9 - Electroweak interactions

Summary

One of the most dramatic events in the history of elementary particle physics was the unification of the electromagnetic and the weak interactions into a single, beautiful gauge theory, which was created by Weinberg, Salam and Glashow and which is nowadays referred to as the ‘Standard Model’ (SM). For a detailed pedagogical account of the need for and development of such a theory, the reader is referred to Leader and Predazzi (1996). We simply recall that this tightly knit theory contains the astounding and incredible prediction of the existence of a set of three vector bosons, W^±, Z⁰, with huge masses, m_w ≈ 80 GeV/c², m_z ≈ 90 GeV/c², and that these unlikely objects were eventually discovered. (The W was identified at CERN in January 1983 and the Z⁰, also at CERN, a few months later.) A test for the spin of the W is described in subsection 8.2.1(ix).

In the Standard Model the electroweak interactions are mediated by the exchange of photons, Zs and Ws, whose coupling to the basic fermions (leptons and quarks) is a mixture of vector and axial-vector. To begin with all particles are massless, and their masses are generated by spontaneous symmetry breaking. The usual mechanism of symmetry breaking requires a neutral scalar particle, the Higgs meson H, whose mass is not determined by the theory. H has not yet been detected experimentally and is the most serious missing link in the theory. But in every other respect the theory has been remarkably successful.