Subject Grouping: Praxis

Peter Higgs, an iconic figure in modern science who in 1964 postulated the existence of the eponymous Higgs boson, passed away on 8 April 2024 at the age of 94.

Peter Higgs was born in Newcastle upon Tyne in the UK on 29 May 1929. His family moved around when he was young and he suffered from childhood asthma, so he was often taught at home. However, from 1941 to 1946 he attended Cotham Grammar School in Bristol, one of whose alumni was Paul Dirac. He went on to study physics at King’s College London, where he got his bachelor’s degree in 1950 and his PhD for research in molecular physics in 1954. After periods at the University of Edinburgh, Imperial College and University College London, in 1960 he settled at the University of Edinburgh where he remained for the rest of his career.

Seeds of success

Following his PhD, Higgs’s research interests shifted to field theory, with a first paper on vacuum expectation values of fields in 1956, followed by a couple of papers on general relativity. Then, in 1964, came his two famous papers introducing spontaneous gauge symmetry breaking into relativistic quantum field theory and showing how a vector boson could acquire a mass in a consistent manner – as long as it was accompanied by a massive scalar boson.

Related ideas had been discussed previously by Phillip Anderson and Yoichiro Nambu in the context of non-relativistic condensed-matter physics, namely in models of superconductivity, where a condensate of electron pairs enables a photon to acquire an effective mass. Anderson conjectured that a similar mechanism should be possible in a relativistic theory, but he did not develop the idea. On the other hand, Nambu used spontaneous symmetry breaking to describe the properties of the pion, but also did not discuss the extension to a relativistic vector boson.

In early 1964 Walter Gilbert (later a winner of the Nobel Prize in Chemistry) wrote a paper arguing that Anderson and Nambu’s ideas for generating mass for a vector boson could not work in a relativistic theory. This was Higgs’s cue: a few weeks later he wrote a first paper pointing out a potential loophole in Gilbert’s argument (though not a specific model). He sent his paper to the journal Physics Letters, which quickly accepted it for publication. A few days later, he wrote a second paper, which contained an explicit model for mass generation, but was taken aback when the same journal rejected this paper as not being of practical interest. Undeterred, Higgs tweaked his paper to make his message more explicit, and submitted it to Physical Review Letters, where it was accepted.

Unknown to Higgs, François Englert and Robert Brout had already sent a paper describing a similar model to the same journal, where it was published ahead of Higgs’s paper. Both papers postulated a scalar field with a non-zero vacuum expectation value that gave mass to a vector boson. However, there was a key difference: Higgs pointed out explicitly that his model predicted the existence of a massive scalar boson, whereas this was not mentioned in the Englert–Brout paper. For this reason, the particle he predicted became known as the Higgs boson. Shortly after the publication of the Higgs and Englert–Brout papers, Gerry Guralnik, Carl Hagen and Tom Kibble published an article referring to their papers and filling in some aspects of the theory, but also not mentioning the existence of the massive scalar boson.

In 1965 Higgs went for a sabbatical to the University of North Carolina, where he continued working on his theory. Remarkably prescient, he wrote a third paper discussing how his boson could decay into a pair of massive gauge bosons as well as calculating associated scattering processes. However, he encountered scepticism about the validity of his theory, and neither his nor the other pioneering mass-generation papers garnered significant attention for several years.

This started to change in 1967 and 1968 when Steven Weinberg and Abdus Salam incorporated the mass-generation mechanism into their formulation of the electroweak sector of the Standard Model. But interest only really took off a few years later, after Gerard ’t Hooft and Martinus Veltman showed that spontaneously broken gauge theories are renormalisable and hence could be used to make accurate and reliable predictions for comparison with experiment, and when neutral weak interactions were discovered in the Gargamelle bubble chamber at CERN in 1973.

The search begins

During the 1970s interest in the experimental community moved towards searches for the massive intermediate vector bosons, the W and Z. However, it seemed to Mary Gaillard, Dimitri Nanopoulos and myself that the key long-term target should be the Higgs boson, the capstone of the structure of the Standard Model, and in 1975 we wrote a paper describing its phenomenology. At the time the existence of the Higgs boson was still regarded with some scepticism, and we ended our paper by writing that “We do not want to encourage big experimental searches for the Higgs boson, but we do feel that people performing experiments vulnerable to the Higgs boson should know how it may turn up.” I met Peter Higgs for the first time around 1980, and he was clearly flattered by our interest in his boson, but unprepared for the subsequent interest in the big experimental searches that followed.

Higgs pointed out explicitly that his model predicted the existence of a massive scalar boson

Searching for the Higgs boson moved to the top of the agenda following the discovery of the W and Z at CERN in 1983, when the Superconducting Super Collider project was launched in the US, followed by the first LHC workshop in 1984. Being a profoundly modest man, Higgs followed these developments from a distance as a somewhat bemused spectator. In the 1990s, precision experiments at LEP and elsewhere confirmed predictions of the Standard Model with high accuracy – if and only if the Higgs boson (or something very like it) was included in the theoretical calculations. Higgs became quietly confident in the reality of his boson. By the time the LHC started accumulating collisions at an energy of 7 and 8 TeV, anticipation of its possible discovery was growing.

Following early hints at the end of 2011, the word went around that on 4 July 2012 the ATLAS and CMS experiments would give a joint seminar presenting their latest results. I was tasked with locating Higgs and persuading him that he might find the results interesting. Somewhat reluctantly, he decided to come to CERN for the seminar, and he had no cause to regret it. He wiped tears from his eyes when the discovery of a new particle resembling his boson was announced, and confessed that he had never expected to see it in his lifetime.

Famously, in October 2013 as the Nobel Prize was being announced, Higgs went missing, in order to avoid being thronged by the media. Some months previously, in a pub in Edinburgh, he had told me that the existence of the Higgs boson was not a “big deal”, but I assured him that it was. Without his theory, electrons would fly away from nuclei at the speed of light and atoms would not exist, and radioactivity would be a force as strong as electricity and magnetism. His prediction of the existence of the particle that bears his name was a deep insight, and its discovery was the crowning moment that confirmed his understanding of the way the universe works.

Peter Higgs is survived by his two sons, a daughter-in-law and two grandchildren.

Experimental physicist Giuseppe Fidecaro, who joined CERN in 1956 and continued there long into his retirement, passed away on 28 March.

Born in Messina, Italy in 1926, Giuseppe studied physics at the University of Rome, graduating in 1947 under the supervision of Edoardo Amaldi. Amaldi had become interested in cosmic rays and asked young “Pippo” to help him build a large detector to study the scattering of mesons on an iron target to explore the nuclear force. Between 1952 and 1954, Giuseppe continued to work on cosmic rays at the Tête Grise laboratory, 3500 m above Cervinia, where Maria Cervasi, whom he had met during his studies at Rome, also worked.

In 1953 Amaldi, who had become secretary general of the provisional CERN, suggested that Giuseppe spend time at the University of Liverpool to learn from the new synchrocyclotron being built there. He went to CERN with Maria, by then his wife, in 1956 and began preparing experiments for the 600 MeV Synchrocyclotron (SC), which came into operation in August 1957.

In January 1958, during a conference in New York, Giuseppe attended a presentation by Feynman describing the universal “V–A” theory of weak interactions. He heard that the theory lacked a key experimental ingredient: the decay of a pion into an electron and neutrino, predicted to occur 10 000 times less frequently than to a muon and a neutrino, which had not been observed in two experiments performed by well-known physicists. Upon his return to CERN, Pippo decided with the other members of the SC group that this would be the target of the next experiment. A device was immediately designed and built, and 40 events in perfect agreement with the V–A prediction were presented by Pippo in September 1958. The news put the newly born CERN on the map of the world of particle physics and laid the groundwork for the future discoveries of neutral currents, the W and Z bosons, and the Higgs field.

In 1960, with the start-up of the PS, Giuseppe led his group to measure – using a system of precision scintillators – the antiproton–proton cross section. The following year, he became professor at the University of Trieste and established a group that carried out a series of important scattering measurements at the PS and the SPS, in particular using polarised targets, during the 1970s. Following the proposal and execution of an experiment at the ILL in Grenoble searching for possible neutron–antineutron oscillations, in 1990 he presented an article “Fixed target B-physics at the Large Hadron Collider” at the LHC workshop in Aachen, which proposed, among other things, the use of a very intense proton beam extracted from the accelerator with a crystal, similar to what had been envisaged for the Superconducting Super Collider. This, and discussions with Giovanni Carboni and Walter Scandale, were at the origin of the RD22 collaboration, which for the first time proved the possibility of high-efficiency proton extraction from an accelerator using a bent crystal – a technique that is now used in LHC beam collimation.

Outside physics, Giuseppe made numerous contributions to CERN. In the early 1960s he was a member of the founding committee of the International Center for Theoretical Physics. In 1975 he was appointed as co-chair of a joint scientific committee set up under a collaboration agreement between CERN and the former USSR concerning the use of atomic energy, a responsibility he held until 1986. He was also tasked with coordinating cooperation with JINR in Dubna.

Giuseppe officially retired in 1991 but, together with Maria, continued his work at CERN as an honorary member of the personnel until as recently as 2020, during which time he devoted himself to research in the history of physics. He produced reports of rare beauty and precision, notably three well-documented articles on the contributions of Bruno Pontecorvo, whose friend he became in Dubna in 1989. Giuseppe was also known to CERN visitors, featuring prominently in the film shown in the Synchrocyclotron exhibition. Maria Fidecaro, with whom his rich human and scientific journey was deeply entwined, passed away in September 2023.