Is the end is nigh for WIMPs?
Image: CH Faham/LUX
After 20-month search period, a key dark matter detection experiment has officially come up empty-handed, casting doubt on the existence of weakly interacting massive particles (WIMPS), which have been far and away the leading explanation for one of the biggest mysteries in astrophysics. This is according to new results from South Dakota's Large Underground Xenon (LUX) detector presented Thursday at the Identification of Dark Matter Conference (IDM 2016) in Sheffield, England.
"With this final result from the 2014-2016 search, the scientists of the LUX Collaboration have pushed the sensitivity of the instrument to a final performance level that is four times better than the original project goals," offered Rick Gaitskell, professor of physics at Brown University and co-spokesperson for the LUX experiment, in a statement. "It would have been marvelous if the improved sensitivity had also delivered a clear dark matter signal. However, what we have observed is consistent with background alone."
To be clear, this doesn't say anything about the existence of dark matter itself, just one of many possible explanations for dark matter. And, given that dark matter accounts for some 85 percent of all of the mass in the universe and is responsible for guiding and nurturing the development of galaxies, this is an explanation that's ultimately at the very heart of how the universe wound up as we see it today. Far from a cosmic curiosity, dark matter and its surrounding mystery explains why we're even here.
Astrophysicists know dark matter exists because its mass has gravitational effects on galaxies. Simply, we can take everything in space that we can see, add it all up, compute the resulting gravitational effects, and then compare the expected effects against the observed effects. The difference between the expected gravitational effects of all of the universe's visible matter and the observed gravitational effects is the thing we call dark matter. We don't see the stuff directly, but nonetheless know it's there because if it wasn't, space would look dramatically different.
So, the catch is that while we can see these gravitational effects, the source remains hidden. This is because dark matter only interacts with other matter and energy through the gravitational force—with no electromagnetic interactions between it and everything else, dark matter basically makes up a parallel ghost universe. It sails through all other matter unimpeded while remaining invisible our telescopes. This is the "weakly interacting" part of our weakly interacting massive particles.
Besides WIMPs, there are other candidates for dark matter, including MACHOS (dim stars or black holes that give off little or no radiation), axions (theorized chargeless, very low mass particles), sterile neutrinos, and gravitinos. WIMPs are favored, however, because of a spectacular coincidence known as the "WIMP miracle."
Said would-be miracle has to do with some very elegant supersymmetric theories of particle physics that imagine every fundamental particle as having a partner superparticle. It turns out this might solve a great deal of astrophysical mysteries—from explaining the apparent weakness of the gravitational force to the existence of the Higgs boson—but supersymmetry also predicts a dark matter particle that has the exact properties of a WIMP. In other words, we have a very good, very deep theory of astrophysics that accounts for dark matter perfectly given the existence of a particle matching perfectly the theorized properties of WIMPs.
The LUX experiment searches for WIMPs via collisions with atoms within a third-of-a-ton tank of cooled liquid xenon surrounded by super-sensitive photodetectors. LUX lives beneath a mile of rock in a repurposed South Dakota mine and is surrounded by a protective outer tank of highly purified water. The idea is to minimize as much as possible collisions within the tank of non-WIMP particles, such as cosmic rays. Since WIMPs don't interact normally within matter, all of the otherwise intervening water and rock might as well not even exist.
The WIMP model remains alive and viable, according to the LUX group. By 2020, LUX will be upgraded to LUX-ZEPLIN (LZ), which will feature a 10-ton xenon tank. The LHC, meanwhile, is conducting experiments that should produce cross-sections of particles that may point to the presence of WIMPs, but it has so far come up empty-handed as well.
"We viewed this as a David and Goliath race between ourselves and the much larger Large Hadron Collider at CERN in Geneva," Gaitskell said. "LUX was racing over the last three years to get first evidence for a dark matter signal. We will now have to wait and see if the new run this year at the LHC will show evidence of dark matter particles, or if the discovery occurs in the next generation of larger direct detectors."
Still, left unsaid is that it's now vastly less likely that the LHC or other WIMP detection experiments will register a hit. "I think we are getting to the point where the limits are excluding so much of the parameter space that we should rethink," Harvard-astrophysicist Avi Loeb told New Scientist. "Perhaps the dark matter is not WIMPs."
WIMPs are running out of places to hide, in other words, which is not necessarily a bad thing. In physics and science in general, eliminating the most obvious and intuitive explanations so often points to deeper, possibly much stranger explanations.