In a disappointment to theorists, neutrino physicists at the IceCube neutrino detector in Antarctica drew a blank in their search for a “sterile” neutrino. If discovered, this particle would have been a fourth type of neutrino and would have indicated the existence of new physics beyond the “standard model” of physics which predicts that there are only three varieties of neutrino. Also, it would have explained the mismatch between quantities of matter and antimatter in the universe, while giving a clue as to what forms dark matter.
After analysing about 100,000 events — a year’s data collected by the IceCube array of detectors at the South Pole — scientists failed to see evidence for such a weakly interacting fourth “flavour” of neutrino. The results were published on August 8, in Physical Review Letters.
It was, in fact, not just the theorists who were keen on seeing the sterile neutrino, so called because it does not interact with matter at all, except for gravity. Its presence was hinted at by experiments at Los Alamos National Laboratory in the 1990s and recently at the Daya Bay nuclear reactor facility in Hong Kong.
IceCube is a very sensitive detector consisting of a 5,160 light sensitive detectors frozen in pristine ice stretching up to a cubic kilometre in the Antarctic. The location helps detection of neutrinos incident on the earth from the northern hemisphere. Of all particles incident on the earth here, only neutrinos are so weakly interacting that they do not get filtered by the intervening bulk of the earth before they reach the South Pole. Normally, neutrinos are detected by these sensors when they crash into a nucleus, releasing a flash of what is called blue Cerenkov light.
The sterile neutrino is even more difficult to detect and elusive than the ordinary neutrinos. The only way to detect it is when it is in the process of changing into another type of neutrino — electron neutrino, tau neutrino or muon neutrino.
The IceCube detector scanned the wide range of energies from 320 GeV to 20 TeV. In all this range, it failed to detect any indication of the sterile neutrino.
So, though it would have elegantly explained many questions in particle physics, the failure to obtain evidence for the sterile neutrino’s presence leaves physicists in the dark once more about, for instance, why neutrinos have such a small mass. It also leaves behind many open questions.
