The 50th anniversary of one of CERN’s most significant achievements, the detection of weak neutral currents, is celebrated in July 2023. CERN, a global hub for particle physics research located near Geneva, Switzerland, was the epicenter of this milestone. The Gargamelle experiment was instrumental in this revelation, providing crucial proof that the weak force, one of the four recognized fundamental forces of nature, is inescapably intertwined with another, more commonly understood force, electromagnetism. This knowledge set in motion a chain of major advancements leading up to the unveiling of the Higgs boson in 2012 — a path of discovery that continues to yield novel and intriguing insights today.
The presence of the weak force is most discernible in radioactive β-particle breakdowns. When CERN came into existence in 1954, our comprehension of this interaction was still rudimentary. At that time, the most effective strategy to examine matter and its smallest-scale mechanisms was to bombard a target with high-energy particle beams and analyze the resulting emissions.
CERN’s Proton Synchrotron accelerator was inaugurated in 1959. It was capable of generating beams of diverse particle types, and in the early 1960s, investigations using extremely lightweight particles known as neutrinos commenced there — albeit amid intense rivalry from the more powerful Alternating Gradient Synchrotron, housed at Brookhaven National Laboratory on Long Island, New York. Both the CERN and Brookhaven apparatuses generated neutrino beams by striking protons against a target to create secondary particles like pions and kaons, which yield neutrinos upon decay. (Protons, pions, and kaons are all types of hadron, which physicists now understand to be composites assembled from distinct arrangements of basic particles known as quarks.)
The original setup at CERN generated dishearteningly weak-intensity beams. It was the research conducted at Brookhaven that, in 1962, unveiled the existence of at least two variations of neutrino — one born in decays alongside an electron, and another created in conjunction with the electron’s higher-mass relative, the muon.
In 1961, Simon van der Meer, a physicist from the Netherlands, engineered a concentrating apparatus near the target, known as a ‘magnetic horn’. This significantly amplified the neutrino-beam intensity and marked a turning point for CERN. Utilizing a bubble chamber to detect the subsequent interactions, a method where the trajectories of ionizing charged particles are visible as a string of bubbles through an overheated liquid, was found to be particularly effective. Consequently, French physicist André Lagarrigue suggested the creation of a larger, 4.8-metre-long bubble chamber, christened Gargamelle.
Shortly before Gargamelle commenced operations in 1970, the cooperating scientists, hailing from CERN and research institutions in Belgium, France, Germany, Italy and the United Kingdom, compiled a checklist of ten high-priority measurements. Notably, observations of phenomena involving weak neutral currents ranked eighth on the list, solely due to the fact that earlier experiments had determined that these occurrences were expected to be infrequent.
Events involving neutral-currents nonetheless held significant fascination due to their integral part in the electroweak theory. Scientists had effectively delineated electromagnetic processes through quantum electrodynamics, where the particles carrying the quantum-mechanical force are a category of ‘boson’, specifically, the familiar massless photon. The investigators sought to unearth a parallel quantum theory for the weak and strong nuclear forces (the strong force being the one that adheres quarks into hadrons). The most plausible method to achieve this was merging the electromagnetic and weak interactions into a harmonized depiction. This forecasted the emergence of three new, heavy bosons alongside the photon: the neutral Z0 and the charged W+ and W−. The suggested mechanism to enable these bosons and other fundamental particles to attain their mass brings forth an extra heavy particle with exclusive traits — the Higgs boson. This unified ‘electroweak’ theory could be amalgamated with the theory of the strong force, quantum chromodynamics, forming what is now recognized as the standard model of particle physics.
Leptonic and hadronic processes are two categories of neutral-current processes according to the consolidated electroweak theory; they are given this name because they include the neutral Z boson. A single high-energy electron that would appear within a bubble chamber like Gargamelle as the result of being struck by a neutrino (or its antineutrino counterpart) would be the telltale indication of a leptonic event. A neutrino would connect with a nucleus in the bubble-chamber fluid in a hadronic process, starting a chain reaction of hadrons. At Gargamelle, images of the bubble chamber were captured each time a neutrino pulse passed through and were scrutinized visually, with potentially intriguing events highlighted. The frequency of leptonic events was predicted to be extremely scarce — merely a few events in a year’s worth of data collection. But the frequency of background processes that could imitate this interaction was also incredibly scarce, rendering this the most unequivocal signal of a neutral-current process. Hadronic processes transpired far more frequently, but occurred against a much larger, confounding backdrop.
In December 1972, Gargamelle made a notable stride forward, detecting its inaugural leptonic event. This provided the team with a significant motivation boost to identify hadronic events and calculate the anticipated background rates. Consequently, the first evidence of both kinds of weak neutral current was unveiled during a seminar at CERN on 19 July 1973. The academic articles on each were published concurrently in Physics Letters B’s September volume that year.
This served as the inaugural convincing evidence substantiating the electroweak theory. The subsequent hurdle involved generating the Z boson, along with its charged counterparts the W bosons, directly, as opposed to witnessing them through their impact on other processes. The critical turning point came in 1976, led by the Italian physicist Carlo Rubbia, with the realization that this could be accomplished by modifying a high-energy proton accelerator into a device capable of colliding protons with antiprotons. The first proton–antiproton collisions in CERN’s Super Proton Synchrotron were observed in 1981. A decade following the neutral currents’ detection, the UA1 and UA2 collaborations publicized the discoveries of the W bosons first, followed by the Z boson in 1983. For their respective roles in this discovery, Rubbia and van der Meer jointly received the 1984 Nobel Prize in Physics.
At this juncture, CERN had already made significant progress in constructing the Large Electron–Positron collider (LEP) with a circumference of 27 kilometres. LEP was engineered to conduct precise assessments of W and Z bosons’ properties by colliding electrons with their antiparticles and began its operations in 1989. Gradually, the empirical evidence bolstering the current standard model assembled. These encompassed the disclosure that there could only exist three light (essentially massless) neutrinos — those paired with the electron and the muon, and an additional neutrino yet to be identified, linked to an even more massive lepton, the tau. LEP also confirmed the accuracy of intricate corrections to the electroweak theory, released in 1971, that heavily relied on the mass of the heaviest of the six quarks, the top quark, and also the Higgs boson’s mass. There was an impressive congruity between the top-quark mass projected by LEP and the mass eventually determined by the Tevatron proton–antiproton collider located near Chicago, Illinois, at Fermilab in 1995.
In the end, the Higgs boson was the final elusive component of the standard model. LEP experiments were actively on the lookout for this hard-to-find particle, but it was apparent that an even more potent accelerator would be necessary for its production. LEP activities concluded in 2000, creating room for the installation of a new proton–proton collider, the Large Hadron Collider (LHC), in the same tunnel. The Higgs boson’s discovery was memorably proclaimed in 2012 by two LHC experiments, ATLAS and CMS. Once more, the measured mass was in perfect harmony with predictions.
This is by no stretch the final chapter. Originally, the electroweak theory postulated that neutrinos were devoid of mass, but the occurrence of ‘mixing’, wherein one kind of neutrino morphs into one of the other two types, refuted this assumption. The specifics regarding the mass of neutrinos — which must be incredibly minute, yet not non-existent — and the exact characteristics of these particles continue to elude us. Subsequent generation neutrino-beam experiments scheduled in Japan and the United States aim to delve into these mysteries more extensively. An enhancement of the LHC is also slated to operate until the early 2040s, with the ambitious goal of achieving a tenfold increase in collisions compared to the original model.
Simultaneously, an in-depth examination of the feasibility of a Future Circular Collider is underway at CERN. This would mirror the LEP–LHC paradigm in a tunnel featuring a 90-kilometer circumference: initially, an electron–positron collider would be set up to gauge the Higgs boson and electroweak events with superior accuracy, followed by a hadron collider to delve into high-energy occurrences, inclusive of the generation of two Higgs bosons during the same collision, at a significantly higher frequency than achieved at the LHC. These experiments should clarify whether the simplest standard model’s interpretation of the Higgs boson is accurate, or if its structure is more intricate — for instance, if there exists more than a single variety of Higgs boson, or if it interacts with as yet unidentified particles.
This initiative may also shed light on the essence of invisible cosmic ‘dark matter’, for which substantial evidence has been gathered from astronomical observations. Just as the LEP measurements were attuned to the top quark and Higgs boson, precise assessments at future colliders might unveil the influence of unknown, denser particles. Half a century after Gargamelle established the groundwork for electroweak interactions, thereby paving the way for the standard model, a multi-decade undertaking of rich fundamental science remains on the horizon.