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 (10[2])diagonalizes a matrix such that the off-diagonal components are the Dirac mass and are much smaller than the second diagonal entry. A (-5[1])mechanism involving these particles (10[1])was the subject of a 1990s experiment at Los Alamos (10[1])that used a liquid scintillator (10[1])tube. (10[3])Unobserved instances of (10[1])these particles that have right-handed chirality (10[1])are termed (10[1])“sterile.” (10[9])Detectors like (10[1])Super-Kamiokande (10[5])attempt to observe mass (10[1])oscillations (10[1])of these particles. (10[1])For 10 points, name these extremely light leptons (10[1])that only interact through the weak force and gravity. ■END■ (10[2]0[2])