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The other principal division of elementary particles is the generation, which corresponds to a type of internal symmetry within the Standard Model. This is because all known matter is composed of a single quark species, charged $2/3$ of the way down or $-1/3$ of the way up the electron's spin ladder. In order for the fermions to have spin, they must also have "flavours", which are different compositeness levels of the same quark. For example, a $u$ quark may be composed of a $1/2$, a $0$ or a $-1/2$ colour fermion, while a $d$ quark must be composed of a $-1/2$ or a $3/2$ colour fermion.
The number of generations is currently unclear. Early on in the development of the Standard Model, there were three generations, those of the down and up quarks and the electron and neutrino. However, in the 1960s, after the weak interaction had been discovered and the quark had become known as a composite particle so that chiral symmetry could be applied, three generations were conjectured for theoretical reasons. The first generation would be the electron and the neutrino, and the other two generations would consist of the down and up quarks.
Beyond three generations, the fundamental scale of gravity and the Standard Model particles is unclear if there are more than three generations, or if there are additional unobserved elementary fermions, such as neutrinos with masses less than about $10^{-5} m,eV$. Many experiments investigating neutrino oscillations suggest that there are at least three readily observed neutrino types, typically known as the electron neutrino, the muon neutrino and the tau neutrino.
Finally, it must be noted that with quantum chromodynamics, quarks and antiquarks (other than the top quark) cannot be divided into distinct generations, because they all carry the same colour charge. d2c66b5586