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Deborah & William Hillyard
Deborah & William Hillyard
Deborah & William Hillyard
Deborah & William Hillyard
Deborah & William Hillyard

Science - Standard Model

Quarks

Quarks come in three pairs or generations, as they are known: Up & Down, Charm & Strange, Top & Bottom, together with their anti-quarks.  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 anti-quark 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.  Originally, the top and bottom quarks were called "truth" and "beauty" respectively, but this fell out of use.  Personally, I prefer the older terms!  Quark masses vary enormously with the heaviest, the "top" being around 60,000 times heavier than the lightest.  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.
Charm Quark
Strange Quark
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. 
Top Quark
The top quark is the heaviest quark 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 via the strong interaction.  It decays via the weak interaction to a W-boson and a bottom-type quark (down, strange, or, most often, bottom).  Kobayashi and Maskawa received the 2008 Nobel Prize for their discovery. 
Bottom Quark
The bottom 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 decays, via the weak interaction, into either an up or charm quark.