Where are they?
— Enrico Fermi
When famous physicist Enrico Fermi posed the question that formed the basis of the famous Paradox that took his name, he didn’t have dark matter in mind. Yet, with the latest results from the Large Underground Xenon Detector coming in, Fermi’s question suddenly becomes more relevant to the search for the ever-elusive dark matter particles as well.
While studying how stars and interstellar gas move within galaxies and galaxy clusters, astronomers in the early 20th century soon realised that the velocities they were observing were too high for the structures of galaxies to remain stable. If all the matter being observed and accounted for was all there was, then galaxies should have flown apart—its contents should have been hurled into the intergalactic void. There either was something flawed with the current understanding of how gravity worked on cosmic scales, or there was something else whose gravity was holding galaxies together, something that was evading observations. Thus the term “dark matter” was born.
Of all the proposed explanations for the nature of dark matter, the leading one between astronomers is that its composed of non-baryonic Weakly Interacting Massive Particles, or WIMPS. Whereas all the visible matter in the Universe, like planets, stars, and galaxies, is composed of ordinary baryonic matter (protons and neutrons), non-baryonic WIMPS are hypothesized particles that don’t emit or absorb any electromagnetic radiation, thus making themselves invisible to observations. It is hypothesized, though, that in rare occasions dark matter would collide with ordinary atoms, a collision that would be visible to suitable detectors. Since the surface of the Earth is teeming with natural radiation sources and cosmic radiation particles coming from space, creating a background of radiation “noise,” the only way to detect those rare WIMP interactions is to build detector facilities underground, shielding them from all this ‘noise’. One such facility is the Large Underground Xenon Detector, or LUX, located 1.5 km below the surface at the Sanford Underground Laboratory in South Dakota, inside the now abandoned Homestake gold mine.
The LUX detector consists of a 2-meter-tall cylindrical tank made of titanium and filled with 370 kg of liquid xenon, a chemically inert element that is cooled at a temperature of -180 degrees Celsius. In order for the detector to be adequately shielded from natural radiation coming from the surrounding walls of rock as well, the xenon-filled tank is further embedded inside a 70,000-gallon tank of water. Inside the liquid xenon runs an electric field, and both at the bottom and above the surface of the liquid, two arrays each consisting of 61 photomultiplier tubes, are installed which can detect the passing of photons through them. As predicted by theory, the vast majority of WIMPs travelling through the detector would leave no traces. But in the rare event where a WIMP would collide with a xenon atom, a pair of photons and electrons would be emitted. The photons would then be detected by the photomultiplier tubes and the electrons would be carried upward, towards the surface of the liquid xenon by the electric field that runs inside the tank. There, a second, stronger electric field above the surface would excite these electrons, forcing them to produce a second pair of photons that would also be detected by the photomultiplier tubes. By comparing the time between the emission of the first and second pair of photons, scientists can identify the type of collisions that took place inside the xenon tank and determine if they were due to the presence of WIMPS.
Following its first three months of operation this year, scientists announced the experiment’s initial results on Oct. 30. Even though LUX is currently the most sensitive dark matter operational detector, the scientists reported no dark matter findings at all. This comes as a surprise, considering the fact that independent experiments being run earlier this year by the Cryogenic Dark Matter Search detector, located in Minnesota, reported hints of three WIMP particles being discovered, with masses that were much lower than those predicted by theory. If those hints were indicative of low-mass dark matter particles, then LUX, due to its higher sensitivity, should have been able to spot thousands more of them. The absence of any such evidence in the first LUX results not only contradicts those previous observations, but also puts a limit to how low the WIMP’s mass must actually be, helping scientists to refine their searches accordingly.
“Those ‘hints’ – which were at best controversial – motivated several theories to explain them, which in turn can lend undue credence to those results,” says astrophysicist Henrique Araujo, leader of a participating team at LUX, from London’s Imperial College department of physics. “LUX is by far the most sensitive instrument in this hunt, and our very clean data contradict that interpretation emphatically: there may well be other light WIMPs out there, but we are drawing a line under those existing claims.”
Although LUX is scheduled to continue searching for another two years, scientists already plan to upgrade it with a seven-ton liquid xenon tank. This would make the detector hundreds of times more sensitive, allowing it to search for WIMPs orders of magnitude more massive—if they exist. This higher sensitivity might hopefully shed some light on the ever-mysterious nature of dark matter, helping to solve one of cosmology’s greatest riddles. Whatever the answer to the dark matter mystery may be, it is bound to be monumental, forever changing the whole field of physics and our entire understanding of the Universe along with it.