Quarks come in three pairs or generations, as they are usually known: Up & Down, Charm & Strange, Top & Bottom, together with their antiquarks. We saw in the section on Quantum Physics how quarks, in addition to charge, mass & spin, carry the color charge. Hadrons are particles that comprise quarks. The proton, for example, is made of three quarks, up, up & down or uud, while the Neutron is udd. Mesons are made from a quark and an antiquark pair, for example, the pion (Õ+) is an up and anti-down pair. The anti-pion (Õ-) is a down and anti-up pair. The Up, Down and Strange quarks are considered "light"; the other three are considered "heavy".
Murray Gell-Mann and George Zweig first suggested, in 1964, that the many particles then known could be made up from just three fundamental particles; the up, down and strange quarks. Gell-Mann suggested the name "quarks", from the James Joyce novel Finnegan's Wake: "Three quarks for Muster Mark!", but he intended it to sound like "kwork". Subsequently, another three were identified giving us the six quarks. Quark rest masses vary enormously with the heaviest, the top quark at 172.9 GeV, being around 85,000 times heavier than the lightest, the up quark at 2.01Mev. Why? No one knows - yet!
Up & Down Quarks
The up quark is the lightest quark and the down is slightly heavier. Having been proposed in 1964 by Gell-Mann and Zweig, they were first observed experimentally at the Stanford Linear Accelerator Center (SLAC) in 1968. The scattering experiments indicated protons had a substructure comprising more fundamental particles. While their "bare" masses are only a few MeV, inside a composite particle like a meson or baryon, the mass equivalent of the gluon energy (from the strong nuclear force) increases this to around 330 MeV.
Particles containing a charm quark are described as "charmed", and if they have two, as "double charmed". If a particle includes both a charm and an anti-charm quark, it is part of a group called Charmonium. It was first suggested in 1964, but a firm prediction came in 1970 from Sheldon Glashow, John Iliopoulos and Luciano Maiani to eliminate flavor-changing decay modes involving the Z-Boson that were predicted but not observed to happen. It decays into a strange quark about 95% of the time, otherwise the decay product is a down quark. The first charmed particle, the the J/Y (J/Psi) meson, was discovered simultaneously at the Stanford Linear Accelerator Center (SLAC) and at the Brookhaven National Laboratory. They both announced their discoveries on 11 November 1974, and shared the 1976 Nobel Prize.
The property of strangeness predates the quark itself. The first strange particle, the Kaon, was found in 1947, before quarks were ever suggested. The strange quark was first proposed in 1964, along with the up and down quarks, by Murray Gell-Mann and George Zweig. A number of particles, like the kaon, decayed much more slowly than expected, and the property giving this effect was dubbed strangeness to explain the strange long decays. It was then found that the decay was via the weak force rather than the strong force.
The top quark, known originally as the Truth quark, is the heaviest weighing around 190 proton masses, or almost as much as an atom of tungsten. Its lifetime is so short, that it cannot form hadrons, permitting very short views of a "naked" quark. It was first suggested in 1973, by Makoto Kobayashi and Toshihide Maskawa, at the same time as the bottom quark, but not found experimentally until 1995 at Fermilab. Generally, they are produced as a top/anti-top pair from the decay of a gluon via the strong interaction.
Their short half-lives prevent them from forming a meson. It is also possible to produce a top quark via the weak interaction through two different channels. In the first, the s-channel, an intermediate W-boson decays into a top quark and an anti-bottom quark. Alternatively, in the t-channel, a bottom quark can transform into a top quark by exchanging a W-boson with an up or down quark. It decays via the weak interaction to a W-boson and a bottom-type quark (usually the bottom quark, rarely a strange quark, and incredibly seldom a down quark). Kobayashi and Maskawa received the 2008 Nobel Prize for their discovery.
The bottom quark, known briefly as the Beauty quark, is the second heaviest of the quarks at more than four times the mass of a proton. If a particle includes both a bottom and an anti-bottom quark, it is part of a group called, rather delightfully, Bottomonium. Like the top quark, it was first suggested in 1973 by Kobayashi and Maskawa, and found experimentally at Fermilab in 1977 as a bottom-antibottom quark pair which became known as the Upsilon meson, which is the heaviest known meson. It is a decay product in top quark decays, and could be a decay product of the Higgs, depending on its mass. The bottom quark itself decays, via the weak interaction, into either an up or charm quark.