An elementary particle that is a constituent of a subatomic particle, such as the proton. The theory of quarks was advanced by Murray Gell-Mann (b. 1929) in 1955. Gell-Mann got the name from James Joyce\'s book, Finnegan\'s Wake, and pronounced it to rhyme with ‘pork’, not lsquo;park’. All particles known before 1974 could be explained in terms of just three quarks (up (u), down (d) and strange (s), the names of which should be treated simply as labels) combined in a couple of different ways. Hadrons are composed of three quarks, the proton being uud and the neutron udd; mesons are composed of a quark and an antiquark. All particles had a logical explanation and all expected combinations existed, with one exception. Gell-Mann\'s detailed prediction of the nature of the omega-minus particle, subsequently discovered in 1964, vindicated the quark model. In 1974 a new particle was discovered called the J/Ψ particle. Theoreticians quickly explained that this was a meson formed from a new, fourth quark named charm (c). There were thus four quarks - u, d, s and c - plus four leptons (with no internal structure) the electron, the muon and their related neutrinos. A few years later, evidence was found for another quark, called bottom (b), and another lepton, called the tauon. A sixth quark, called the top (t) quark, was discovered in 1995. There are good theoretical reasons for expecting a sixth lepton, the tauon neutrino; experimental proof is currently limited.
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quark
symbol
approx. mass
charge/e
up|
u|
5 MeV/c{TAG(tag=>sup)}2{TAG}|
+â…”|
down|
d|
10 MeV/c{TAG(tag=>sup)}2{TAG}|
-â…“|
strange|
s|
100 MeV/c{TAG(tag=>sup)}2{TAG}|
-â…“|
charm|
c|
1.5 GeV/c{TAG(tag=>sup)}2{TAG}|
+â…”|
bottom|
b|
4.7 GeV/c{TAG(tag=>sup)}2{TAG}|
-â…“|
top|
t|
30 GeV/c{TAG(tag=>sup)}2{TAG}|
+â…”|
| |
|
