50 years of gigantic electroweak breakthroughs

Half a century ago, a series of tiny traces in a bubble chamber at CERN changed the course of particle physics. The observation of weak neutral currents, announced on July 19, 1973 by Paul Musset of the Gargamelle collaboration, suggested that the weak electromagnetic forces were facets of a more fundamental electroweak interaction that reigned in the early Universe. Since then, the exploration of this new sector of nature has been a core activity of CERN, leading to the discovery of the W and Z bosons in 1983 and culminating in the discovery of the Higgs boson in 2012.

The weak force is one of the four fundamental forces of nature, responsible for crucial processes such as radioactive beta decay. While the electromagnetic force was well understood as the result of the exchange of neutral photons between charged particles, the weak force was more difficult to interpret in the language of quantum theory. In the 1960s, theorists postulated that the weak interaction was mediated by massive versions of the photon: the charged W boson and the neutral Z boson, both of which are inextricably linked to the photon of electromagnetism. The W boson allowed for weak interactions involving a rearrangement of electric charge, while the Z boson was how charged particles interacted via the weak force. While the former were already known, the latter had never been seen before.

As physicists mastered the art of firing intense beams of neutrinos into detectors to study fundamental interactions, the search for neutral currents became possible. In the late 1960s Andr Lagarrigue of LAL Orsay had proposed the largest bubble chamber in the world, Gargamelle, named after an imaginary giantess. The chamber was built by Cole Polytechnique Paris in 1968 and assembled into one of the beamlines of CERN’s Proton Synchrotron. Data acquisition began in 1970, with the first results arriving shortly thereafter. Reflecting the attention of experimentalists of the time, the search for neutral currents ranked only eighth among Gargamel’s top ten physics goals.

Choosing experimental evidence for neutral currents among numerous similar-looking events was not easy, especially with the technology of the day. The researchers needed to see both leptonic events (where a neutrino interacted with an electron in the dense gas filling Gargamel) and hadronic events (where a neutrino was scattered by a proton or neutron). I remember spending evenings with my colleagues scanning the films on special projectors, which allowed us to observe the eight views of the chamber, recalls Donatella Cavalli, a Gargamelle member of the University of Milan, who was a PhD student at the time. When the first lepton event was discovered in December 1972, we were convinced that there were neutral currents.

Further data would reveal possible neutral current hadronic events, but it took some time for the community to become convinced. Initially, the independent HarvardPennsylvaniaWisconsinFemilab experiment in the United States confirmed Gargamel’s findings, but when they changed their experimental setup, the traces vanished. Only in 1974, after further analysis by both collaborations, the existence of neutral currents was universally accepted leading to the award of the 1979 Nobel Prize in Physics to electroweak architects Sheldon Glashow, Abdus Salam and Steven Weinberg.

Gargamelle is now an exhibit in CERN’s Van Hove Square, but physicists are still pursuing the path it paved. In providing the first evidence of the electroweak theory, Gargamelle’s results led CERN to convert the Super Proton Synchrotron into a proton-antiproton collider powerful enough for the UA1 and UA2 collaborations to directly discover the W and Z bosons, a feat recognized by the award of the 1984 Nobel Prize in Physics to CERN’s Carlo Rubbia and Simon van der Meer. During the 1990s, precision measurements of the W and Z bosons at the Large ElectronPositron collider confirmed important quantum corrections to the electroweak theory (which, together with the strong force theory, quantum chromodynamics, constitutes the Standard Model of particle physics). This led physicists to the discovery of the final piece of the electroweak Higgs boson puzzle at the Large Hadron Collider (LHC) in 2012, which led to theorists François Englert and Peter Higgs being awarded the Nobel Prize in Physics in 2013.

But the journey doesn’t end there. As the LHC’s ATLAS and CMS experiments continue to probe the Higgs boson and other mysterious domains of the Standard Model to increasing levels of accuracy, physicists are studying the feasibility of a successor to the collider at CERN, the proposed Future Circular Collider, which would go much further, opening the next chapter in electroweak exploration.

Read more in the CERN Courier:

CERN’s neutrino odyssey

Higgs after LHC

A scientific symposium marking 50 years of the neutral currents and 40 years of the W and Z bosons will be held at CERN on 31 October 2023 at the Science Gateway Auditorium.