Higgs10: A boson is born
By: Matthew Chalmers
28 April, 2022 · Voir en français
In the first part of the Higgs10 series, we look at the how the Higgs boson came to be
On 4 July 2012, half a century’s wait came to an end as the ATLAS and CMS experiments announced the discovery of the Higgs boson. Celebrate 10 years since this extraordinary achievement by learning more about the history that led up to it, the next steps in understanding the mysterious particle, and CERN’s role in this endeavour. The “Higgs10” series will walk you through this journey, starting with an account by CERN Courier editor, Matthew Chalmers, of the theorisation of the Higgs boson in the 1960s.
It’s every theoretical physicist’s dream to conjure a new particle from mathematics and have it observed by an experiment. Few have scaled such heights, let alone had a particle named after them. In the CERN auditorium on 4 July 2012, Peter Higgs wiped a tear from his eye when the ATLAS and CMS results came in. The Higgs boson holds the record (48 years) among elementary particles for the time between prediction and discovery, going from an esoteric technicality to commanding the global spotlight at the world’s most powerful collider.
Revealing that the universe is pervaded by a stark “scalar” field responsible for generating the masses of elementary particles was never something Robert Brout and François Englert, and independently Peter Higgs, set out to do. Their short 1964 papers – one by Brout and Englert, two others by Higgs – concerned an important but niche problem of the day. “Of no obvious relevance to physics” was how an editor of Physics Letters is said to have remarked on rejecting one of Higgs’ manuscripts. The papers went from fewer than 50 citations by the turn of the decade to around 18 000 today.
At the time the “BEH” mechanism was being dreamt up independently in Brussels and Edinburgh – and in London by Gerald Guralnik, Carl Hagen and Tom Kibble – the Standard Model of particle physics was years away. Physicists were still trying to understand the menagerie of hadrons seen in cosmic-ray and early accelerator experiments, and the nature of the weak force. The success of quantum electrodynamics (QED) in describing electromagnetism drove theorists to seek similar “gauge-invariant” quantum field theories to describe the weak and strong interactions. But the equations ran into a problem: how to make the carriers of these short-range forces massive, and keep the photon of electromagnetism massless, without spoiling the all-important gauge symmetry underpinning QED.
It took a phenomenon called spontaneous symmetry breaking, inherent in superconductivity and superfluidity, to break the impasse. In 1960, Yoichiro Nambu showed how the “BCS” theory of superconductivity developed three years earlier by John Bardeen, Leon Cooper and John R. Schrieffer could be used to create masses for elementary particles, and Jeffrey Goldstone brought elementary scalar fields to the party, picturing the vacuum of the universe as a “Mexican hat” in which the lowest-energy state is not at the most symmetrical point at the peak of the hat but in its rim. It was an abstraction too far for soon-to-be CERN Director-General Viki Weisskopf, who is said by Brout to have quipped: “Particle physicists are so desperate these days that they have to borrow from the new things coming up in many-body theory like BCS. Perhaps something will come of it.”
Four years later, Brout, Englert and Higgs added the final piece of the puzzle by showing that a mathematical block called the Goldstone theorem, which had beset initial applications of spontaneous symmetry breaking to particle physics by implying the existence of unobserved massless particles, does not apply to gauge theories such as QED. Unaware that others were on the trail, Higgs sent a short paper on the idea to Physics Letters in July 1964 where it was accepted by Jacques Prentki, the editor based at CERN. In a second paper sent one week later, Higgs demonstrated the mathematics – but it was rejected. Shocked, Higgs sent it to Physical Review Letters, and added crucial material, in particular : “it is worth noting that an essential feature of this type of theory is the prediction of incomplete multiplets of scalar and vector bosons” – a reference to the Higgs boson that was almost never published. In a further twist of fate, Higgs’ second paper was received and accepted the same day (31 August 1964) that Physical Review Letters published Brout and Englert’s similarly titled work. Today, the scalar field that switched on a fraction of a nanosecond after the Big Bang, giving the universe a non-zero “vacuum expectation value”, is generally referred to as the BEH field, while the particle representing the quantum excitation of this field is popularly known as the Higgs boson.
In further Nobel-calibre feats, Steven Weinberg incorporated the BEH mechanism into electroweak theory developed also by Abdus Salam and Sheldon Glashow, which predicted the W and Z bosons, and Gerard ‘t Hooft and Martinus Veltman put the unified theory on solid mathematical foundations. The discovery of neutral currents in 1973 in Gargamelle at CERN and of the charm quark at Brookhaven and SLAC in 1974 gave rise to the electroweak Standard Model. Flushing out and measuring its bosons took three major projects at CERN spanning three decades – the SPS proton-antiproton collider, LEP and the LHC. In the mid-1970s, John Ellis, Mary Gaillard and Dimitri Nanopoulos described how the Higgs boson might reveal itself, and experimentalists accepted the challenge.
The discovery of the Higgs boson at the LHC in 2012 ended one journey, but opened another fascinating adventure. Understanding this unique particle will take every last drop of LHC data, in addition to that of a “Higgs factory” that may follow. Is it elementary or composite? Is it alone, or does it have siblings? And what are the roles of the mysterious BEH field in the beginning and the fate of the universe?
“We’ve scratched the surface,” said Peter Higgs in 2019. “But we have clearly much more to discover.”