Mysterious Dark Matter Remains Elusive


The hunt for the elusive dark matter received yet another blow earlier today at an international conference in Sheffield, England. Scientists with the Large Underground Xenon (LUX) dark matter experiment announced that they found no hints of dark matter particles in their latest analysis, despite increasing the sensitivity of the experiment fourfold for its final run.

“We built an experiment that has delivered world-leading sensitivity in multiple new results over the last three years,” Brown University’s Rick Gaitskell, co-spokesperson for the LUX collaboration, told Symmetry. “We gave dark matter every opportunity to show up in our experiment, but it chose not to.”

Stupid dark matter. At least these results have resolved most of the troubling discrepancies between various dark matter experiments over the last several years.

The planets, stars, galaxies, and everything else that we see makes up just 4.9 percent of the stuff in the universe. Roughly 26.8 percent is dark matter. (The rest — 68.3 percent — is made up of dark energy.) We’re not exactly sure what it is, but physicists can tell it’s there because of its indirect effects, like the famous “bullet cluster” composite image that made headlines several years ago. Physicists have been searching for years to directly detect dark matter particles, using instruments in the sky and deep underground.

The leading contender for a dark matter particle is a class of weakly interacting massive particle (WIMP), which is similar to another subatomic particle called a neutrino in that it rarely interacts with other matter. There are lots of different experiments around the world, with a dizzying array of acronyms, all looking to be the first to directly detect WIMPs. How difficult that is likely to be depends on whether the WIMPs are heavy or light. As I wrote at Quanta in 2013:

These kinds of experiments are usually housed deep underground — the better to block out cosmic rays, which can easily be confused with a dark matter signal — and feature a detector housing a carefully chosen target material, such as germanium or silicon crystals, or liquid xenon. [LUX uses xenon.] Then physicists wait for a rare collision between an incoming dark matter particle and the nucleus of an atom in the target material. This should give rise to a tiny flash of light, and if that flash is strong enough, it will be recorded by the detector. [I]n order to be detected, the dark matter particle must transfer enough energy when it knocks the nucleus for the resulting signal to go above the detector’s energy threshold.

A lighter WIMP would be much more difficult to detect, which is why physicists originally favored models that predicted a heavier WIMP. “Kinematically, it’s much easier for a heavier particle to transmit that energy than a lighter particle,” New York University physicist Neal Weiner told me at the time, comparing the two scenarios to bowling balls and ping-pong balls. As experiment after experiment has failed to detect a dark matter particle, the light WIMP scenario has started looking more likely—assuming it’s even a WIMP at all and not some other more exotic possibility.

There have been tantalizing hints now and then, but none met the threshold required to claim a bona fide discovery. Most controversial is the DAMA/LIBRA experiment (Dark Matter/Large Sodium Iodide Bulk for Rare Processes), located deep underground in the Gran Sasso mountain in central Italy. Over a decade ago, that collaboration claimed to detect tiny fluctuations in the rate of collision events over the course of a year. But others doubted it was a dark matter signal, especially since the similar XENON10 Cryogenic Dark Matter Search II (CDMSII) experiments failed to detect any signal in that energy range.

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SOURCE: Gizmodo, Jennifer Ouellette