Neutrino oscillation arises from mixing between the flavor and mass eigenstates of neutrinos. Fermions include elementary particles such as quarks, electrons and neutrinos. Thus all muons decay to at least an electron, and two neutrinos. Thus electron, muon, and tau neutrinos are denoted and . Efforts to design an accelerator facility where neutrinos are produced through muon decays are ongoing. If they do, their properties are substantially different from observable neutrinos and antineutrinos. This might include neutrinos and free electrons, dark matter, supersymmetric particles, axions, and black holes. The above discussion is valid for massless neutrinos, which are always relativistic. The exchanged neutrinos are virtual particles. These thermal neutrinos are several times more abundant than the electron-capture neutrinos. All emitted neutrinos are of the same energy. The energy of supernova neutrinos ranges from a few to several tens of MeV. Salam introduced chiral symmetry in the theory of neutrinos. A pair of high energy neutrinos were detected in 2013. This eliminated some possible errors related to matching detected neutrinos to their creation time.