3.5.10 Symmetry
In the beginning of the 1950s, QED become a reliable theory, but it took more time until Quantum Field Theory could be applied successfully to important physical problems in a systematic way. The theories explored relied on a rich variety of symmetries pioneered and articulated by Murray Gell-Mann (b. 1929). This made it possible to apply Quantum Field Theory to new particles and new interactions and fully explain their structure.
In the following decades, Quantum Field Theory was extended to describe the electromagnetic force and also weak and strong interaction, so that new Lagrangians were found which contain new classes of particles or quantum fields. The search still continues for a unified theory of all interactions that more comprehensively explain all matter and energy, including gravity.
The new focus on symmetry led to the triumph of non-Abelian gauge theories, pioneered by the work of Chen-Ning Yang (b. 1922) and Robert Mills (1927-1999), and spontaneous symmetry breaking, by Yoichiro Nambu (1921-2015).
A combined re-normalizable theory associated with the gauge group
is dubbed the standard model of elementary particle physics, that was assembled by Sheldon Glashow (b. 1932), Steven Weinberg (b. 1933) and Abdus Salam (1926-1996), between 1959 and 1967, in addition to many other scientists, including: Peter Higgs, Francois Englert, Robert Brout, Martin Veltman, Gerard ’t Hooft, Frank Wilczek, David Gross and David Politzer.
According to the standard model, described further in section 5.18, there are six types of leptons and six types of quarks, where the members of both groups are all fermions with spin-half. On the other hand, there are four spin-one particles, or bosons, that mediate the interaction between elementary particles and the fundamental forces. The linchpin of the symmetry breaking mechanism of the theory is the spin-zero Higgs boson which was discovered in 2012, after 40 years of its prediction.