Mile-deep experiment narrows the search for dark matter particles

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A view inside the LUX detector(Credit: Matthew Kapust/Sanford Underground Research Facility)

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Buried under a mile of rock in an old South Dakotan gold mine, the Large Underground Xenon (LUX) experiment has been looking for the telltale signs of elusive dark matter particles for the past 20 months. The search has now concluded and, although no direct signs of dark matter were found, the results are still an important step that helps close in on the search for this elusive particle.

Clues from the way in which light bends as it travels through space have led scientists to formulate the existence of "dark matter." This substance, so called because it is not easily detected, is believed to make up more than 80 percent of the mass of our Universe.

Scientists have so far been unable to detect dark matter directly. The leading theory focuses on WIMPs (for "weakly interacting massive particles"), and states that dark matter particles are all around us, but cannot be easily isolated because they interact only very weakly with ordinary matter. To stand a chance at detecting them, scientists must perform their experiments far away from any external source of interference and use extremely sensitive instruments.

LUX is just such an instrument, capable of measuring the extremely weak flashes of light and electrical charges that would occur as a dark matter particle collides with atoms of xenon gas. To shield from cosmic rays and other radiation, the detector is protected by a mile of rock and submerged in 72,000 gallons (280 cubic meters) of high-purity water.

This sophisticated instrument is not the only route that scientists can take to detect a dark matter particle: the Large Hadron Collider (LHC), for instance, might produce dark matter by accelerating particles to near the speed of light and colliding them. But the road taken by the LUX detector offers some important advantages over the LHC.

"One of the strengths of direct detection is that we are able to check models over a large range of possible particle masses, Prof. Richard Gaitskell told us." For an accelerator search such as the LHC producing the particles becomes very difficult at higher masses since there is not enough energy in the beam."

The 20-month search undertaken by LUX ran between October 2014 and last May. The detector benefitted from a sensitivity four times greater than initially expected, in part due to a range of calibration measures that were taken by the scientists – including compensating for a static-charge buildup, exposing the detector to radiation to better characterize a radioactive interference, and using neutrons as stand-ins for dark matter to quantify the signal that would be produced by a WIMP collision.

However, the observations were consistent with background noise – in other words, no dark matter particles were detected.

The negative result is not what the researchers were hoping for, but the high sensitivity of the instrument still allows scientists to state with confidence that dark matter particles do not feature within the ranges of mass or interaction strength analyzed in the experiment.

The results do not disprove the WIMP model, but they exclude many potential models for dark matter particles and offer a guideline for the next generation of detector experiments. Because no interaction was found, scientists can be confident that the interaction strength of a dark matter particle with ordinary matter must be below the sensitivity of LUX.

LUX scientists will be pouring over the data over the next few months, analyzing it to help shape future experiments and maximize the chance of detecting the elusive dark matter particle.

The LUX-ZEPLIN detector – part of the next generation of instrumentation – is expected to come online by the end of the decade: according to Gaitskell, it will be an impressive 70 times more sensitive than LUX.

But scientists and enthusiasts need not wait that long for the next clues on the nature of dark matter: in just two weeks' time, LHC scientists are expected to release their latest findings on their own search for new particles.

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