The Search for Dark Matter

Underneath the Black Hills of South Dakota lies the Sanford Underground Research Facility (SURF) which conducts the Large Underground Xenon dark matter experiment (also known as LUX). It has proven to be the most sensitive detector of dark matter, and now a new set of calibration techniques has dramatically improved on its sensitivity.

But to take a step back for a moment, dark matter is a non-light emitting substance that makes up a large portion of  the mass of our universe. When we use the term "light," what we are referring to is photons. This means that dark matter doesn't emit anything detectable from the lowest part of the electromagnetic spectrum (radio waves) to the highest part (gamma rays).

So dark matter, which makes up some 85% of all matter in our universe, is in many regards "missing." We can't see it.

But that doesn't mean that we don't know it's there. It's just that, so far, the only way we've been able to detect dark matter is through its influence on physical matter (thanks to gravity). In this way, it's sort of like the wind—looking out of a window, we can see the leaves and trees moving, but we can't see the air blowing against it.

To help detect dark matter, LUX is designed to identify very rare occasions when dark matter particles collide with a xenon atom within the detector, during which point the xenon atom recoils and emits a small flash of light, detectable by LUX’s light sensors.

Specifically, LUX researchers are searching for weakly interacting massive particles (WIMPs), one of leading candidates for dark matter. These are very, very heavy particles don't experience the electromagnetic force. As a result, they don't interact with light. There are no WIMP atoms or compounds or chemical reactions, just clumps of deeply, profoundly inert mass.

In the press release, Rick Gaitskell, professor of physics at Brown University and co-spokesperson for the LUX experiment, said, "We have improved the sensitivity of LUX by more than a factor of 20 for low-mass dark matter particles, significantly enhancing our ability to look for WIMPs.

Gaitskell added, "We can track the neutron to deduce the details of the xenon recoil, and calibrate the response of LUX better than anything previously possible."

The Elusive Dark Matter

Researchers reexamine data collected in LUX’s first three-month run back in 2013 in a new research submitted to Physical Review Letters, an open-access preprint version of the report was posted to the arXiv server last week.

In the end, these latest LUX results come from measurements so sensitive that we can eliminate certain ranges of possible dark matter particle masses with a great deal of confidence. This find is notable, as other experiments had previously reported potential hints of dark matter here.

While this may seem like a loss; however, not finding dark matter in this respect is another way of saying that we are zeroing in on some more definite conclusion and characteristics.

"And so the search continues," Dan McKinsey, a UC Berkeley physics professor and co-spokesperson for LUX, said. "LUX is once again in dark matter detection mode at Sanford Lab. The latest run began in late 2014 and is expected to continue until June 2016. This run will represent an increase in exposure of more than four times compared to our previous 2013 run. We will be very excited to see if any dark matter particles have shown themselves in the new data."

And if it doesn't show up, it will still be no great blow. The LUX project is set to be decommissioned and upgraded to the LUX-ZEPLIN experiment, which will have a staggering 10 tons of liquid xenon to work with to try and register a dark matter collision.The current LUX is based on just one third of a ton.

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