Neutrino oscillations provide hint of antimatter imbalance answer

Nature : One explanation for the dominance of matter over antimatter in the universe is that a superheavy primordial particle cousin to neutrinos decayed in a matter-favored process. The Tokai to Kamioka (T2K) experiment is designed to test that proposal by measuring neutrino oscillations—when one of the three flavors of neutrino turns into another. Muon neutrinos and antineutrinos are shot from the Japan Proton Accelerator Research Complex to the Super-Kamiokande detector 295 km away. Over six years of operation, the researchers expected to see 24 electron neutrinos and 7 electron antineutrinos. (The disparity exists because antimatter is harder to produce and detect.) Instead, T2K scientists have seen 32 electron neutrinos and 4 electron antineutrinos. That signal is intriguing but well below the threshold needed to rule out statistical variation. T2K would need to produce 13 times as much data to come to a solid conclusion but is slated for only five more years of operation. However, a second, similar experiment called NOvA at Fermilab will switch from shooting neutrinos to antineutrinos next year. The two groups intend to combine their data and expect to have enough to produce a 3 σ signal by 2020. It will take the next generation of neutrino experiments to reach the 5 σ level required to announce a discovery.