The Standard Model
Nearly thirty years and many experiments later,
the quark idea has been confirmed. It is now part of the Standard Model of Fundamental Particles and Interactions. Discoveries
have shown that there are six types of quarks (given the odd
names of up,
down,
strange,
charm,
bottom,
and top,
in order of increasing mass). Also, there are six types of
particles including the electron, called leptons.
The Standard Model accounts for the strong,
weak,
and electromagnetic interaction of the quarks and leptons, and thus explains
the patterns of nuclear bindings and decays.
The Particles Made from Quarks
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The reason that fractional electric charges like those
of quarks have not been seen is that the quarks are never
found separately, but only inside composite particles
called hadrons.
There are two classes of hadrons: baryons,
which contain three quarks, and mesons,
which contain one quark and one antiquark. The sample
meson table gives some examples. Particles made from the
first five quark types have been produced and studied
at accelerators. The top quark is so massive it took many
years and very high-energy accelerators to produce it.
The top quark was finally discovered in April 1995 at
Fermilab. |
The Leptons
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In contrast to the quarks, any of the six leptons may
be found by itself. The electron
is the best known lepton. Two other charged leptons,
the muon,
(discovered in 1936) and the tau
(discovered in 1975) differ from the electron only in
that they are more massive than it is.
The other three leptons are very elusive particles
called neutrino,
which have no electric charge and very little, if any,
mass. There is one type of neutrino corresponding to
each of the three types of electrically charged leptons.
For each of the six leptons there is an antilepton with
equal mass and opposite charge. |
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