Higgs:The invention and discovery of the 'God Particle'

by Higgs- The Invention

  The particle curved more slowly than an electron, and more sharply than a proton of similar speed would curve (in the opposite direction). There was no alternative but to conclude that this was a new ‘heavy’ electron, with a mass about 200 times that of an ordinary electron. It could not be a proton, as the proton mass is about 2000 times that of an electron.*

  The new particle was initially called the mesotron, subsequently shortened to meson. It was an unwelcome discovery. A heavy version of the electron? This did not fit with any theories or preconceptions of how the fundamental building blocks of nature should be organized.

  Incensed, Galician-born American physicist Isidor Rabi demanded to know: ‘Who ordered that?’2 Willis Lamb echoed this sense of frustration in his 1955 Nobel lecture when he said: ‘…the finder of a new elementary particle used to be rewarded by a Nobel Prize, but such a discovery now ought to be punished by a $10,000 fine.’3

  In 1947 another new particle was discovered in cosmic rays atop the Pic du Midi in the French Pyrenees by Bristol University physicist Cecil Powell and his team. The new particle was found to have a slightly larger mass than the meson; 273 times that of the electron. It came in positive, negative, and, subsequently, neutral varieties.

  The physicists were now running into trouble with names. The meson was renamed the mu-meson, subsequently shortened to muon.* The new particle was called the pi-meson (pion). As techniques for detecting particles produced by cosmic rays became more sophisticated, the floodgates opened. The pion was quickly followed by the positive and negative K-meson (kaon) and the neutral lambda particle. New names proliferated. Responding to a question from one young physicist, Fermi remarked: ‘Young man, if I could remember the names of these particles, I would have been a botanist.’4

  The kaons and the lambda behaved rather oddly. These particles were produced in abundance, a signature of strong-force interactions. They were often produced in pairs which formed characteristic ‘V’-shaped tracks. They would then travel on through the detector before disintegrating. Their disintegration took a lot longer than their production, suggesting that although they were being produced by the strong force, their decay modes were governed by a much weaker force, the same force, in fact, that governs radioactive beta-decay.

  Isospin could not help to explain the strange behaviour of the kaons and the lambda. It seemed as if these new particles possessed some additional, hitherto unknown, property.

  American physicist Murray Gell-Mann was puzzled. He realized that he could account for the behaviour of these new particles using isospin provided he assumed that the isospins were for some reason ‘shifted’ by one unit. This made no sense physically, so he proposed a new property, which he subsequently called strangeness, to account for this shift.* He later immortalized the term with the words of Francis Bacon: ‘There is no excellent beauty that hath not some strangeness in the proportion.’5

  Gell-Mann argued that, whatever it is, strangeness is, like isospin, conserved in strong-force interactions. In a strong-force interaction involving an ‘ordinary’ (i.e. non-strange) particle, production of a strange particle with a strangeness value of +1 had to be accompanied by another strange particle with a strangeness value of –1, so that the total strangeness was conserved. This was why the particles tended to be produced in pairs.

  Conservation of strangeness also explained why the strange particles took so long to decay. Once formed, transformation of each strange particle back into ordinary particles was not possible through strong-force interactions, which could be expected to happen quickly, because this would require a change in strangeness (from +1 or –1 to zero). The strange particles therefore hung around long enough to succumb to the weak force, which does not respect the conservation of strangeness.

  Nobody knew why.

  ____________

  In his landmark paper on beta-radioactivity, Fermi had drawn an analogy between the weak force and electromagnetism. He had made his estimates of the relative strengths of the forces involved in the interaction using the mass of the electron as a yardstick. In 1941, Julian Schwinger had wondered what the consequences would be if he assumed that the weak force is carried by a much, much larger particle. He had estimated that if this field particle was actually a couple of hundred times the proton mass, then the strengths of weak-force and electromagnetic interactions might actually be the same. This was the first hint that it might be possible to unify the weak and electromagnetic forces into a single, ‘electro-weak’ force.

  Yang and Mills had discovered that, to take account of all the different ways that neutrons and protons can interact in the nucleus, they needed three different kinds of force particles. In 1957, Schwinger came to much the same conclusion regarding weak-force interactions. He published an article in which he speculated that the weak force is carried by three field particles. Two of these particles, the W+ and W– (in modern parlance) are necessary to account for the transmission of electric charge in weak interactions. A third, neutral particle is needed to account for instances in which no charge is transferred. Schwinger believed that this third particle was the photon.

  According to Schwinger’s scheme beta-radioactivity would now work like this. A neutron would decay, emitting a massive W− particle and turning into a proton. The short-lived W− particle would in its turn decay into a high-speed electron (the beta-particle) and an anti-neutrino (see Figure 8).

  FIGURE 8 The mechanism of nuclear beta-decay could now be explained in terms of the decay of a neutron (n) into a proton (p), with the emission of a virtual W− particle. The W− particle goes on to decay into an electron (e−) and an anti-neutrino

  Schwinger asked one of his Harvard graduate students to work on the problem.

  Sheldon Glashow was an American-born son of Russian Jewish immigrants. He graduated from the Bronx High School of Science in 1950 together with his classmate Steven Weinberg. He had gone with Weinberg to Cornell University, securing his bachelor’s degree in 1954, before moving on to become one of Schwinger’s graduate students at Harvard.

  The heavy W particles that Schwinger had hypothesized were obliged to carry electric charge. Glashow soon realized that this simple fact meant that it was actually impossible to separate the theory of the weak force from that of electromagnetism. ‘We should care to suggest,’ he wrote in an appendix to his PhD thesis, ‘that a fully acceptable theory of these interactions may only be achieved if they are treated together…’6

  Glashow now reached for the same SU(2) quantum field theory that Yang and Mills had developed, taking on faith Schwinger’s assertion that the three field particles of the weak force were the two heavy W particles and the photon. For a time he believed he had succeeded in developing a unified theory of the weak and electromagnetic forces. What’s more, he believed that his theory was renormalizable.

  But in truth he had made a series of errors. When these were revealed, he realized that the theory was demanding too much from the photon. His solution was to enlarge the symmetry by combining the Yang–Mills SU(2) gauge field with the U(1) gauge field of electromagnetism, in a product written SU(2) × U(1). This represents more of a ‘mixture’ of weak and electromagnetic forces rather than a fully unified electro-weak force, but it had the advantage that it freed the photon from its burden of responsibility for aspects of weak-force interactions.

  The theory still demanded a neutral carrier for the weak force. Glashow now had three massive weak-force particles equivalent to the triplet of B particles first introduced by Yang and Mills. These were the W+, W–, and Z0.*

  In March 1960 Glashow lectured in Paris. Here he encountered Gell-Mann, on sabbatical leave from the California Institute of Technology (Caltech) and working as a visiting professor at the Collège de France. Glashow described his SU(2) × U(1) theory over lunch. Gell-Mann offered encouragement. ‘What you’re doing is good,’ Gell-Mann told him, ‘But people will be very stupid about it.’7

  Stupid or not, the physics community was la
rgely unimpressed with Glashow’s theory. Just as Yang and Mills had discovered, the SU(2)×U(1) field theory predicted that the carriers of the weak force should be massless, like the photon. Inserting the masses ‘by hand’ into the equations would ensure that the theory remained unrenormalizable. Like Yang and Mills before him, Glashow could not figure out how the field particles were supposed to acquire their mass.

  There was more trouble. Elementary-particle interactions involve one or more particles decaying or reacting together to produce new particles. When such interactions involve charged intermediaries their reactions are referred to as charged ‘currents’, as they involve the ‘flow’ of charge from the starting to the finishing particles. It was anticipated that a neutral weak-force carrier – the Z0 – would manifest itself experimentally in the form of interactions involving no change in charge, called ‘neutral currents’. No evidence for any such currents could be found in the strange-particle decays, which by now had become the particle physicists’ principal hunting ground for data on weak-force interactions.

  Glashow waved his arms. He argued that the Z0 was simply so much more massive than the charged W particles that interactions involving the Z0 were out of reach of contemporary experiments. The experimentalists were not impressed.

  ____________

  Murray Gell-Mann had been born in New York in 1929. A child prodigy, he entered Yale University to study for a bachelor’s degree when he was just fifteen. He secured his doctorate at the Massachusetts Institute of Technology (MIT) in 1951, aged just 21. He worked for a short time at the Institute for Advanced Study in Princeton before moving first to the University of Illinois at Urbana-Champaign, then Columbia University in New York, then the University of Chicago where he worked with Fermi and puzzled over the properties of the strange particles.

  In 1955 he took a professorship at Caltech where he worked with Feynman on the theory of the weak nuclear force. He also began to turn his attention to the problem of classifying the ‘zoo’ of elementary particles that had by now been discovered. It was possible to discern small patterns in the zoo – particles that clearly belonged to the same species, for example – but the individual patterns did not fit together to give a coherent picture.

  Particle physicists had by this time introduced a taxonomy to lend the zoo at least some sense of order. There were two principal classes. These were the hadrons (from the Greek hadros, meaning thick or heavy) and the leptons (from the Greek leptos, meaning small).

  The class of hadrons includes a sub-class of baryons (from the Greek barys, also meaning heavy). These are heavier particles which experience the strong nuclear force and include the proton (p), neutron (n), lambda (Λo), and two further series of particles that had been discovered in the 1950s and named sigma (Σ+, Σo, and Σ–) and xi (Ξo, Ξ–). The class of hadrons also includes the sub-class of mesons (from the Greek mésos), meaning ‘middle’). These particles experience the strong force but are of intermediate mass, such as the pions (π+, πo, π–) and the kaons (Κ+, Κo, and Κ–).

  The class of leptons includes the electron (e–), muon (μ–), and the neutrino (ν). These are light particles which do not experience the strong nuclear force. Both the baryons and the leptons are fermions, named for Enrico Fermi. They are characterized by half-integral spins. The baryons and leptons listed above all possess a spin of ½, and can therefore take up two spin orientations, given as +½ (spin-up) and –½ (spin-down). Fermions obey Pauli’s exclusion principle.

  Sitting outside the classes of hadrons and leptons was the photon, the carrier of the electromagnetic force. The photon is a boson, named for Indian physicist Satyendra Nath Bose. Bosons are characterized by integral spin quantum numbers and are not subject to Pauli’s exclusion principle. Other force carriers, such as the hypothetical W+, W–, and Z0 particles, were expected to be bosons with integral spins. Bosons with zero spins are also possible, but these are not force particles. Mesons are examples of bosons with zero spin. The classification of particles known around 1960 is summarized in Figure 9.

  It was clear that amidst this confusion there must be a pattern, a particle equivalent of Dmitri Mendeleev’s periodic table of the elements. The question was: What is this pattern and does it have an underlying explanation?

  Gell-Mann initially tried to construct a pattern from a fundamental triplet of particles consisting of the proton, neutron, and lambda, using these as building blocks to construct all the other hadrons. But it was a big mess. It was never really clear why these particles should be regarded as more ‘fundamental’ than the others. He realized that he was reaching for the underlying explanation before a proper pattern had been established. This was a bit like trying to figure out the fundamental building blocks of the chemical elements without first appreciating the position that each element occupies in the periodic table.

  FIGURE 9 The taxonomy adopted by particle physicists around 1960 had helped to organise the known particles into different classes. These were hadrons (baryons and mesons) and leptons. Sitting outside this classification was the photon, the force particle of electromagnetism.

  Gell-Mann believed that the framework for such a pattern could be provided by a global symmetry group, a way of organizing the particles so that the pattern of their interrelationships could be revealed. He was at this stage searching only for a way of reclassifying the particles, rather than seeking to develop a Yang–Mills field theory, which would have required a local symmetry.

  He knew he needed a larger continuous symmetry group than U(1) or SU(2) to accommodate the range and variety of particles that were then known, but he was quite unsure how to proceed. By this time he was working as a visiting professor at the Collège de France in Paris. Perhaps not surprisingly, copious quantities of good French wine consumed over lunch with his French colleagues did not immediately help to point the way to a solution.

  Glashow’s visit to Paris in March 1960 therefore prompted more than just noises of encouragement. Gell-Mann was intrigued by Glashow’s SU(2) × U(1) theory. He began to understand how it might be possible to expand the symmetry group to higher dimensions. Thus inspired, he now tried theories with more and more dimensions. He tried three, four, five, six, and seven dimensions, trying to find a structure that did not correspond to the product of SU(2) and U(1). ‘At that point, I said, ‘That’s enough!’ I did not have the strength after drinking all that wine to try eight dimensions.’8

  It seemed that the wine had not aided conversation either. The colleagues with whom Gell-Mann was drinking at lunch were mathematicians who could have solved his problem almost immediately. But he never discussed it with them.

  Glashow chose to accept Gell-Mann’s offer to join him at Caltech and, shortly after his return from Paris, the two physicists searched for a solution together. But it was only after a chance discussion with Caltech mathematician Richard Block that Gell-Mann discovered that the Lie group SU(3) offered the structure he had been searching for. In Paris he had given up just as he was about to discover this for himself.

  The simplest, or so-called ‘irreducible’ representation of SU(3) is a fundamental triplet. Other theorists had actually tried to construct a model based on the SU(3) symmetry group and had used the proton, neutron, and lambda particles as the fundamental representation. Gell-Mann had already been down this road, and had no wish to repeat his experiences. He simply skipped over the fundamental representation and turned his attention to the next.

  One of the representations of SU(3) consists of eight dimensions. ‘Rotating’ a particle in one dimension transforms it into a particle in another dimension, just as ‘rotating’ the isospin of the neutron in the SU(2) symmetry group turns it into a proton. If Gell-Mann could somehow place a particle in each dimension, then perhaps he could begin to understand the nature of their underlying relationships. It was surely no coincidence that there were eight baryons – the proton, neutron, lambda, three sigma, and two xi particles?

  These particles could be di
stinguished by their values of electric charge, isospin, and strangeness. Plot strangeness value against either charge or isospin on a graph and a hexagonal pattern emerges with a particle at each apex and two particles in the centre (see Figure 10). The pattern demanded that the proton, neutron, and lambda particles be included in the scheme, and Gell-Mann must have felt justified in his decision to resist ascribing these to the fundamental representation.

  When Gell-Mann produced a similar analysis for the mesons, he found that he needed to include the anti-Κo but was still short by one particle. The meson equivalent of the lambda was ‘missing’. Emboldened, he speculated that there must exist an eighth meson with an electric charge of zero and zero strangeness.

  FIGURE 10 The Eightfold Way. Gell-Mann found that he could fit the baryons, including the neutron (n), and proton (p) and the mesons into two octet representations of the global symmetry group SU(3). But there were only seven particles in the representation for the mesons. One particle, the meson equivalent of the Λo, was missing. This particle was found a few months later by Luis Alvarez and his team in Berkeley. They called it the eta, η.

  Gell-Mann had discovered the patterns in two ‘octets’ of particles based on an eight-dimensional representation of the global SU(3) symmetry group. He called it the ‘Eightfold Way’, a tongue-in-cheek reference to the teachings of Buddha on the eight steps to Nirvana.* He completed his work on the Eightfold Way during Christmas 1960 and it was published as a Caltech preprint in early 1961. The particle he had predicted to complete the meson octet was found a few months later by American physicist Luis Alvarez and his team in Berkeley, California. They called the new particle the eta, η.