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In the current understanding of particle physics, the set of fundamental constituents of matter consists of six quarks and six leptons. The quarks are named u,d; c,s; t,b and the leptons consist of electrons (e), muons, tau's and their corresponding neutrinos (denoted with the greek letter "nu") are all spin ½ fermions and are grouped in three families, each consisting of a quark pair and a lepton pair. It has been known for a long time that the quarks exhibit small quantum mechanical mixing among themselves which allows weak decays between different quark families as well as time reversal-violating transitions. The masses, coupling constants and mixing angles are parameters of the Standard Model which have to be determined experimentally. Quite recently experimental evidence was obtained that also the neutrinos in the lepton sector have masses and mix among themselves. The quarks make up hadrons such as the protons, neutrons and pions and interact through the strong, weak and electromagnetic forces. The leptons interact only through the weak and, if charged, the electromagnetic force.
In the Standard Model the strong, weak and electromagnetic interactions are mediated through the exchange of spin-one particles (vector bosons) in a way precisely prescribed by the gauge symmetry of the theory. Different vector bosons are responsible for the different types of interaction: Eight gluons (g) for the strong interaction described by quantum chromodynamics (QCD), W+,- and Z bosons for the weak interaction and photons (denoted with the greek letter "gamma") for the electromagnetic interaction (quantum electrodynamics, QED). While the gluons and photons are found to be massless, the W and Z have large masses.
In the Standard Model the fundamental particles are initially massless. The masses are then generated through interactions with hypothetical scalar fields called Higgs fields without violating the gauge symmetry. At least one of these Higgs fields should be visible as a massive scalar boson called the Higgs particle (Ho) which is yet to be discovered. This is the major aim at the large hadron collider LHC under construction at CERN. The heaviest quark (t) was found a few years ago and the existence of the tau neutrino was confirmed in July 2000, completing our experimental knowledge of all three quark-lepton families.
Processes involving weak and electromagnetic interactions can be calculated using perturbation theory. This allows precise predictions which can be compared with experiments. In particular, the impressive precision measurements at the electron-positron collider LEP have been matched by very careful evaluations of electro-weak radiative corrections to which particles contribute virtually even if they cannot be produced at the actual energy of the experiment. In this way the mass of the top quark was predicted before it was found and bounds for the Higgs mass have been obtained. At high energies the strong interaction can also be treated perturbatively. However, the strong interaction has a special feature (confinement), which makes it impossible to create an isolated quark or gluon. What is observed as hadrons are bound states of quarks held together by gluons, and such states cannot be treated using perturbation theory.
The Standard Model has been successfully tested both at high and low energies and at present the only hint of a deviation from its predictions are the neutrino oscillations mentioned above. Nevertheless there are still many open questions: What is the origin of the particle masses? Is there a Higgs boson? Why do quarks and leptons have such different masses? How small are the neutrino masses? Why are there three and only three families? What causes the pattern of mixing between families? Why are the discrete symmetries C (particle <-> antiparticle), P (space reflection), T (time reversal) or the combination CP violated for part of the interactions? It is generally expected that phenomena outside the Standard Model will be found at the TeV scale, which may answer some of the above questions. The existing elementary particles might again turn out to be composite but the most promising option for an extension of the Standard Model at present seems to be the existence of a "supersymmetry" which connects bosons and fermions. Supersymmetry has very attractive theoretical features such as unification of the coupling constants at an energy of about 10^15 GeV and solution of the problem why the Higgs mass is not of that magnitude. Phenomenologically it predicts a light Higgs particle and allows the decay muon -> electron + photon or an electric dipole moment of the neutron. Experiments to look for these phenomena are being planned in LTP. However, supersymmetry also implies the existence of many additional particles, the supersymmetric partners to the known fermions and bosons. They have not been seen experimentally up to now but are searched for in present and future colliders.
Editor: Haselhurst
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