Electroweak interaction

Encyclopedia : E : EL : ELE : Electroweak interaction

In particle physics, the electroweak interaction is the unified description of two of the four fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very different at everyday low energies, the theory models them as two different aspects of the same force. Above the unification energy, on the order of 10² GeV, they would merge into a single electroweak force.

Mathematically, the unification is accomplished under an SU(2) × U(1) gauge group. The corresponding gauge bosons are the photon of electromagnetism and the W and Z bosons of the weak force. In the Standard Model, the weak gauge bosons get their mass from the spontaneous symmetry breaking of the electroweak symmetry from SU(2) × U(1)_Y to U(1)_em, caused by the Higgs mechanism (see also Higgs boson). The subscripts are used to indicate that these are different copies of U(1); the generator of U(1)_em is given by Q = Y/2 + I₃, where Y is the generator of U(1)_Y (called the hypercharge), and I₃ is one of the SU(2) generators (a component of isospin). The distinction between electromagnetism and the weak force arises because there is a (nontrivial) linear combination of Y and I₃ that vanishes for the Higgs boson (it is an eigenstate of both Y and I₃, so the coefficients may be taken as −I₃ and Y): U(1)_em is defined to be the group generated by this linear combination, and is unbroken because it doesn't interact with the Higgs.

For contributions to the unification of the weak and electromagnetic interaction between elementary particles Sheldon Glashow, Abdus Salam, and Steven Weinberg were awarded the Nobel Prize in Physics in 1979. The existence of the electroweak interactions was experimentally established in two stages: the first being the discovery of neutral currents in neutrino scattering by the Gargamelle collaboration in 1973, and the second in 1983 by the UA1 and the UA2 colaborations that involved the discovery of the W and Z gauge bosons in proton-antiproton collisions at the converted Super Proton Synchrotron.

Electroweak interaction

See also