" 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 . Since the masses of the first generation quarks are significantly below the QCD scale, the uncertainties here are pretty large. In fact, current QCD lattice models seem to suggest a significantly lower mass of these quarks from that of this table. The fermions can be arranged in three generations, the first one consisting of the electron, the up and down quarks, and the electron neutrino. All ordinary matter is made from first generation particles; the higher generation particles decay quickly into the first generation ones and can only be generated for a short time in high-energy experiments. The reason for arranging them in generations is that the four fermions in each generation behave almost exactly like their counterparts in the other generations; the only difference is in their masses. Model Airplane Picture For example, the electron and the muon both have half-integer spin and unit electric charge, but the muon is about 200 times more massive. The electron and the electron-neutrino, and their counterparts in the other generations, are called "leptons", "weakly interacting particles". "

" 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. 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. (1987) Introduction to Elementary Particles, Wiley, John & Sons, Inc. ISBN 0471603864 Journal Articles Y. Airplane Desktop Model Hayato et al. , Search for Proton Decay through p → νK+ in a Large Water Cherenkov Detector. "