" This implied the existence of non-zero neutrino masses since massless particles travel at the speed of light and so do not experience the passage of time. The Standard Model did not accommodate massive neutrinos, because it assumed the existence of only "left-handed" neutrinos, which have spin aligned counter-clockwise to their axis of motion . If neutrinos have non-zero mass, they necessarily travel slower than the speed of light. Therefore, it would be possible to "overtake" a neutrino, choosing a reference frame in which its direction of motion is reversed without affecting its spin (making it right-handed). Since then, physicists have revised the Standard Model to allow neutrinos to have mass, which make up additional free parameters beyond the initial 19. A further extension of the Standard Model can be found in the theory of supersymmetry, which proposes a massive supersymmetric "partner" for every particle in the conventional Standard Model. Supersymmetric particles have been suggested as a candidate for explaining dark matter. Anime Model Kit Although supersymmetric particles have not been observed experimentally to date, the theory is one of the most popular avenues of research in theoretical particle physics. References Textbooks Griffiths, David J. "

"784 GeV Tau antineutrino 0 1 0 0 1 < 70 MeV Top quark t +2/3 2 +1/2 +1/6 3 173 GeV Bottom quark b -1/3 2 -1/2 +1/6 3 ~4. 7 GeV Anti-top antiquark tc -2/3 1 0 -2/3 173 GeV Anti-bottom antiquark bc +1/3 1 0 +1/3 ~4. 7 GeV * - These are not ordinary Abelian charges which can be added together but labels of Group representations of Lie groups. ** - Mass is really a coupling between a left handed fermion and a right handed fermion. For example, the mass of an electron is really a coupling between a left handed electron and a right handed electron, which is the antiparticle of a left handed positron. Also neutrinos show large mixings in their mass coupling, so it's not accurate to talk about neutrino masses in the flavor basis or to suggest a left handed electron neutrino and a right handed electron neutrino have the same mass as this table seems to suggest. *** - What is actually measured experimentally are the masses of baryons and hadrons and various cross section rates. Motorcycle Model Kit Since quarks can't be isolated because of QCD confinement, the quantity here is supposed to be the mass of the quark at the renormalization scale of the QCD phase transition. In order to compute this quantity, physicists have to set up a lattice model and try out various masses for the quarks until the model comes up with a close fit with experimental data. "