Poke a second pair of holes.
The bottom quark, the fifth quark to be detected, is a member of the third and heaviest pair of quarks.
A third, still heavier pair of quarks consists of top (or truth, t) and bottom (or beauty, b), again with charges of +2/3e and −1/3e, respectively.
The muon, and a second variety of neutrino, formed two further leptons, which would have spoiled this symmetry, but by 1974 the first hints had emerged that there was a balancing pair of quarks: the strange and the charm.
In such collisions, which create temperatures reaching trillions of degrees, a pair of quarks can be knocked out of a proton or a neutron.
The number of supposed quark flavors grew to the current six in 1973, when Makoto Kobayashi and Toshihide Maskawa noted that the experimental observation of CP violation could be explained if there were another pair of quarks.
Prior to 1994 one of the main missing ingredients of the Standard Model was the top quark, which was required to complete the set of three pairs of quarks.
This in turn implies that there are probably no more than three pairs of leptons and three pairs of quarks.
This underlying symmetry does not, however, indicate how many pairs of quarks and leptons should exist in total.
They give no explanation, for example, for the number of pairs of quarks and leptons; they even raise the question of why such an enormous gap exists between the masses of the W and Z bosons of the electroweak force and the X bosons of lepton-quark interactions.
