Physicists on the lookout for dark matter have a problem. As a theory, the idea that huge numbers of elusive, invisible particles are knocking about out in the universe, churning in galaxies and galactic clusters, has been tremendously successful. But those particles—should they exist—have so far confounded all attempts to directly detect them.
For the past 30 years, most researchers would have told you that the most likely candidate for dark matter is what’s known as a Weakly Interacting Massive Particle (WIMP). Having a hypothetical mass of between 10 and 1,000 times that of a proton, WIMPs are believed to clump throughout the cosmos, interacting with ordinary matter—the stuff making up stars, cities, and people—only through gravity and the weak force. An idea called supersymmetry, which is physicists’ favorite extension of the Standard Model, the framework governing the behavior of all known particles and forces, also happens to contain theoretical particles with the exact properties of a WIMP.
WIMPs should be flying around us, ghostly and unseen, at all times. They would interact so rarely with ordinary matter that sending one through a lead cube 200 light-years to a side would result in a 50-50 chance of the particle coming out the other end without hitting anything. To hunt for them, experimentalists have set up clever detectors with bazillions of molecules, giving them a slim but quantifiable probability of being struck by a dark matter particle, and placed them deep below the Earth’s surface to prevent interference from cosmic radiation.
But last year, the Large Underground Xenon (LUX) collaboration, one of the most sensitive dark matter direct-detection experiments in the world, released new findings. Their result? Nothing. Around the same time, a Chinese project called the Particle and Astrophysical Xenon Detector (PandaX) produced another negative result. What about the French Expérience pour DEtecter Les WIMPs En Site Souterrain (EDELWEISS) experiment? It saw nothing. The XENON 100 experiment? Nothing. The advanced Cryogenic Dark Matter Search (SuperCDMS)? Nothing.
WIMPs should be flying around us, ghostly and unseen, at all times.
“How many times do you have to repeat an experiment and see nothing before you start wondering that the particle doesn’t exist?” asks physicist Juan Collar of the University of Chicago.
Researchers don’t generally paint null results in such disappointing terms. According to press releases, the experiments above have all placed stricter and stricter limits on where WIMPs might be found. But the mysterious subatomic prey have almost run out of places to hide. Collar is not alone in foreseeing a time when they may have to be abandoned. While multiple astronomical observations point to dark matter’s existence, the experimental evidence is so far negligible, and the Large Hadron Collider (LHC) has yet to see any hint of supersymmetry. The picture that’s emerging is a sobering reminder that in science, as in anything else, the search for answers provides no guarantee that you will actually find them.
Just a few years ago, the situation was quite different. Experiments had begun to gain the sensitivity needed to search in earnest for WIMPs and, while the initial results were often contradictory, most researchers assumed things would clear themselves up fairly soon. Something that looked like a dark matter signal had been spotted coming from the center of the galaxy—just where dark matter should cluster most densely. And Collar’s Coherent Germanium Neutrino Technology (CoGeNT) detector had seen hints that could be interpreted as dark matter particles. It seemed like several lines of evidence were converging, producing a palatable excitement in the community.
Dark matter had been posited as far back as 1933, when astrophysicist Fritz Zwicky noticed that galaxies in clusters seemed to be rotating around one another much faster than would be expected based on their visible mass. In the 1970s and 80s, astronomers Vera Rubin and Kent Ford showed that stars within galaxies were similarly rotating too fast, as if some undetectable material was tugging on them gravitationally and giving them some extra spin. Their calculations showed that dark matter was actually the bulk of a galaxy’s mass, outweighing ordinary visible matter by six to one. It was around this time that supersymmetry became popular, graciously providing physicists with WIMPs to search for. In 1985, physicists Mark Goodman and Edward Witten suggested a way to repurpose a neutrino experiment to first hunt for such particles.
But instead of finally obtaining evidence for slippery WIMPs, experimental results in the last few years seem to have made them disappear. When Collar’s team created a souped-up version of their detector called CoGeNT-4, their earlier dark matter hints went away. “Whatever we were seeing before seems to be a background that mimicked a signal,” he says. PICO, another experiment Collar works on that is 16 times more sensitive than previous detectors, released an analysis in February that didn’t show a single WIMP.
LUX, underground and in the tank. The tank will be filled with water to remove neutrons and external radioactivity byproducts. Credit: C.H. Faham
WIMPs still have at least one experiment in their favor. For more than a decade, the Dark Matter Large Sodium Iodide Bulk for Rare Processes (DAMA/LIBRA) collaboration has been seeing a putative signal that oscillates with the seasons. This happens to be exactly what you’d expect; as the Earth goes around the sun, it will collide with more galactic dark matter particles in June than in December. The trouble is that many other things also vary seasonally—like snow melting on the mountains above DAMA/LIBRA’s detector or cosmic rays hitting the atmosphere—and no other team has managed to corroborate the experiment’s findings.
Larger WIMP detectors have recently come online. The XENON1T, the successor to Xenon 100, began taking data last year and is expected to release results in the next few years. But Collar and others aren’t sure that anything new will turn up. If WIMPs do eventually fall completely out of favor, the question is where to go next?
One option that’s been discussed is a Strongly Interacting Massive Particle, or SIMP. Such particles would be very light, having as little as one-thousandth the mass of a proton, and still interact somewhat weakly with ordinary matter, but would interact strongly with themselves, annihilating and producing ordinary matter particles. SIMP models can be created that match with cosmological observations, sometimes even better than those involving WIMPs, which have discrepancies regarding the small-scale structure of the universe.
View of the bottom of the photomultiplier tube (PMT) holder. Credit: C.H. Faham
Another exotic idea comes from theoretical physicist Erik Verlinde, who posted an online paper last year suggesting that dark matter is actually an illusion arising from the interaction of ordinary matter and dark energy. The conjecture jibes in certain ways with a class of theories that speculate on ways to modify our understanding of gravity, suggesting that perhaps that it works differently that we expect on extremely large scales.
“What you have to never lose sight of is the possibility that we may have already learned everything we can about dark matter,” says Collar. “We know it interacts gravitationally, and we hope it interacts some other way, but perhaps this is the only way, and all we’ll ever learn about dark matter is through its gravity.”
Such comments from a leading experimentalist suggest that dark matter direct-detection methods might be headed towards a gloomy future. Collar says that he and other researchers are starting to apply their wispy particle hunting skills to adjacent fields, like neutrino physics, where “at least there’s a signal.” But physicists are not down for the count yet. The death of one idea often spurs the development of new ones, and Collar thinks that at least a few scientists are starting to move in the right direction and come up with alternative tests.
“Is there any guarantee that we’ll succeed? It might be a matter of luck at the end of the day,” he says. “We could go through another 30 years without anything, or somebody could turn on a new detector tomorrow and finally see something.”