a combination of theory and experiment, a mathematical model that describes
or explains all particle physics observed so far by physicists has been
worked out. This model is called the Standard Model. From the experimental
point of view, the Standard Model is studied and confirmed so well that
things are, well, almost boring.
Standard Model consists of elementary particles grouped into two classes:
bosons (particles that transmit forces) and fermions (particles that make
up matter). The bosons have particle spin that is either 0, 1 or 2. The
fermions have spin 1/2.
Particles that transmit forces
> 78 GeV
table above lists the elementary particles in the Standard Model that
transmit the four forces observed in Nature. Note that the graviton isn't
technically part of the Standard Model but we'll include it anyway. The
Standard Model is from a technical standpoint incompatible with gravity,
and that's why string theory became an active field of theoretical physics.
we say that quarks and gluons are observed "indirectly", we
mean that evidence of their existence inside hadrons exists but these
particles have not been observed singly. In the theory of quarks and gluons,
they are believed to be confined inside hadrons and unobservable as single
particles, except possibly at extremely high temperatures such as could
be found very early in the Big Bang.
Particles that make up matter
fermions in the Standard Model, particles that make up matter, seem to
be grouped into three generations. Notice that the quarks
with charge 2/3 come in a group of three, as do the quarks with charge
-1/3, as do the electron, muon and tau, and the electron, muon and tau
neutrinos. In each group, the heavier particles are shown in the larger
physics has not explained why there are three generations of particles
that make up matter. Maybe string theory will come up with an answer for