Understanding the Higgs 5: The strong dominate the field

By the mid-1960s, the situation in particle physics was not good. Its aim had been to uncover the basic building blocks of matter, and for a time it had seemed that the end was in sight. In 1917 Ernest Rutherford had split the atom and show that it consisted of a positive nucleus surrounded by negative electrons, and in 1932 John Cockcroft and Ernest Walton, working under his direction, had split the nucleus.

The work of the experimenters had stimulated the mathematical physicists to develop a series of powerful theories. First came quantum theory, then quantum field theory which extended the classical concept of a field to the new quantum world that was opening up. With Paul Dirac one of its main creators, it was elegant and beautiful, with various hints in it of something even deeper.

But then the progress started to decline. As the power of the particle accelerators was stepped up, more and more ‘elementary’ particles began to appear. By the 60s it was becoming like a zoo, with several hundred categorised. And worse than that, the refined landscape of quantum field theory started to show up some potholes. Certain calculations produced terms which went to infinity, the equivalent of the ground blowing up under the investigator’s feet. Stopgap methods were deployed, one being just to subtract out the infinities. It worked, but the feeling of mathematical beauty was being lost.

So particle physicists looked for territory that might yield more promising results, and homed in on the strong interaction. There it was possible to do calculations, as techniques had been developed for working out what would happen when one particle collided with another – a process known as scattering. The techniques involved gathering today much data and putting it in an array of numbers called a matrix, and the theory was known as S-matrix theory, the S representing scattering.

There was something of a tidal flow from quantum field theory to S-matrix theory, and the move involved not only a shift in content but a shift in style. S-matrix theory was very much about nuts and bolts and practicalities. Quantum field theory was about a search for abstract structure.

The work of Murray Gell-Mann and Yuval Ne’eman in applying Lie groups to the strong interactions did for a time start to turn the tide back towards the search for mathematical structure, but that wasn’t for long, as their theory also pointed to something for particle calculations.

They had found that the strongly interacting particles seemed to be grouped in templates of the Lie group SU(3). This group produces patterns of eights and tens. But at its deepest level it is formed – generated – out of just three basic units. In 1964 Gell-Mann suggested (as did another physicist, George Zweig, working independently), that as the upper level were fully associated with known particles, then the very lowest level would also have its counterpart in nature.

The counterpart of this lowest level would be three particles with charges that were in multiples of one-third.

At first, the idea seemed quite hypothetical, since the theory showed that these particles were so tightly bound together that it would be impossible to separate them and give any one of them a separate existence. But them in 1968 the Stanford Linear Accelerator showed that the proton did indeed seem to have much smaller, point-like objects within it. It was a bit like finding a layer of rock so hard that no geologist’s hammer could break a piece off and yet noticing that it had a finely-shaped pattern running over it.

So the new particles had to be taken seriously, and calculations could be made – and there was a whole new territory of work to do.

For a time there was caution about using the name Gell-Mann had given the new particles, and the particle-like signs within the proton were termed partons. But as the case for them strengthened, so too did the name – which was quark.

According to Gell-Mann, he came to the name from the sound first, which was ‘kwork’, but he didn’t have a spelling.

‘Then, in one of my occasional perusals of Finnegans Wake, by James Joyce, I came across the word “quark” in the phrase “Three quarks for Muster Mark”.’

This is the passage that reads:

Three quarks for Muster Mark!
Sure he has not got much of a bark
And sure any he has it’s all beside the mark.

So, explained Gell-Mann, ‘Since “quark” (meaning, for one thing, the cry of the gull) was clearly intended to rhyme with “Mark”, as well as “bark” and other such words, I had to find an excuse to pronounce it as “kwork”. But the book represents the dream of a publican named Humphrey Chimpden Earwicker. Words in the text are typically drawn from several sources at once, like the “portmanteau” words in “Through the Looking-Glass”. From time to time, phrases occur in the book that are partially determined by calls for drinks at the bar. I argued, therefore, that perhaps one of the multiple sources of the cry “Three quarks for Muster Mark” might be “Three quarts for Mister Mark”, in which case the pronunciation “kwork” would not be totally unjustified. In any case, the number three fitted perfectly the way quarks occur in nature.’

Zweig preferred the name ace for the particle, but given the nature of the apparent guess that turned out to be brilliantly correct, three quarks trumped three aces and the terminology of Gell-Mann (and James Joyce) is the one which has stuck.

So with Lie groups to steer them and quarks to crunch together, the strong interactions dominated the early 1960s.

Few people continued to work on the weak interactions, where progress had at one time seemed so promising.

The weak interaction and the electromagnetic interaction had been shown to have features in common, a kind of overall symmetry, which might be the hallmark of a more overarching unity, in which they each would become an aspect of a single electroweak interaction.

Some of the differences between them could be accounted for by taking account of a further symmetry, gauge symmetry. This could be applied to a model of an electroweak interaction to break it down into the two types. The approach was developed by Yang, Mills and Shaw, and applied by Sheldon Glashow – who found that it produced two particles. One of these was a familiar one, the photon, a particle-like burst of energy that is formed with the interaction of light with matter. The other one was something new, a kind of ‘weak interaction photon’.

And that is where the snag came in that stopped any further progress for several years. The photon has zero mass. This is because, as we noted some time back, mass increases with speed, so that a mass-possessing particle would go toward infinite mass as it approached the speed of light. So a particle can only travel at light-speed if it has a zero mass to start with.

The masslessness of the photon is also connected with the distance which it can travel – which is infinite. As we know, if nothing gets in its way, light can continue on its way for ever, which is how astronomers can observe the light from stars so far away that it takes billions of years for the light to cover the distance.

But the weak interaction is quite the opposite. It also takes place between particles that are exceptionally close together. Indeed, it is so incredibly localised that the distance over which it can act is no more than a fraction of the diameter of a proton.

So if the weak force is to somehow involve the appearance of some kind of particulate energy-burst, that particle has to be born and die within this incredible short distance. This means that on the one hand its lifetime has to be incredibly brief, and on the other that it must have a substantial mass, in order to carry sufficient energy with that ultra-short life.

So a theory that proposes a zero-mass photon-like weak particle is simply flying in the face of the facts.

It became even worse than that when the British-born physicist Jeffrey Goldstone showed mathematically that it was not possible to get mass into the weak particles out of this approach. The hope had been that you could start with the symmetry and break it a little to get particles with mass. But Goldstone showed that mass would only come if you broke it completely – and thereby defeated the purpose of starting with symmetry in the first place.

The challenge looked hopeless – which is why Glashow’s work was put on the shelf and sat there, while the flow of the tide was to the strong rather than the weak.

It was the solution of this challenge that was the genesis of the Higgs particle.

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