Understanding the Higgs 7: Three roads converge

By 1964 the challenge was clear. The familiar phenomena of electricity and magnetism looked as if they could be part of something more comprehensive, an electroweak interaction, with the electromagnetic field one of its aspects and the weak interaction another.

The reasoning came from symmetry. When the electroweak field was looked at from one perspective it appeared as electromagnetism. When viewed from another perspective, it appeared as the weak interaction which causes radioactivity.

But because the weak interaction in the world is very different from the electromagnetic one, the symmetry could not be complete. So somehow the symmetry had to be broken.

Can we somehow keep symmetry and yet break it? The answer at first might seem to be no, if we think of symmetry as something like a vase which we break when we drop it. But we don’t have to go that far in the concept of breaking things. We could think of the symmetry of a circle or a five-pointed star and allow it to break a little – and get a rose. The original symmetry is still there, but with edges that are somehow ‘softer’.

The problem was that Goldstone’s theorem seemed to force the choice to be between a perfect vase or a smashed one, ie no vase at all. But Philip Anderson’s 1963 paper had suggested that in superconductivity a similar problem was in fact resolved by a process of condensation. Particles in a crystal had their behaviour shaped by the lattice of the crystal.

That suggestion from Anderson was enough to encourage physicists at three centres to take up the challenge anew – and each independently to come up with a solution.

Breaking it gently

The way to resolve the problem is to invoke a new field which fills all of space – today called the Higgs field. This field obeys the rules of symmetry, just as does the existing field of the electroweak theory. But it somehow condenses into a single state – in the same way as the theory of a bar magnet tells us that the jumbled-up magnetic domains drop down into place in a single direction which locks in. Or in the way in which the electrons in a superconductor lock in to a single coordinated structure.

So the field itself is symmetric – but the fact that it condenses into a single fixed state breaks the symmetry.

And it is this field that provides the mass. It links with the Yang-Mills field (the field which has to exist to maintain gauge symmetry) and the particles of the Yang-Mills field acquire mass as a result. The outcome is not only the photon for electromagnetism but no less than four other particles. Three of these are concentrations of the weak interaction, now given mass by the condensing of the Higgs field. And there is a fourth, a kind of by-product of the process, something new – the particle which has become known as the Higgs.

In Brussels

The first to publish were at the Free University of Brussels – Francois Englert and Robert Brout.

Englert’s first degree had been in engineering, but he switched to physics and went to work at Cornell as Brout’s research assistant. They found that their approaches to physics were so similar that when the time came for Englert to go back to Belgium, Brout decided to move there with his family, and went on to acquire Belgian citizenship and to play a big part in the further development of physics in the country, up till his death last year.

One of the features that Englert and Brout shared was a readiness to look afresh at any problem, and that, says Englert today, is how they made their breakthrough with what has come to be known as the Higgs particle.

‘The generality of our results is largely attributable to the use of quantum field theory, which at the time was largely ignored in elementary-particle physics. Its use in deriving the mechanism was no accident. Driven by his unusual faculty to translate abstract concepts into tangible intuitive images, Robert always conspicuously disregarded academic knowledge and favored entering any subject from scratch. For him, the fact that he was no expert on particle physics was an advantage: He could easily free himself from fashionable trends in the quest for a consistent theory of short-range fundamental forces.’

At Imperial

Imperial College London was the base of Tom Kibble, who had taken his physics degree and doctorate at Edinburgh. He was of Scots descent and born in Madras in India, where his grandfather had been a medical officer and his grandmother, Helen Bannerman, had become a well-known writer of children’s books. Working with him at Imperial were two visiting Americans, Gerry Guralnik and Dick Hagen, who had both attended lecture courses by Julian Schwinger during their earlier studies.

The paper written by the three was just being put in the post for publication when the papers by Englert and Brout and by Peter Higgs arrived.

In Edinburgh

Meanwhile at Edinburgh Peter Higgs had been following the arguments about Anderson’s work on superconductivity. In particularly, he noted a criticism made by Wally Gilbert, a remarkable man who started out as a physicist but switched to biology after meeting the James Watson (of double helix fame) and went on to win the Nobel Prize for his work in sequencing DNA. Today in retirement, he is an artist-photographer whose work is exhibited in galleries across the US, with the images including an Orkney series.

In 1964 Gilbert had argued that there was a fault in the argument that symmetry-breaking could lead to mass. Peter Higgs thought at first that this was indeed the end of any hope of a way forward, but then realised that a method developed by Julian Schwinger could be applied to show Gilbert was wrong. He sent a letter accordingly to the journal Physics Letters which is published at CERN, the particle accelerator centre near Geneva.

He then worked on a second paper, to put together the actual way in which the symmetry-breaking occurs, using the Higgs field. He sent off this paper to Physics Letters as well – only to have it returned as not suitable for publication. It turned out afterwards that the editor of the journal had thought the paper ‘not to have any relevance to particle physics’.

‘I was rather shocked,’ he says. ‘I did not see why they would accept a paper that said this is a possible way to evade the Goldstone theorem, and then reject a paper that showed how you actually do it.’

So during August he revised the paper and with the encouragement of one of his colleagues, Euan Squires, added on a paragraph that drew attention to the fact that the theory had practical experimental consequences – that it predicted a new particle which could be looked for. This emphasis was one of the factors that associated Peter Higgs’s name with the search for the particle.

He sent the paper to the editor of another journal, Physical Review Letters, who confirmed that he would accept it and mentioned the paper by Englert and Brout which he was also about to publish.

The Edinburgh connection

Peter Higgs had come to Edinburgh four years before to take up a lecturing post in the Tait Institute of Mathematical Physics. Its graduates include Sir David Wallace (seen above on the left with Peter Higgs) who subsequently became President of the Institute of Physics and Master of Churchill College, Cambridge, and Dennis Canavan, who went on to become first a teacher and then one of Scotland’s most highly regarded political figures.

The Institute took its name from James Clerk Maxwell’s friend and colleague, Peter Guthrie Tait, and was housed in Roxburgh Street, just across the road from the Physics Department.

The Tait Chair in Natural Philosophy had been established in 1922, with the aim of developing the study of mathematical physics, and its first incumbent was Sir Charles G. Darwin, the physicist grandson of the great naturalist. After him came Max Born, one of the founding fathers of quantum theory, who won the Nobel Prize for the probabilistic interpretation.

Born had begun his career at Göttingen as assistant to the mathematician David Hilbert and his own assistants included Werner Heisenberg, Wolfgang Pauli and Enrico Fermi.

In 1953 Born was succeeded by one of his former students, Nicholas Kemmer, who established the Tait Institute itself. Kemmer, born in St Petersburg, had been educated in Germany when his family moved there after the Russian Revolution. He had worked in Zurich as Pauli’s assistant, then on wartime atomic energy research, and after the war at Cambridge. His own research had developed the use of symmetry for protons, neutrons and mesons, and was a first stage in the developments that would grow into a core concept in particle physics today. At Cambridge, he gave priority to his teaching work, for which he was in much demand, with his students including Ron Shaw and Freeman Dyson, who speaks of the high quality of Kemmer’s teaching of quantum field theory.

And indeed it was another one of Kemmer’s Cambridge research students – Abdus Salam – who along with Steven Weinberg took the step of using the Higgs approach to solve the problem of electroweak theory.

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