protons and neutrons were made of quarks was still relatively new: The experiment providing the first evidence for this occurred in 1969.

Although physicists did not realize it at the time, the field of particle physics was about to undergo a revolutionary change caused by the combination of new accelerators and the new theoretical tools called gauge theories. In 1972, the proton accelerator at Brookhaven was delivering beams of unprecedented intensity. The proton accelerator at CERN (the European Laboratory for Particle Physics) was being used to produce intense beams of neutrinos. A new high-energy proton accelerator at Fermilab was just beginning to produce new physics results, and SPEAR, an electron-positron collider, was starting to operate at the Stanford Linear Accelerator Center (SLAC).

THE FORCES

There have been two major developments in the understanding of fundamental forces over the past 25 years. One was the establishment of the idea that electromagnetic and weak forces are unified into a single force that could be described by a theory formulated around the principle of gauge invariance: a gauge theory. The other was the discovery of a theory of the strong force, the force that holds quarks together in the proton, that was similarly a gauge theory.

The Electroweak Force

A central idea of the Standard Model is that the electromagnetic force and the weak force are different manifestations of a single unified force called electroweak. This was not evident for many years because the weak force acts over only very short distances and is completely negligible at the atomic distance scales at which the electromagnetic force acts to bind electrons to the nucleus. Electroweak theory, developed in the 1960s, gave a natural explanation for this difference. The reason for the difference is that the force carriers for the weak force, W and Z bosons, are extremely massive, whereas the force carrier for the electromagnetic force, the photon, is massless. This theory predicted many new phenomena that could be explored with the new, higher-energy accelerators whose use was beginning in the early 1970s.

Demonstrating the Unification of Weak and Electromagnetic Forces

A dramatic prediction of electroweak theory was that there would be a new kind of interaction involving quarks and leptons, called the neutral weak current. One manifestation of this new interaction was that a neutrino could strike a quark or an electron and recoil, remaining a neutrino. Up to this point, the only neutrino interactions observed were those in which a neutrino was transformed



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