Sheldon Glashow, Abdus Salam, and Steven Weinberg were awarded the 1979 Nobel Prize in Physics for their contributions to the unification of the weak and electromagnetic interaction between elementary particles. 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 collaborations that involved the discovery of the W and Z gauge bosons in proton–antiproton collisions at the converted Super Proton Synchrotron. In 1999, Gerardus 't Hooft and Martinus Veltman were awarded the Nobel prize for showing that the electroweak theory is renormalizable.
Mathematically, the unification is accomplished under an SU(2) × U(1) gauge group. The corresponding gauge bosons are the three W bosons of weak isospin from SU(2) (W1, W2, and W3), and the B boson of weak hypercharge from U(1), respectively, all of which are massless.
In the Standard Model, the
bosons, and the photon, are produced by 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). U(1)Y and U(1)em are different copies of U(1); the generator of U(1)em is given by Q = Y/2 + T3, where Y is the generator of U(1)Y (called the weak hypercharge), and T3 is one of the SU(2) generators (a component of weak isospin).
The spontaneous symmetry breaking makes the W3 and B bosons coalesce into two different bosons – the
boson, and the photon (γ),
where θW is the weak mixing angle. The axes representing the particles have essentially just been rotated, in the (W3, B) plane, by the angle θW. This also introduces a mismatch between the mass of the
and the mass of the
particles (denoted as MZ and MW, respectively),
The W1 and W2 bosons, in turn, combine to give massive charged bosons
The distinction between electromagnetism and the weak force arises because there is a (nontrivial) linear combination of Y and T3 that vanishes for the Higgs boson (it is an eigenstate of both Y and T3, so the coefficients may be taken as −T3 and Y): U(1)em is defined to be the group generated by this linear combination, and is unbroken because it does not interact with the Higgs.