November 1, 2013
The Large Underground Xenon (LUX) detector. Credit: Sanford Lab
The search for Weak Interacting Massive Particles or WIMPs continues as the latest results from the worlds most sensitive detector LUX show no sign of dark matter.
Dark matter, similarly to dark energy, is hypothesized matter that makes 26.8% of the universe. We don't know what dark matter exactly is, but that doesn't stop us from searching for candidates. Right now, we think it has to exist because according to current theories, without dark matter, outermost parts of spiral, rotating galaxies should separate from the rest of the stars that make the galaxy.
Simply put, galaxies should have five times more matter than we can't see than that which we can see in order to behave like they do. So, scientists think that it is made of some sort of subatomic particle and they have been searching for it for decades. The particle is named Weak Interacting Massive Particle (WIMP) and it should only interact through gravity and the weak force.
Since they don't interact through electromagnetism, we can't see them. Yet, there are methods scientists believe will one day help us discover the dark matter. Although these methods resemble those that are used to detect neutrinos, another subatomic particle, and WIMPs are similarly evasive, it should be noted that they are expected to be much slower and much heavier than neutrinos. Just in case you were wondering if neutrinos are what dark matter is made of...
To detect WIMPs, there are many underground detectors which should observe the resulting photons as a WIMP collides with an atom of medium inside the detector. One such detector uses liquid xenon as a medium and it is called Large Underground Xenon (LUX) detector. It is buried a kilometer and a half in an old gold mine in South Dakota, USA. It is currently the most sensitive installation of this kind.
You may find it disappointing to hear that in the first three months the LUX has found no trace of WIMPs. "We looked hard for these dark matter particles and we didn't see anything," says physicist Rick Gaitskell of Brown University, Providence, USA. However, the results rule out a number of possible masses and characteristics WIMPs should posses. Because the LUX is the most sensitive detector, no WIMPs means that their mass is at least 5,000 to 10,000 times the mass of a proton.
The results also contradict some earlier claims that dark matter may have been detected. The Dark Matter (DAMA) project in Italy claimed over a decade ago that they have seen signs of WIMPs. Also, two other experiments, Cryogenic Dark Matter Search (CDMS) and Coherent Germanium Neutrino Technology (CoGeNT) observed a couple of events that hinted at dark matter detection.
Looking at the history of neutrino detection, a few bumps on the road are to be expected. Because of the time needed to gather data and money needed to build ever larger underground detectors, many scientist would be happy just to see the discovery of WIMP in their lifetime, despite their rivalries. "At this point in my career, I am much less concerned with which experiment discovers the dark matter particles, and much more I would like to know within my lifetime," says Blas Cabrera of Stanford University, Stanford, USA. He is the leader of competing CDMS project.
It is certain that bigger detectors will be needed in the future, like LUX-ZEPLIN. "We're just so happy the instrument is performing so well," Gaitskell says."Hopefully, next time we'll be announcing a [positive] result. That, as they say, we’ll leave in the lap of the gods."