UA1: first Z event recorded
This image taken by the UA1 experiment on 30 April 1983 was the first detection of a Z0 particle. (Image: CERN)

Discovered in 1983 by physicists at the Super Proton Synchrotron (SPS) at CERN, the Z boson is a neutral elementary particle. Like its electrically charged cousin, the W boson, the Z boson carries the weak force.

The weak force is essentially as strong as the electromagnetic force, but it appears weak because its influence is limited by the large mass of the W and Z bosons. Their mass limits the range of the weak force to about 10-18 metres, and it vanishes altogether beyond the radius of a single proton.

Enrico Fermi was the first to put forth a theory of the weak force in 1933, but it was not until the 1960s that Sheldon Glashow, Abdus Salam and Steven Weinberg developed the theory in its present form, when they proposed that the weak and electromagnetic forces are actually different manifestations of one electroweak force. 

By emitting an electrically charged W boson, the weak force can cause a particle such as the proton to change its charge by changing the flavour of its quarks. In 1958, Sidney Bludman suggested that there might be another arm of the weak force, the so-called "weak neutral current," mediated by an uncharged partner of the W bosons, which later became known as the Z boson.

Physicists working with the Gargamelle bubble chamber experiment at CERN presented the first convincing evidence to support this idea in 1973. Neutrinos are particles that interact only via the weak interaction, and when the physicists examined collisions they were able to detect evidence of the weak neutral current, and hence indirect evidence for the Z boson.

At the end of the 1970s, physicists Carlo Rubbia, Peter McIntyre and David Cline suggested converting what was then CERN’s biggest accelerator, the Super Proton Synchrotron, to operate as a proton-antiproton collider, with the aim of producing W and Z bosons directly. Both types of particle were observed there for the first time in 1983 by the UA1 and UA2 experiments.

The bosons were then studied in more detail at CERN and at Fermi National Accelerator Laboratory in the US. In 1989, first physics results from the Large Electron-Positron collider at CERN measured the width of the Z boson and confirmed that there are only three neutrino types in Nature: electron, muon and tau. During the 1990s, LEP and the SLAC Linear Collider in the US produced millions of Z bosons for further study. These results culminated in the need to search for the final piece of the Standard Model – the Higgs boson, which was discovered in July 2012.

The discovery of W and Z bosons was an extraordinary technical triumph, confirming a critical aspect of the Standard Model. As a result, the 1984 Nobel prize in physics was awarded to Carlo Rubbia and Simon van der Meer.