Neutrinos only interact with matter through the feeblest of forces, the weak nuclear force and gravity, yet they play critical roles in an incredible range of phenomena. They influenced the formation of the early Universe and may be the reason why matter came to dominate over antimatter shortly after the big bang. They are also integral to the inner workings of stars, including during their explosive demise as a supernova. Moreover, neutrinos are practically everywhere: even a single banana emits a million neutrinos a day from the unstable potassium isotopes it contains.
Although only three types of neutrino are known to exist, hints of a new kind of neutrino that solely interacts with matter through gravity have appeared in several experiments. If such a “sterile” neutrino does indeed exist, it might also play an important role in the evolution of the Universe. The hunt for sterile neutrinos has gone on for decades and has been full of twists and turns, with tantalizing positive signals that were later found to be in tension with null results in follow-up experiments. Now the world’s largest neutrino detector, the IceCube experiment at the South Pole, has released an analysis that eliminates a large portion of the parameter space in which sterile neutrinos could exist .
Standard neutrinos come in three flavors, each of which is associated with a charged partner: the electron, muon, or tau particle. The discovery that neutrinos oscillate, meaning one type of neutrino can transform into another, led to the realization that each flavor state is a linear superposition of three mass states with masses m1, m2, and m3—a beautiful example of basic quantum mechanics at work (see 7 October 2015 Focus story.) Based on precision oscillation measurements, we know that the mixing between neutrinos is quite large compared to similar effects among the quarks. Also, the distance needed for one neutrino type to turn into another, the neutrino oscillation wavelength, is determined by the difference between the squared masses of the participating mass states. These differences, m22−m21m22−m12 and m23−m22m32−m22, are known with good precision for the standard neutrinos.
APS Viewpoint: Hunting the Sterile Neutrino, David W. Schmitz