July 2023 marks the 50th anniversary of one of the greatest discoveries made at CERN, the international particle-physics research centre near Geneva, Switzerland: namely, weak neutral currents. The discovery, made by the Gargamelle experiment, provided key evidence that one of four known fundamental forces in nature, the weak interaction, is inextricably entwined with another, the more familiar force of electromagnetism. That finding opened a path of exploration that led, by way of numerous breakthroughs, to the discovery of the Higgs boson in 2012 — and it is still revealing new and exciting perspectives today.

The influence of the weak interaction is seen most obviously in radioactive β-particle decays. When CERN was founded in 1954, particle physicists’ understanding of the interaction was in its infancy. Back then, the best way to study matter and its workings at the smallest scales was to fire high-energy beams of particles into a target and measure what emerged.

In 1959, CERN’s Proton Synchrotron accelerator started up. This could produce beams of various particle types, and in the early 1960s, experiments began there with the extremely light particles known as neutrinos — albeit with fierce competition from the higher-energy Alternating Gradient Synchrotron, situated at Brookhaven National Laboratory on Long Island, New York. The CERN and Brookhaven machines both made neutrino beams by colliding protons with a target to make secondary particles such as pions and kaons, which produce neutrinos when they decay. (Protons, pions and kaons are all varieties of hadron, particles physicists now know to be composites constructed from different configurations of elementary particles called quarks.)

The initial CERN configuration yielded disappointingly low-intensity beams. It was the Brookhaven experiments that, in 1962, demonstrated that there were at least two types of neutrino — one produced in decays together with an electron, and another produced together with the electron’s higher-mass cousin, the muon.

In 1961, Dutch physicist Simon van der Meer invented a focusing device around the target called a ‘magnetic horn’, which helped to increase the neutrino-beam intensity. This was a game-changer for CERN. Detecting the resulting interactions using a bubble chamber, in which the paths of ionizing charged particles can be seen as a line of bubbles through a superheated liquid, proved a particularly promising method. This led French physicist André Lagarrigue to propose a larger, 4.8-metre-long bubble chamber — dubbed Gargamelle.

Shortly before Gargamelle started up in 1970 (Fig. 1), the collaborating scientists, based at CERN and at institutions in Belgium, France, Germany, Italy and the United Kingdom, made a list of ten priority measurements. Observations of processes involving weak neutral currents made it only to number eight, merely because previous experiments had set low limits on how often they were expected to occur.

Scientists in lab coats working on the installation of the Gargamelle bubble chamber in the PS.

Figure 1 | Installation of the Gargamelle bubble-chamber experiment at CERN. Commissioned in 1970, Gargamelle discovered its first ‘weak neutral current’ event in December 1972 — a breakthrough announced to the world on 19 July 1973.Credit: CERN

Neutral-current events were nevertheless of great interest because of their role in the electroweak theory. Physicists had successfully described electromagnetic processes through the theory of quantum electrodynamics, in which the particles transmitting the quantum-mechanical force are a type of ‘boson’, specifically, the familiar massless photon. The researchers wanted to find a similar quantum theory for the weak and strong nuclear forces (the strong force being that which binds quarks together into hadrons). The most promising way to do this was to combine the electromagnetic and weak interactions in a unified description1–3. This predicted the existence of three new, heavy bosons in addition to the photon: the neutral Z0 and the charged W+ and W−. The mechanism that was proposed to allow these bosons and other fundamental particles to acquire their mass results in an additional heavy particle with unique properties — the Higgs boson4–6. This unified ‘electroweak’ theory could be combined with the theory of the strong force, quantum chromodynamics, to form what is now known as the standard model of particle physics.

According to the unified electroweak theory, there are two types of neutral-current process, so called because they involve the neutral Z boson: leptonic and hadronic processes. In a bubble chamber such as Gargamelle, the signal of a leptonic process would be a single high-energy electron that would appear after being struck by a neutrino (or its antiparticle, an antineutrino). In a hadronic process, a neutrino would interact with a nucleus in the bubble-chamber fluid, producing a shower of hadrons.

At Gargamelle, photographs of the bubble chamber were taken each time a neutrino pulse went through and were analysed by eye, with potentially interesting events flagged. The frequency of leptonic events was expected to be very low — only a handful of events in a year’s data-taking. But the frequency of background processes that could mimic this interaction was also very low, making this the most unambiguous signal of a neutral-current process. Hadronic processes happened much more often, but occurred against a much higher, confounding background.

Gargamelle eventually made the breakthrough in December 1972, finding its first leptonic event. This gave the team an even greater incentive to identify hadronic events and evaluate the expected background rates. So it was that, at a seminar held at CERN on 19 July 1973, the first evidence for both types of weak neutral current was presented. Papers on each were published in the same volume of Physics Letters B in September that year7,8.

This was the first compelling evidence that the electroweak theory was correct. The next challenge was to produce the Z boson, and its charged siblings the W bosons, directly, rather than observing them through their influence on other processes. The decisive step, led by the Italian physicist Carlo Rubbia9 in 1976, was the realization that this could be achieved by transforming a sufficiently high-energy proton accelerator into a machine to collide protons and antiprotons. CERN’s Super Proton Synchrotron saw its first proton–antiproton collisions in 1981. In 1983 — ten years after the observation of neutral currents — the UA1 and UA2 collaborations announced the discoveries first of the W bosons and then of the Z boson. Rubbia and van der Meer shared the 1984 Nobel Prize in Physics for their parts in this discovery.

At that time, preparations were well under way at CERN to build the 27-kilometre-circumference Large Electron–Positron collider (LEP). LEP was designed to make detailed measurements of the properties of W and Z bosons by colliding electrons with their antiparticles, and started operating in 1989. Piece by piece, the experimental evidence in support of today’s standard model came together. These included the revelation that there could be three, and only three, light (that is, near-massless) neutrinos — those associated with the electron and the muon, and another neutrino, yet to be discovered, linked to an even heavier lepton, the tau. LEP also demonstrated the validity of subtle corrections to the electroweak theory, published10 in 1971, that depended strongly on the mass of the heaviest of the six quarks, the top quark, and also on the mass of the Higgs boson. There was remarkable agreement between the top-quark mass predicted by LEP and the mass eventually measured by the Tevatron proton–antiproton collider at Fermilab near Chicago, Illinois, in 1995.

Eventually, the last missing ingredient of the standard model was the Higgs boson. Experiments at LEP actively searched for the elusive particle, but it became clear that an even more powerful accelerator was needed to produce it. LEP operations ended in 2000, paving the way for the installation of a new proton–proton collider, the Large Hadron Collider (LHC), in the same tunnel. The discovery of the Higgs boson was famously announced in 2012 by two LHC experiments, ATLAS and CMS. Again, its measured mass was in excellent agreement with predictions.

This is by no means the end of the story. In the original electroweak theory, neutrinos were assumed to be massless, but the phenomenon of ‘mixing’, in which one type of neutrino transforms into one of the other two types, proved this could not be the case. The details of the neutrino masses — which must be very small, but not zero — and the exact nature of these particles are still not known. Next-generation neutrino-beam experiments planned in Japan and the United States will explore these questions in richer detail. An upgrade of the LHC will also continue running until the early 2040s, ultimately aiming to deliver ten times more collisions than the original design.

Meanwhile, a detailed feasibility study of a Future Circular Collider is in progress at CERN. This would replicate the LEP–LHC model in a tunnel with a 90-kilometre circumference: first, an electron–positron collider would be installed to measure the Higgs boson and electroweak processes with even greater precision, and then a hadron collider would explore even higher-energy phenomena, including the production of two Higgs bosons in the same collision, at a much higher rate than that obtained at the LHC. Such experiments should reveal whether the simplest standard-model description of the Higgs boson is correct, or if there is a more complex structure to it — whether there is more than one type of Higgs boson, for example, or whether it interacts with other unknown particles.

This programme might also give clues to the nature of unseen cosmic ‘dark matter’, for which there is strong evidence from astronomical observations. Just as the LEP measurements were sensitive to the top quark and Higgs boson, precise measurements at future colliders might reveal the influence of as yet unknown, heavier particles. Fifty years after Gargamelle laid the foundations of electroweak interactions, and with it the standard model, a decades-long programme of rich fundamental science still lies ahead.