Neutrinos — once described as “the most tiny quantity of reality ever imagined by a human being” — are perhaps the most exotic and least understood of all known subatomic particles. Produced by the decay of radioactive elements, these particles rarely, if ever, interact with matter, making them extremely hard to detect and study. Every second, billions of neutrinos traveling at nearly the speed of light pass through the Earth.
Currently, neutrinos are known to have three different types, or “flavors” — the electron neutrino, the muon neutrino and the tau neutrino. Scientists also speculate over the existence of a fourth flavor known as the “sterile” neutrino. What makes this type of neutrino unique is the fact that unlike its “active” counterparts that, every now and then, interact with other matter via the electromagnetic force, it only interacts gravitationally.
The discovery of sterile neutrinos could “answer questions such as why neutrinos have mass or if neutrinos are important contributors to the dark matter pool in the universe,” the IceCube Neutrino Observatory in Antarctica, which, on Monday, announced that its two-year hunt for the particles had drawn a blank, said in a statement.
Dark matter accounts for roughly 27 percent of the mass and energy in the observable universe, and 85 percent of all mass in the universe. Currently, the hypothetical Weakly Interacting Massive Particles, which are believed to interact with normal matter through gravity and the weak nuclear force, are the leading candidates to explain the composition of dark matter, but what class of particles these WIMPs belong to is not yet known.
Although the detectors at IceCube, which includes thousands of optical sensors buried deep beneath the Antarctic ice, have so far failed to detect sterile neutrinos — which, if they exist, may very well make up dark matter — scientists have now been able to narrow down the mass-energy range, or the "parameter space," in which these particle may be hiding.
For the purpose of the study, IceCube researchers looked for sterile neutrinos in the energy range of 320 gigaelectronvolts to 20 teraelectronvolts.
“IceCube’s search for sterile neutrinos is an example of something that experimental physicists strive for — taking a phenomenon that is weak and difficult to study and examining it using a method where its effects would be amplified,” Ben Jones, who co-led an independent study that analyzed the IceCube data, said in the statement. “By not seeing sterile neutrinos in this way, we have excluded much of the parameter space that has been inaccessible to previous sterile neutrino experiments.”