It was on this day, June 1, 1983, that physicists at the CERN facility in Switzerland first confirmed the existence of the Z-boson and provided full confirmation of the GWS model of weak nuclear forces (also known as electroweak theory) first predicted twenty years earlier. Although it had been inferred from other experiments in the 1970s, this was the first direct observations of its existence. (The CERN press release can be viewed here.)

As I have written in past articles, the Universe is known to contain only four basic forces: (1) electromagnetic forces that govern molecular binding, light and radio waves and electricity (among other things), (2) weak nuclear forces that control the radioactive decay of some nuclei, (3) strong nuclear forces that bind nuclei together, and (4) gravity, which keeps the Universe together.

Throughout the 1950s, it was realized that electromagnetic forces could be predicted in quantum mechanics by requiring particles to be symmetric under a certain type of mathematical rotation (called a gauge transformation). This idea was further advanced by Glashow, Weinberg, and Salam by further allowing certain pairs of particles to be interchanged as well as rotated. The result was a theory that not only predicted electromagnetic forces, but also predicted something very similar to the weak nuclear forces. The primary problems were that this new force should operate over large scales than just sub-atomic scales, and that it predicted three new particles that had never been observed experimentally. In particular, it predicted the existence of a second species of photon that would be identical to the first, but had never been observed.

Eventually a solution was found by allowing the three new particles to have a mass so large that they could not be produced in the particle experiments of the time. (This solution has other problems, which were resolved by the Higgs mechanism, but that is a separate topic). So then the question was, when higher energy particle accelerators are constructed would the GWS model be proven right by the observation of these new particles. 

As it turned out, the W+ and W- were observed in the 1970s, with masses roughly 88 times greater than that of a hydrogen atom. The Z-boson was the last to be discovered, weighing in at over 90 GeV (for comparison, a single proton weighs a mere 0.93 GeV). The GWS model had been confirmed and electroweak forces were united.

The discovery of the Z-boson also lead to the creation of the Large Electron Positron collider in the 1989, which ran for more than a decade and produced billions of Z-bosons. The benefit of this project was that with so many particles produced, physicists could perform very precise measurements of the properties of not just the Z-boson, but of many other Standard Model parameters as well.

Perhaps it is not an anniversary that is celebrated by the masses, but then the history of science in general and physics in particular is filled with little publicized discoveries that lead to major advances in technology. And as with so much of modern particle physics, we can only wonder where it will lead us in the centuries to come.