Reading through the coverage of the first results from the world’s most sensitive search for dark matter, the Large Underground Xenon (LUX), you’d be forgiven for thinking that researchers working on dark matter had either discovered nothing at all or were on the verge of laying their (metaphorical) hands on the stuff. As with so much scientific research though, things are never as simple as they seem.
With the Higgs boson safely tucked under one arm, the next great hunt for physicists is the search for dark matter, even if scientists really don’t know very much about it at all. With their typical caution, the most any physicist will commit to is that there is something out there that is dark – in that it doesn’t show up in visible light or other electromagnetic waves – and that interacts with the universe in a similar way to visible matter.
After that, we’re firmly in the realm of theory. Dark matter might be made up of particles and those particles might be Weakly Interacting Massive Particles (WIMPs). WIMPs interact through the weak force and gravity, but not through electromagnetism – so they can’t be seen – and not through the strong nuclear force. Then again, dark matter might be made up of axions, a hypothetical elementary particle. Or it might be made up of something else entirely.
According to Dr. Chamkaur Ghag, an astroparticle physicist from University College London who works with the LUX project, WIMPs might be a “favoured theory”, but they’re far from the only theory out there.
“With experimental direct dark matter searches such as LUX, we try to be as model independent and “broadband” as possible, probing as many models as are feasible in the hunt for a signal,” he explained. “There are still many other models of course, and this is as it should be.”
So what exactly have LUX’ first results proved? Well, they have gone some way towards disproving other recent sets of results from the Coherent Germanium Neutrino Technology (CoGeNT) and the Cryogenic Dark Matter Search(CDMS) experiments, even if scientists at those projects aren’t entirely sure they agree yet.
for signals from dark matter in underground mines, to help eliminate background interference. The CoGeNT experiment and the CDMS experiment are located in an old mine in Minnesota, while LUX is searching for signals from dark matter particles colliding with atoms in liquid xenon in an old gold mine in South Dakota.
The theory goes that dark matter particles like WIMPs usually just pass through atoms without having any effect, but ever so rarely, they should hit visible matter atoms instead of passing through the spaces in between them. These are the events that experiments are searching for. In LUX’ case, researchers are hoping that they’ll be able to measure the photons given off by a xenon atom when it gets bumped by dark matter. They stuff the experiments down into mines and heavily shield them to stop anything else, like cosmic rays or background radioactivity, from causing the photons.
Brown University physicist and LUX co-spokesperson Rick Gaitskell talks about the dark matter experiment ( Credit: Matt Kapust, Sanford Underground Research Facility)
So far, LUX hasn’t seen a single hit, suggesting that in the low mass range it is looking in – between five and 10,000 times the mass of a proton – interactions happen extremely rarely. Both CDMS and CoGeNT had claimed some events in that range, but Professor Richard Gaitskell of Brown University, who works on the LUX project, said that these results make that unlikely.
“LUX rules out CoGeNT and CDMS very definitively. LUX sees absolutely no sign of a low mass WIMP signal with the probability of interacting that those earlier experiments prefer,” he said.
When the LUX results were publicised last week, friendly rivalries flared, with both Juan Collar of the University of Chicago, who leads CoGeNT, andStanford University’s Blas Cabrera, leader of CDMS, telling Scientific American that what they had seen could still turn out to be dark matter. But Gaitskell is pretty confident.
result and the CDMS/CoGeNT result is very high. LUX expected to see 1550 events in its latest run, if CDMS II’s favoured WIMP model was correct. LUX saw nothing,” he explained.
“CDMS/CoGeNT are now taking the opportunity to review the LUX data since it was first made available at the talks last Wednesday, so I believe they will begin to understand soon the strength of the new LUX result.”
Dr Simon Peeters, a physicist from the University of Sussex who’s also involved in dark matter research but not in any of these experiments, said that it was “extremely difficult” to rule out any results.
“Many aspects about dark matter are unknown and the comparison from one experiment to another has many technical difficulties,” he said. “[But] I would definitely say that the CoGeNT and CDMS results are challenged.”
Even if CDMS and CoGeNT have been ruled out, that doesn’t mean that all WIMPs are now totally out of the picture, only that low-mass WIMPs are starting to look really unlikely. To any non-scientists, that might seem like a bit of a dud result, but in this search, any narrowing of the field is a big help.
“We do not know what the WIMP mass or interaction cross-section will be, so we could be just around the corner from dark matter discovery,” Ghag pointed out. “Indeed, many theoretical models predict the existence of dark matter within reach of LUX, and certainly within reach of the eventual successor experiment LUX-ZEPLIN (LZ).”
LUX researchers work on the interior of the dark matter detector ( Credit: Matt Kapust, Sanford Underground Research Facility)
Even mistakes in results can ultimately help physicists onto the right track, as Peeters said.
“Despite their best efforts, the result of these experiments might be incorrect, hence searches like this do need independent confirmation – the history of neutrino physics is full of examples of this.
you should not be too afraid of making mistakes. In hindsight, the mistakes in neutrino physics were needed to make progress.”
The temptation is to compare this stage of the search for dark matter to the tail end of the search for the Higgs boson. Dark matter can easily be seen as the next great physics quest after the Higgs boson, but it’s a very different kind of search with very different parameters.
“The challenge compared to the Higgs is that there is currently no “Standard Model” for WIMP dark matter. The Higgs was the last jigsaw piece in the Standard Model of Particle Physics – a theory that had been with us for over 60 years, and that all the other pieces of had already been found,” Gaitskell said.
“The Standard Model is unable to provide a solution to dark matter. This is one of the reasons that looking for dark matter particles is so compelling. Dark matter particles have to come from an entirely new theory – one that is beyond any one that is currently verified.”
Although that doesn’t mean that LUX won’t be the one to find it – when you’re looking everywhere for something, it can sometimes be in the first, or the second, place you look.
“It is clear we need to push on to test even more weakly interacting dark matter particles,” Gaitskell said. “For WIMPs we can do this in the next run of LUX, which will last a year and increase the sensitivity by a further factor of five.
“We can also increase sensitivity by over a factor of 100, using the new experiment LUX-ZEPLIN, which is 20 times the size of LUX and which we would like to run in 2016.
“The discovery may be around the corner, or it may take another ten years. We must continue to look, and should not flinch simply because it remains elusive. They do call it weakly interacting for a reason…” he added.
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