| The Standard Model of particle physics is the theory which
describes the interactions between elementary particles, excluding
gravitational interactions. The fundamental constituents of matter are
the leptons (electron, muon, tau, and their neutrinos)
and the quarks (up, down, charm,
strange, top, and bottom).
All of these particles interact through the electromagnetic force
(except the chargeless neutrinos) through photon exchange.
They also interact through the weak force by exchanging a massive
vector boson, W or Z. In fact these two forces mix
at an energy scale of about 1 trillion electron volts (1 TeV) and
are called the electroweak force.
We refer to the apparent difference
of the forces at everyday low energies as electroweak symmetry breaking
which occurs spontaneously.
The mechanism which drives this symmetry breaking is referred to
as the Higgs mechanism, and requires the existence of a spinless
boson called the Higgs particle.
Finally, the quarks interact among each other through the
strong force. This subsection of the Standard Model is called
quantum chromodynamics, or QCD. Here the quarks exchange gluons
to form baryons such as the proton or neutron, or a
quark and an anti-quark can pair to form a mesons such as the pion.
These bound states are generically referred to as hadrons.
In fact the strong force in its normal phase is confining, i.e.
one cannot observe an unbound quark. Experiments at the
Relativistic Heavy Ion Collider (RHIC) are searching for a new state of
matter at high temperatures where quarks would be deconfined.
Setting aside the fact that gravity is not part of this theory,
the Standard Model on its own leaves many questions unanswered.
For example, why is the top quark so heavy? It is nearly 40 times
more massive than the next heaviest, the bottom, and over 40000
times more massive than the light up and down quarks.
Another problem is that fundamental spinless bosons are kept in
a stable theory only through very tenous fine tuning of the
theory's parameters. Could the Higgs be a composite particles?
Is there a more suitable mechanism for electroweak symmetry breaking?
Or can some extra supersymmetry stablize the theory?
It should be emphasized that the Standard Model describes all of
the observed interactions of elementary particles to date (Sep. 2000).
Some of these experiments are the most precise experiments ever performed.
Therefore, it is highly unlikely that it will be disproved.
The present searches for new physics beyond the Standard Model
all have strong theoretical motivation, but are not inspired by
a conflict between experimental results and theoretical calculations.
So any new theory of fundamental particles and their interactions
must necessarily subsume the Standard Model.
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