Fermilab’s Tevatron, the largest particle accelerator in the United States, was shut down on September 30 after a celebrated career of 28 years that has provided us with some of the greatest discoveries in particle physics. This leaves the European lab CERN (see photo on left) to lead the way into future discoveries with its Large Hadron Collider.This landmark in experimental physics is an opportunity to reexamine the theoretical model physicists have constructed and relied on in their search to understand the workings of the universe: the standard model of particle physics. The standard model is a comprehensive theory about nature’s elementary particles and the forces that control their behavior, and it has been constructed over a half-century of intensive work by many theoretical physicists as well as experimentalists. The model has worked amazingly well, harmoniously combining theory and experiments and producing extremely accurate predictions about the behavior of particles and forces. But could the model now be beginning to show some cracks?
It all started on a wintry evening in 1928. While staring at the flames in the fireplace at St. John’s College, Cambridge, Paul Dirac made one of the most important discoveries in the history of science when he saw how to combine the Schrödinger equation of quantum mechanics with Einstein’s special (but not general) theory of relativity. This achievement launched relativistic quantum field theory—which forms the theoretical basis for the standard model—and produced two immediate consequences: an explanation of the spin of the electron, and Dirac’s stunning prediction of the existence of antimatter (confirmed a few years later with the discovery of the positron).
In the late 1940s, Richard Feynman, Julian Schwinger, and Sin-Itiro Tomonaga, all working independently, presented the first quantum field theory, called quantum electrodynamics, which explained the electromagnetic interactions of electrons and photons. It forms the first part of the standard model by handling interactions that are controlled by the electromagnetic field. The theory’s success inspired other theoretical physicists to construct similar quantum field theories for addressing the actions of the weak and strong nuclear forces—thus together accounting for everything in particle physics except for the action of gravity, the subject of Einstein’s general theory of relativity. By the 1970s, the result, the standard model, was ready: a quantum field theory of all elementary particles—leptons and quarks and their interactions through the actions of particles (such as the photon) called bosons.