CERN Accelerating science

4 July 2012 wasn’t the first time physicists had packed themselves into the CERN auditorium to witness the discovery of a new elementary particle. To rapturous applause on 20 January 1983, Carlo Rubbia, spokesperson of the UA1 experiment at the Spp-barS collider, presented six candidate events for the W boson, the electrically charged carrier of the weak interaction responsible for radioactive decay. In similar scenes the following afternoon, Luigi Di Lella of the UA2 experiment announced four W candidates. Along with the Z boson and massless photon, the W boson is one of three “gauge” bosons of a unified electroweak interaction that demands the existence of a fourth “scalar” particle called the Higgs boson.

Indirect evidence for the Z boson had been obtained a decade earlier at Gargamelle, driving the community to seek a direct discovery of the massive electroweak bosons. But their predicted masses – around 80 and 90 GeV for the W and Z, respectively – were beyond the reach of experiments at the time. In 1976, Rubbia, Peter McIntyre and David Cline suggested modifying the CERN SPS from a one-beam accelerator into a machine that would collide beams of protons and antiprotons, greatly increasing the available energy. Simon van der Meer had already invented a way of producing and storing dense beams of protons or antiprotons, while his “stochastic cooling” method to reduce the energy spread and angular divergence of the beams had been honed at the Intersecting Storage Rings (the world’s first hadron collider). Many doubted the wisdom of the decision, however, especially as CERN was keen to push its visionary Large Electron–Positron (LEP) collider.

As former UA2 spokesperson Pierre Darriulat wrote in CERN Courier in 2004: “The pressure to discover the W and Z was so strong that the long design, development and construction time of the LEP project left most of us, even the most patient, dissatisfied. A quick (but hopefully not dirty) look at the new bosons would have been highly welcome. But when proton–proton colliders such as the Superconducting Intersecting Storage Rings were proposed in this spirit, they were ‘killed in the egg’ by the management at CERN, with the argument that they would delay – or, even worse, endanger – the LEP project. The same argument did not apply to the proton–antiproton collider, as it did not require the construction of a new collider ring and could be proposed as an experiment … Another argument also made it possible for the proton–antiproton project to break the LEP taboo: if CERN did not buy Carlo’s idea, it was most likely that he would sell it to Fermilab.”

Two detectors, UA1 and UA2, built around the Spp-barS beam pipe to search for signatures of the W and Z particles, started taking collision data in 1981. When they confirmed the existence of the W boson – which was announced at a press conference at CERN on 25 January 1983, followed by the discovery of the Z boson a few months later and the Nobel Prize in Physics for Rubbia and Van der Meer the following year – the case for the existence of the Higgs boson grew stronger.

That’s because all three bosons hail from the same “Mexican hat”-shaped Brout-Englert-Higgs (BEH) field that broke the electroweak symmetry a fraction of a nanosecond after the Big Bang and left the universe with a non-zero vacuum expectation value. As the universe transitioned from a symmetrical state at the top of the hat to a more stable configuration in the rim, three of the BEH field’s four mathematical components were absorbed to generate masses for the W and Z bosons (while keeping the photon massless); the fourth, corresponding to an otherworldly oscillation up and down the rim of the Mexican hat, is the Higgs boson.

In 1983, assuming that the electroweak Standard Model and BEH mechanism were correct, three quarters of the BEH field had been discovered. LEP went on to measure the properties of the W and Z bosons in great detail, helping to constrain the possible hiding places for the “remaining quarter”. The Standard Model does not predict the mass of the Higgs boson. Finding it would require an even more powerful machine. Thanks to the foresight of CERN Director-General John Adams in 1977, the LEP tunnel was designed to be large enough to accommodate the proton–proton collider that, 35 years later, would uncover the final quarter of the mysterious scalar field that pervades the universe and gives mass to elementary particles.