thanks to this coincidence, but again and again, direct detection experiments have come up empty handed. Some astrophysicists have begun to look more favorably at alternate dark matter candidates, like "hidden light" particles.
The LUX detector is the world's most sensitive dark matter detector, but it's not the end of things. In a sense, the latest LUX results are a victory because they come from measurements so sensitive that we can now comfortably eliminate certain ranges of possible dark matter particle masses. Not finding dark matter is just another way of saying that we're zeroing in on some more definite conclusion.
In any case, the latest results are based on 2013 data. The LUX experiment has been in its second run since 2014, which will continue until 2016, when the detector will be decommissioned and upgraded to the LUX-ZEPLIN experiment, featuring a full 10 tons of liquid xenon within which a dark matter collision might be registered. The current LUX is based on a mere third-ton of the stuff.