A type of these particles has an X charge of negative five or a “B minus L” of negative one in the SO(10) (“S-O-ten”) grand unification theory. A mechanism that predicts unobserved “heavy” instances of these particles diagonalizes a matrix such that the off-diagonal components are the Dirac mass and are much smaller than the second diagonal entry. A mechanism involving these particles was the subject of a 1990s experiment at Los Alamos that used a liquid scintillator tube. (10[1])Unobserved instances of these particles that have right-handed chirality are termed “sterile.” Detectors like Super-Kamiokande attempt to observe mass oscillations of these particles. For 10 points, name these extremely light leptons that only interact through the weak force and gravity. ■END■