Exchange interaction independently by physicists Werner Heisenberg and Paul Dirac in 1926. The exchange interaction is sometimes called the "exchange force". However, it is not a true force and should not be confused with the exchange forces produced by the exchange of force carriers, such as the electromagnetic force produced between two electrons by the exchange of a photon, or the strong force between two quarks produced by the exchange of a gluon. Although sometimes erroneously described as a force, the exchange interaction is a purely quantum mechanical "effect" unlike other forces. Quantum mechanical particles are classified as bosons or fermions. The
Technicolor (physics) "M", and the coupling of quarks and leptons to the new particles must be suppressed relative to their coupling to the gauge bosons. This is the case with technicolor, so long as the lightest technivector mesons, "ρ" and "a", are heavier than 200–300 GeV. The "S"-parameter is sensitive to all new physics at the TeV scale, while "T" is a measure of weak-isospin breaking effects. The "U"-parameter is generally not useful; most new-physics theories, including technicolor theories, give negligible contributions to it. The "S" and "T"-parameters are determined by global fit to experimental data including "Z"-pole data from LEP at