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Glashow visited Brookhaven in August 1974, once more to urge the experimentalists to search for the charm-quark. American physicist Samuel Ting was listening. He was preparing to use the 30 GeV Alternating Gradient Synchrotron (AGS) to study high-energy proton–proton collisions and watch carefully for electron–positron pairs emerging amidst the chaos of hadrons produced.
When the data revealed that electron–positron pairs were piling up in a narrow ‘resonance’ at an energy of around 3 GeV, the experimentalists were not sure what to make of this. They sought to eliminate obvious sources of error and re-checked their analysis. It made no difference. The peak remained stubbornly fixed at 3.1 GeV, and stubbornly narrow. They began to suspect that this might be new physics.
Ting was cautious. He had acquired a reputation for showing up errors in other physicists’ experiments, and he didn’t want to fall victim to the same treatment. He resisted the pressure to publish the results until they had had a chance to reconfirm the data.
Meanwhile, over on the West Coast, Stanford University physicist Roy Schwitters had a problem. The Stanford Positron Electron Asymmetric Rings (SPEAR), used to collide accelerated electrons and positrons, had come on stream at SLAC in mid-1973. Schwitters had found an error in one of the computer programs used to analyse data from the SPEAR experiments. When he corrected it, data re-analysed from experiments carried out in June 1974 now showed hints of structure – small bumps at 3.1 and 4.2 GeV. The project leader, American physicist Burton Richter, was eventually persuaded to reconfigure SPEAR for collision energies around 3.1 GeV, so that they could go back and have another look.
By November 1974, it was clear that both Ting’s group at Brookhaven and Richter’s group at SLAC had discovered the same new particle, a meson formed from a charm-quark and an anti-charm-quark. Ting’s group had resolved to call it the J-particle, Richter’s group called it the ψ (psi). This joint discovery was subsequently referred to as the ‘November revolution’.
There followed something of a hiatus over priority. Neither group would concede priority by adopting the name given by the other, and the particle is today still called the J/ψ. Ting and Richter shared the 1976 Nobel Prize for physics.
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The discovery of the J/ψ was a triumph for theoretical and experimental physics. It also served to neaten the structure of the fundamental particles – the foundation of what was fast becoming the ‘Standard Model’ of particle physics.
There were now two ‘generations’ of fundamental particles, each consisting of two leptons and two quarks and the particles responsible for carrying forces between them. The electron, electron neutrino, up-quark, and down-quark form the first generation. The muon, muon neutrino, strange-quark, and charm-quark form the second generation, differentiated from the first by their masses. The photon carries the electromagnetic force, the W and Z particles carry the weak nuclear force, and eight coloured gluons carry the strong nuclear force or colour force between the coloured quarks.
But by the spring of 1977, overwhelming evidence had accumulated for an even heavier version of the electron – called the tau lepton. It was not what physicists really wanted to hear.
A tau lepton demanded a tau neutrino and, inevitably, speculation mounted that there are actually three generations of leptons and quarks. American physicist Leon Lederman found the upsilon (Υ) at Fermilab in August 1977. This is a meson consisting of what had by then come to be known as a bottom quark and its anti-quark. With a mass of about 4.2 GeV, the bottom quark is a heavier, third-generation version of the down- and strange-quarks with a charge of . It was assumed that the final member of the third generation – the top quark – was heavier still and would be found as soon as colliders capable of the requisite collision energies could be built.
Although it had made something of a surprise appearance, the third generation of leptons and quarks was readily absorbed into the Standard Model (see Figure 18). At a symposium organized at Fermilab in August 1979, evidence was presented for the appearance of quark and gluon ‘jets’ produced in electron–positron annihilation experiments. These are directed sprays of hadrons produced from the formation of a quark–anti-quark pair in which an energetic gluon is also ‘liberated’ from one of the quarks. Such tell-tale ‘three-jet events’ provide the most striking evidence yet found for both quarks and gluons.
FIGURE 18 The Standard Model of particle physics describes the interactions of three generations of matter particles through three kinds of force, mediated by a collection of field particles or ‘force carriers’.
The top quark was still missing, as was direct evidence for the W and Z particles, the carriers of the weak force. As the Standard Model became the new orthodoxy, Glashow, Weinberg, and Salam learned that they had been awarded the 1979 Nobel Prize for physics for their work on electro-weak unification.
The race was now on to find the remaining particles needed to complete the set. In his Nobel Prize lecture, Weinberg explained that the electro-weak theory predicted masses for the W and Z particles of about 83 GeV and 94 GeV, respectively.*
Back in June 1976, CERN had commissioned its Super Proton Synchrotron (SPS), a 6.9 kilometre circumference proton accelerator capable of generating particle energies up to 400 GeV. A month before its commissioning, these particle energies had already been surpassed by the proton accelerator at Fermilab, which had reached 500 GeV. But smashing particles into stationary targets results in a substantial waste, as energy is carried away by recoiling particles. In this kind of arrangement, the energy that can usefully be channelled into the creation of new particles increases only as the square-root of the particle energy in the beam.
This meant that collisions involving particles accelerated even to the energies now available from the SPS or Fermilab accelerator could be expected to produce new particles only of much lower energy. To reach the energies predicted for the W and Z particles would require an accelerator considerably larger than any yet built.
There was an alternative. The idea of colliding two beams of accelerated particles had been developed in the 1950s. If the accelerated particles were passed into two linked storage rings, in beams travelling in opposite directions, then the beams could be brought into head-on collision. Now all the energy of the accelerated particles could be channelled into the creation of new particles.
Such particle colliders were first constructed in the 1970s. SPEAR was an early example, but it utilized head-on collisions between leptons (electrons and positrons). In 1971 CERN completed construction of the Intersecting Storage Rings (ISR), a hadron (proton–proton) collider which used the 26 GeV proton synchrotron as the source of accelerated protons. The protons would first be accelerated in the synchrotron before being passed into the ISR, where they would be brought into collision. However, the peak collision energy – 52 GeV – was still insufficient to reach the W and Z particles.
In April 1976 a study group was assembled at CERN to report on the next major construction project, called the Large Electron–Positron (LEP) collider. This was to be built in a 27-kilometre circular tunnel passing beneath the Swiss–French border near Geneva. It would use the SPS to accelerate electrons and positrons to speeds close to that of light before injecting them into the collider ring. Collisions would involve particles (in this case electrons) and their anti-particles (positrons), which would be circulated in opposite directions in a single ring. The initial design energy was 45 GeV for each particle beam which, when combined, would produce head-on collision energies of 90 GeV, bringing the LEP just within reach of the W and Z particles.
American physicist Robert Wilson, the director of Fermilab, had an even grander vision. He wanted to build a hadron collider capable of reaching collision energies of 1 TeV (1000 GeV, a terra electron volt or a trillion electron volts). It would eventually become known as the ‘Tevatron’. Such a collider would require as yet untried and untested superconducting magnets to accelerate the particles. And
it was no more than a proposal.
Such was the situation faced by high-energy particle physicists in 1976. CERN’s SPS could accelerate particles to 400 GeV and its ISR could reach collision energies of 52 GeV, neither of which was sufficient to find the W and Z particles. The LEP would in principle be capable of finding them but this machine would not be available until 1989. Fermilab’s Main Ring could accelerate particles to 500 GeV, still insufficient to find the W and Z particles. The Tevatron, capable in theory of reaching collision energies of 1 TeV, was on the drawing board.
The physicists didn’t have the patience to wait. ‘The pressure to discover the W and Z was so strong,’ recalled CERN physicist Pierre Darriulat, ‘that the long design, development and construction time of the LEP project left most of us, even the most patient among us, unsatisfied. A quick (and hopefully not dirty) look at the new bosons would have been highly welcome.’3 Patience was also wearing thin among the physicists at Fermilab.
What the physicists on both sides of the Atlantic needed to do was figure out how they could stretch their existing facilities to the all-important energy regime.
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One possible solution had emerged in the late 1960s. It was possible – in principle – to turn an accelerator into a hadron collider by creating beams of protons and anti-protons that circulate around an accelerator ring in opposite directions. The beams could then be brought into head-on collision. A proton–proton collider required two intersecting rings, with protons in each ring travelling in opposite directions, but proton–anti-proton collisions could be engineered in a single ring. And it would be possible to achieve collision energies equal to twice the highest accelerator energies.
But this was no straightforward matter. Anti-protons are produced by colliding high-energy protons with stationary targets (such as copper). A million such collisions are required to produce a single anti-proton. Worse still, the anti-protons are produced with a broad range of energies, too broad to be accommodated in a storage ring. Only a small fraction of the anti-protons so produced would ‘fit’ in the ring, greatly reducing both the intensity of the anti-proton beam and the beam luminosity, a measure of the number of collisions that the beam can produce.
To make a beam of anti-protons sufficiently luminous for successful proton–anti-proton collider experiments would require that the anti-proton energies be somehow ‘gathered’ and concentrated around the desired beam energy.
Fortunately, Dutch physicist Simon van der Meer had figured out how to do precisely this. Van der Meer had graduated in engineering from the Delft University of Technology in 1952. He had worked for the Philips electronics company in the Netherlands for a few years before joining CERN in 1956. At CERN he became an accelerator theorist, primarily concerned with the practical application of theoretical principles to the design and operation of particle accelerators and colliders.
Van der Meer had performed some speculative experiments using the ISR in 1968 but did not publish an internal report on his findings until four years later. The reason for his tardiness was simple: the physics he was pursuing seemed vaguely mad. In his report he wrote: ‘The idea seemed too far-fetched at the time to justify publication.’4
His 1968 experiments had hinted that it might indeed be possible to concentrate anti-protons with an initial spread of energies to the much narrower energy range needed to fit in a storage ring. The technique involved using ‘pick-up’ electrodes to sense anti-protons with energies that strayed from the desired beam energy and sending a signal to a ‘kicker’ electrode on the other side of the ring to nudge these particles back into line. The signals passed from pick-up to kicker electrodes are like a shepherd’s whistled instructions to a sheep dog. On receiving the instructions, the dog barks the stray sheep back into line, and allows the flock to be neatly escorted into the pen.
Van der Meer called the technique ‘stochastic cooling’. The word stochastic simply means ‘random’, and the cooling refers not to the temperature of the beam but to the random motions and the energy spread of the particles contained within it. By repeating the process many millions of times, the beam would gradually converge on the desired beam energy. In 1974 van der Meer carried out some further tests of stochastic cooling using the ISR. The results were not substantial, but they were sufficient to suggest that the principle worked.
In the meantime, Carlo Rubbia had set aside his disappointment at having been beaten to the discovery of weak neutral currents by CERN physicists. Rubbia had secured his PhD at the Scuola Normale in Pisa, Italy, in 1959. He had worked on aspects of muon physics at Columbia University before joining CERN in 1961. In 1970 he was appointed to a professorship at Harvard, spending one academic term a year there and the balance of his time back at CERN. His globetrotting had attracted the award of a nickname by his Harvard students, who called him the ‘Alitalia professor’.
Rubbia was also stubborn, single-minded, ambitious, and notoriously difficult to work with.* He had resolved that he would not be beaten to the discovery of the W and Z particles.
Together with colleagues from Harvard, in mid-1976 Rubbia had submitted proposals to Wilson to convert Fermilab’s 500 GeV proton synchrotron into a proton–anti-proton collider. Wilson had declined, preferring to focus his energies instead on garnering support for the Tevatron. The stochastic cooling technique seemed like a long-shot. If it didn’t work, potentially valuable time on the synchrotron would be lost. Wilson agreed to a half-million-dollar experiment with a small-scale machine to discover if the technique would work.
Rubbia simply took his proposal back with him to CERN, where it got a much more positive reception from Leon van Hove, then CERN Director-General. By June 1978 further CERN trials of stochastic cooling had yielded results that were greatly encouraging, and van Hove was ready to take a gamble. This was an opportunity for CERN to discover new particles, an achievement that had for some years been the preserve of American facilities. Besides, had van Hove not agreed, Rubbia would most probably have gone back to Leon Lederman, who had taken over at Fermilab following Wilson’s resignation in February.* ‘Most likely, if CERN hadn’t bought Carlo [Rubbia]’s idea, he would have sold it to Fermilab,’ Darriulat explained.5
Rubbia was given the go-ahead to form a collaborative team of physicists to design the elaborate detector facility that would be required to discover the W and Z particles. As this was to be constructed in a large excavated area on the SPS the collaboration was called Underground Area 1, or UA1. The team would grow to include some 130 physicists.
Six months after the decision a second, independent collaboration, UA2, was formed under Darriulat’s leadership. This would be a smaller collaboration, consisting of some 50 physicists, designed to provide friendly competition with UA1. The UA2 detector facility would be less elaborate (it would not be able to detect muons, for example), but would nevertheless be able to provide independent corroboration of the UA1 findings.
Proton and anti-proton beam energies of 270 GeV would combine in the SPS to produce collisions with a total energy of 540 GeV, well in excess of the energies required to reveal the W and Z particles.
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After some delays, UA1 and UA2 finally began logging data in October 1982. It was anticipated that collisions producing the W and Z particles would be very rare, so both detector facilities were set up so that they would respond only to selected collisions meeting pre-programmed criteria. The collider would produce several thousand collisions per second over a period of two months. Only a handful of W- and Z-producing events were expected.
The detector facilities were programmed to identify events involving the ejection of high-energy electrons or positrons at large angles to the beam direction. Electrons carrying energies up to about half the mass of the W would be the signature of the decay of W− particles. High-energy positrons would likewise signal the decay of W+ particles. Measured energy imbalances (differences between the energies of the particles going into the colli
sion versus those coming out) would signal the concomitant production of anti-neutrinos and neutrinos, which could not be detected directly.
Preliminary results were presented at a workshop in Rome in early January 1983. Rubbia, uncharacteristically nervous, made the announcement. From the several thousand million collisions that had been observed, UA1 had identified six events that were candidates for W-particle decays. UA2 had identified four candidates. Though somewhat tentative, Rubbia was convinced: ‘They look like Ws, they smell like Ws, they must be Ws.’6 ‘His talk was spectacular,’ wrote Lederman. ‘He had all the goods and the showmanship to display them with a passionate logic.’7
On 20 January 1983, CERN physicists packed into the auditorium to hear two seminars delivered by Rubbia for UA1 and Luigi Di Lella for UA2. A press conference was called on 25 January. The UA2 collaboration preferred to reserve judgement, but judgement was soon forthcoming. The W particles had been found, with energies close to the predicted 80 GeV.
The UA1 discovery of the Z0, with a mass around 95 GeV, was announced on 1 June 1983. This was based on the observation of five events – four producing electron–positron pairs and one producing a muon pair. The UA2 collaboration had accumulated a few candidate events by this time but preferred to wait for results from a further experimental run before going public. UA2 eventually reported eight events producing electron–positron pairs.
By the end of 1983, UA1 and UA2 between them had logged about a hundred W± events and a dozen Z0 events, revealing masses around 81 GeV and 93 GeV, respectively.
Rubbia and van der Meer shared the 1984 Nobel Prize for physics.
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It had been a long journey, one that was, arguably, begun with Yang and Mills’ seminal 1954 work on an SU(2) quantum field theory of the strong force. This was the theory which predicted the massless bosons that had so irked Pauli. In 1957 Schwinger had speculated that the weak nuclear force is mediated by three field particles, and his student Glashow had subsequently reached for an SU(2) Yang–Mills field theory to accommodate them.
Higgs:The invention and discovery of the 'God Particle' Page 12
