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 particle? 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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