The road to unification

Gargamelle and the discovery of weak neutral currents

In July 1973, a groundbreaking discovery was announced in CERN’s main auditorium: the Gargamelle group of physicists had found the first direct evidence of the weak neutral current, a process that required the existence of a neutral particle to carry the weak fundamental force.

This particle, called the Z boson, and the associated weak neutral currents, were predicted by electroweak theory, according to which the weak force and the electromagnetic force are different versions of the same force. This was the first great discovery to be made at CERN and opened a new door to the future of particle physics, the unification of the forces.

Gargamelle was the name of the particle detector used to make this discovery at the Proton Synchrotron accelerator. It was a large bubble chamber, a type of particle detector that uses a pressurised transparent liquid to detect electrically charged particles passing through it.

Named after the mother of Gargantua (the giant in the story by François Rabelais), Gargamelle measured 4 m long with a 2 m diameter, weighed 1000 tonnes, and contained 18 tonnes of liquid Freon. It was made especially for detecting neutrinos. These particles have no charge, and would leave no tracks in the detector, so the aim was to reveal any charged particles set in motion by the neutrinos and so reveal their interactions indirectly.

It was one of these interactions, in which a neutrino set an electron in motion, that provided the first direct observation of a weak neutral current interaction.

Ten years later, two CERN experiments, UA1 et UA2, discovered W and Z particles, carriers of the eletroweak force, a discovery that was awarded the Nobel Prize.