Before the Standard Model (SM) was developed in the 1970s
(the key elements of the Standard Model known as quarks were
proposed by Gell-Mann and Zweig in 1964), physicists observed
hundreds of different kinds of particles in particle
accelerators. These were organized into relationships on
their physical properties in a largely ad-hoc system of
hierarchies, not entirely unlike the way taxonomy grouped
animals based on their physical features. Not surprisingly,
the huge number of particles was referred to as the "particle
The Standard Model, which is now the prevailing model of particle
physics, dramatically simplified this picture by showing that
most of the observed particles were mesons, which are combinations
of two quarks, or baryons which are combinations of three quarks,
plus a handful of other particles. The particles being seen in
the ever-more-powerful accelerators were, according to the theory,
typically nothing more than combinations of these quarks.
Within the Standard Model, there are several different types of
particles. One of these, the quarks, has six different kinds, of
which there are three varieties in each (dubbed "colors", red,
green, and blue, giving rise to QCD: quantum chromodynamics).
Additionally, there are six different types of what are known as
leptons. Of these six leptons, there are three charged particles:
the electron, muon, and tauon. The neutrinos comprise the other
three leptons, and for each neutrino there is a corresponding
member from the other set of three leptons. In the Standard Model,
there are also bosons, including the photons, W+, W-, and Z
particles, gluons, and a few open spaces left for the graviton
and Higgs boson, which have not yet been discovered. Almost
all of these particles come in "left-handed" and "right-handed"
versions (see chirality). The quarks, leptons and W boson all
have antiparticles with opposite electric charge.