The remaining leptons are the three varieties of neutrino , the electron neutrino, the muon neutrino and the tauon neutrino. These interact so weakly with matter that most pass straight through the Earth when they reach it. Leptons interact through the weak nuclear force, those which act via the strong nuclear force are known as hadrons . Hadrons are heavy particles and, unlike leptons, consist of consituent particles known as quarks . These elementry particles come in six flavours which have come to be known as up, down, charm, strange, top and bottom .
Quarks have fractional electrical charges of either positive two thirds or negative one third.
They can combine into threes and make the elementary particles of protons and neutrons. Protons, with positive charge of one, consist of two up quarks and a down quark. Neutrons, with no electrical charge, consist of one up quark and two down quarks. One other criterion has to be taken into account when quarks join together. Each of these six flavours also has a property known as colour.