Higgs Boson Discovery (2012) at CERN's Large Hadron Collider

The Higgs boson discovery was announced on 4 July 2012 at CERN, the European particle physics laboratory near Geneva, where two huge experiments at the Large Hadron Collider reported finding a new particle with a mass of about 125 GeV. It was the particle predicted in 1964 to explain how fundamental particles get their mass, and the last missing piece of the Standard Model of particle physics. The finding brought the Nobel Prize in Physics in 2013 to François Englert and Peter Higgs.

Background: the mass problem

By the early 1960s physicists had theories describing forces through the exchange of particles, building on the quantum ideas that began with Max Planck and the study of the atom since Rutherford. But elegant theories of the weak nuclear force seemed to require that its carrier particles have no mass, while experiments showed the force acts over very short distances, implying heavy carriers.

In 1964 three groups independently proposed a solution. Robert Brout and François Englert in Brussels, Peter Higgs in Edinburgh, and Gerald Guralnik, Carl Hagen and Tom Kibble in London described how a field filling all space could give particles mass. Higgs explicitly pointed out that the theory predicted a new massive particle, which became known as the Higgs boson.

The idea is often explained by analogy. The Higgs field is pictured as something like a medium filling the universe: particles that interact with it strongly, such as the W and Z bosons that carry the weak force, behave as heavy particles, while those that do not interact with it at all, such as the photon, remain massless. A Higgs boson is a ripple in the field itself, and detecting one was the clearest way to prove that the field exists.

In 1967-68 Steven Weinberg and Abdus Salam built the mechanism into a unified theory of electromagnetic and weak forces. That theory, part of what became the Standard Model, was strongly confirmed in the 1970s and 1980s, but the Higgs boson itself remained unseen.

The long search

Because the theory did not predict the boson’s mass, experimenters had to search across a wide range. CERN’s Large Electron–Positron Collider (LEP) ruled out masses below about 114 GeV before it closed in 2000. Fermilab’s Tevatron near Chicago narrowed the possibilities further before it shut down in 2011.

The Large Hadron Collider (LHC) was built in the 27-kilometre tunnel that had housed LEP, about 100 metres underground. Its first beams circulated on 10 September 2008, but nine days later a faulty electrical connection caused a helium leak that damaged dozens of superconducting magnets. After repairs, it restarted in November 2009 and began high-energy collisions in March 2010.

The discovery

The LHC smashes protons together at nearly the speed of light. Only a tiny fraction of collisions produce a Higgs boson, which decays almost instantly, so physicists looked for its decay products, such as pairs of photons or four leptons, standing out above a large background.

Two independent general-purpose detectors, ATLAS and CMS, each run by collaborations of around 3,000 scientists, searched separately so that each could check the other. Hints of an excess near 125 GeV appeared in late 2011. To avoid fooling themselves, both teams analysed much of their data “blind”, fixing their methods before looking at the region where a signal might appear. By mid-2012 the LHC was colliding protons at 8 TeV, and the extra data from that year strengthened the signal in both experiments.

At a seminar at CERN on 4 July 2012, ATLAS spokesperson Fabiola Gianotti and CMS spokesperson Joe Incandela each reported a new particle at a mass of about 125-126 GeV, with a statistical significance of about five sigma, the standard required to claim a discovery. Five sigma means that, if no new particle existed, random fluctuations in the background would produce a signal at least that strong only about once in 3.5 million times. Peter Higgs and François Englert were in the audience, and Higgs was seen wiping away tears as the results were presented. CERN’s Director-General, Rolf-Dieter Heuer, summed up: “As a layman I would now say, I think we have it.”

The two papers were published in Physics Letters B in September 2012. Further data showed that the new particle had zero spin and behaved as the Standard Model predicted, and in March 2013 CERN said it was looking more and more like a Higgs boson.

By the numbers

After the discovery

The LHC was upgraded and resumed operation in 2015 at a higher collision energy of 13 TeV, producing many more Higgs bosons. In 2018 ATLAS and CMS observed the Higgs boson interacting with the heaviest known particle, the top quark, and decaying into bottom quarks, its most common decay. So far, every measurement has matched the Standard Model within the uncertainties.

Physicists hope that more precise measurements will reveal small differences pointing to new physics, for example linked to dark matter or to why the Higgs mass is so much smaller than some theories suggest. A High-Luminosity upgrade of the LHC, intended to increase the number of collisions about tenfold, is planned to run from around 2030, and proposals for even larger colliders are under discussion.

Key people

Peter Higgs (1929-2024), a modest theorist at the University of Edinburgh, became the public face of the discovery. François Englert, a Belgian physicist, shared the Nobel Prize with him; their colleague Robert Brout died in 2011, before the discovery. Guralnik, Hagen and Kibble, who also published in 1964, were widely honoured but not included in the Nobel award, which is limited to three people.

Thousands of experimental physicists, engineers and technicians worked on the LHC and its detectors. Gianotti later became CERN’s first female Director-General in 2016.

Myths and facts

The nickname “God particle” comes from the title of Leon Lederman’s 1993 popular science book and has no religious meaning; physicists generally avoid it. The Higgs boson also does not give all mass to the universe: most of the mass of ordinary matter comes from the energy binding quarks inside protons and neutrons, not directly from the Higgs field.

Timeline

• 1964, August to November: Brout and Englert, Higgs, and Guralnik, Hagen and Kibble publish their papers.
• 1967-1968: Weinberg and Salam build the mechanism into the electroweak theory.
• 2000, November: LEP shuts down without a confirmed discovery.
• 2008, 10 September: The first beams circulate in the LHC.
• 2008, 19 September: A magnet fault halts the LHC for over a year.
• 2010, 30 March: The LHC begins high-energy collisions.
• 2012, 4 July: ATLAS and CMS announce a new boson at about 125 GeV.
• 2013, 8 October: Englert and Higgs are awarded the Nobel Prize in Physics.

Why it matters

The Higgs boson discovery confirmed a 48-year-old prediction about why matter has mass, completing the Standard Model. It showed what large international collaborations could achieve at CERN, the laboratory where the World Wide Web was also born, and it opened precise studies of the Higgs field that may reveal physics beyond the Standard Model.