A Nobel discovery

Hunting the heavyweights with UA1 and UA2

UA1 and UA2 were two experiments at CERN’s Super Proton Synchrotron (SPS) accelerator which started taking data in 1981 when the SPS first operated as a proton–antiproton collider. At the time, one of the hottest challenges in particle physics was the hunt for the force-carrier particles predicted by electroweak theory. Named the W and Z bosons, these were heavy particles, so finding them would require an accelerator that could reach an unprecedented level of energy.

Physicists David Cline, Peter McIntyre and Carlo Rubbia suggested modifying CERN’s biggest and newest accelerator at the time, the SPS, from a one-beam accelerator into a two-beam collider. This would collide a beam of protons with a beam of antiprotons, greatly increasing the available energy in comparison with a single beam colliding against a fixed target. Simon Van de Meer at CERN had already invented a way of producing and storing dense beams of protons or antiprotons. The two ideas, combined with the huge effort to develop new technology and engineering at CERN, eventually led to the historic discovery of the W and Z bosons in 1983 by UA1 and UA2.

The discovery was so important that the two key scientists behind the discovery received the Nobel Prize in Physics only a year later. The prize went to Carlo Rubbia, instigator of the accelerator’s conversion and spokesperson of the UA1 experiment, and to Simon van der Meer, whose technology was vital to the collider’s operation.

This was a significant achievement in physics that further validated the electroweak theory. It also helped to secure the decision to build CERN’s next big accelerator, the Large Electron Positron collider, whose job was to mass-produce Z and W bosons for further studies.

Origins of the hunt

In the 1960s three physicists, Steven Weinberg, Abdus Salam and Sheldon Glashow, proposed a theory. They believed that two of the four fundamental forces – the electromagnetic force and the weak force – were in fact different facets of the same force. Under high-energy conditions (such as in a particle accelerator), the two would merge into the electroweak force.

No scientific theory can become established without a solid grounding of experimental proof. The first evidence in support of the theory emerged when the Gargamelle detector at CERN found the neutral current, an essential ingredient to the electroweak theory. Further observations followed to secure the three theorists a Nobel Prize in 1979. However, there were still three hypothetical force-carrier particles described by the theory that no one had managed to find. The W⁺, W^- and the Z⁰ bosons remained tantalisingly out of reach until an accelerator could be built with high enough energy to carry out the search – a problem solved by the conversion of the SPS accelerator.