Leptons also come in three generations; Electron & Electron neutrino, Muon & Muon Neutrino, and Tau & Tau Neutrino, each with their associated anti-particle.  Muons and Taus are, essentially, identical to electrons except for their masses.  Leptons are fundamental particles that do not combine into other particles.  All the leptons have ½ Spin, and, while neutrinos carry no charge, the other three carry identical charge of -1.  Electrons and the three neutrinos are inherently stable, and thus the types we commonly see around us.  The muon and the tau are much heavier than the other leptons.  When they are produced in particle accelerators, they decay very quickly into lighter particles

The existence of neutrinos was inferred from neutron decay.  A neutron decays into a proton and an electron, but experimenters found that momentum was not conserved, so they postulated the existence of the neutrino, in fact an anti-neutrino, to carry away the excess momentum.  Subsequently, it was found experimentally.  Neutrinos hardly react at all with other particles, and in fact every second about 50 trillion (50,000,000,000,000) pass harmlessly thorough your body.  Initially thought to have zero mass, and therefore travel at the speed of light, recent experiments have confirmed neutrino oscillation, which requires them to have mass.  The latest research, as of August 2010, indicates that the minimum mass is 0.05 eV, while the sum of the three neutrino masses must be less than 0.28 eV.  The electron neutrino is the lightest, then the muon neutrino, then the tau neutrino, but all three masses are similar. 

In September of 2011, the media published many articles about CERN claiming they had found neutrinos traveled at faster than light speed.  In fact, the OPERA experiment in Italy, that uses muon neutrinos generated by CERN, reported anomalies in their measurements and asked the scientific community for assistance.  While, to date, they have made no claims, it seems unfortuanate that they had a press conference so early in the procedings, before any peer review!.  The observations indicated that the difference was of the order of 20 parts per million.  The neutrinos seen after the supernova SN 1987A arrived at almost exactly the same time as the radiation.  Assuming the OPERA/CERN observation is correct, neutrinos should have arrived some four years earlier.  Having said that, the neutrinos in the OPERA/CERN experiments are of much higher energy.  I remain unconvinced about the results. 


Standard Model

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