If the theory was correct, then neutral kaons could be expected to exhibit weak neutral currents which also involved a change in strangeness. The rather embarrassing absence of such strangeness-changing currents was now explained by invoking the GIM mechanism and the existence of a fourth, charm-quark.
The theorists turned their attentions to other sources of weak neutral currents which did not involve a change in strangeness number and began to urge the experimentalists to search for them. The best candidate events appeared to involve interactions between muon neutrinos and nucleons: protons and neutrons. In the collision of a muon neutrino and a neutron, for example, the exchange of a virtual W− particle turns the muon neutrino into a negative muon and the neutron into a proton. This is a charged current. Exchange of a virtual Z0 particle leaves both the muon neutrino and the neutron intact – a neutral current (see Figure 16). If both processes occur then evidence for weak neutral currents could be obtained by scattering muon neutrinos from nucleons and looking for events in which no muon is produced. Weinberg estimated that for every 100 charged current events there should be somewhere between 14 and 33 neutral current events.
The problem was that neutrinos are extremely light, neutral particles which leave no trace in particle detectors. Such detectors depend on the passage of charged particles dislodging electrons from the atoms of the detector material, leaving a tell-tale trail of charged ions in their wake. The first detector of this type was invented by Scottish physicist Charles Wilson in 1911. In Wilson’s ‘cloud chamber’ the particle tracks are made visible through the condensation of water vapour around the ions that are left behind.
FIGURE 16 (a) A neutron collides with a muon neutrino and a virtual W− particle is exchanged. This turns the neutron into a proton and the neutrino into a muon. This is a charged ‘current’. However, the same collision may also involve exchange of a virtual Z0 particle, (b). No particle changes identity and no muon is produced. This ‘muonless’ event is a neutral current.
The cloud chamber was superseded in the early 1950s by the bubble chamber, invented by American physicist Donald Glaser, but the principles are very similar. A bubble chamber is filled with liquid held close to its boiling point. A charged particle passing through the liquid again leaves a trail of ions and electrons in its wake. If the pressure above the liquid is then lowered, the liquid begins to boil. But it will boil first along the trail of ions left behind, forming a series of bubbles which make the track visible. The tracks can then be photographed and the pressure increased to stop any further boiling of the liquid.
The advantage of the bubble chamber is that the chamber liquid can also serve as the target for particles from an accelerator. Most bubble chambers used liquid hydrogen, but heavier liquids such as propane and freons (the liquids used in older refrigerators) could also be used.
The only signature of a ‘muonless’ event of the kind Weinberg was seeking would be a spray of hadrons suddenly appearing in the detector, seemingly out of nowhere. But there were many other, rather mundane, explanations for such mysterious sprays of hadrons. Muon neutrinos might strike atoms in the detector walls, chipping off stray neutrons which could go on to produce random hadrons in the detector. Events occurring ‘upstream’ of the detector could produce neutrons which then produce hadrons. And if a muon produced in a charged current event was scattered with a large recoil angle, there was a good chance that it would be missed altogether. Background events such as these could easily be miscounted as genuine muonless events, and therefore erroneously identified as weak neutral currents.
The experimentalists were extremely wary of the difficulties involved in any such search. A list of experimental priorities drawn up by CERN physicists in November 1968 put the W particles at the top, but the search for weak neutral currents was a humble eighth. ‘The fact is that, up until 1973, there was no firm evidence in favour of neutral currents and plenty of evidence against them,’ wrote Oxford physicist Donald Perkins.4
However, by the spring of 1972 the enormous theoretical advances that had been achieved pushed the search to the top of the agenda. The physicists began to think that it might be possible to provide a definitive answer.
A large and growing international collaboration led by CERN physicist Paul Musset, Andre Lagarrigue from the accelerator laboratory in Orsay, and Donald Perkins utilized ‘Gargamelle’, the world’s largest heavy liquid bubble chamber. Gargamelle had been funded by the French atomic energy commission, built in France and installed at CERN in 1970 alongside the 26 GeV proton synchrotron.* It had taken six years to construct, and was designed specifically to study collisions involving neutrinos.
Gargamelle had been in operation for almost a year, and had thrown up lots of muonless events that had been dismissed as background ‘noise’ produced by stray neutrons. The experimentalists now began to look at these events with renewed interest.
The challenge was to distinguish genuine muonless events produced by weak neutral currents from those produced by background neutrons, large-angle muon scattering, and misidentification. It was a painstaking and rather thankless task, but by late 1972 many physicists in the Gargamelle collaboration, which by now included physicists from seven European laboratories and guests from America, Japan, and Russia, were beginning to believe that they had found something. But opinion within the collaboration was divided, not so much about the reality or otherwise of the neutral currents themselves, but rather about whether the evidence they had gathered was sufficiently compelling.
In the meantime, a second search had begun in America. The world’s largest proton synchrotron had been constructed at the National Accelerator Laboratory (NAL)* in Chicago, reaching its design energy of 200 GeV in March 1972. Italian physicist Carlo Rubbia at Harvard, Alfred Mann at the University of Pennsylvania, and David Cline at the University of Wisconsin now used beams of muon neutrinos generated by the synchrotron to look for muonless events. The CERN team was ahead, but their preliminary reports were inconclusive. Rubbia was ambitious and determined to get there first.
Finding muonless events was easy. Proving that they derived from weak neutral currents was hard. When Musset presented further preliminary data in early 1973 there was no fanfare, no claim to have made the discovery that they were all pursuing.
The NAL team’s advantage allowed them an opportunity to catch up. Their synchrotron was more powerful, able to create more muon neutrino scattering events in a shorter time. Their detector also provided a larger target mass than Gargamelle, improving the chances of detecting the scattering events. These factors helped to reduce the impact of background neutrons, but there was nothing that could be done about muons scattered at large angles ‘escaping’ without being detected. Rubbia and his team at Harvard sought to account for this contribution using a computer simulation, subtracting a theoretical estimate of the contribution from the number of muonless events measured experimentally, and so arriving at the number of genuine muonless events.
It was a clumsy compromise, and Mann and Cline were deeply suspicious. Rubbia, aware that the CERN physicists were building their body of evidence, was in a hurry.* Mann and Cline understood only too well how such pressures could easily lead the physicists to self-delusion, to convince themselves of the existence of something that actually wasn’t there. They urged caution.
News of the NAL physicists’ progress reached CERN in July 1973. Rubbia wrote to Lagarrigue claiming that they had accumulated ‘approximately one hundred unambiguous [neutral current] events’.5 He went on to suggest that the groups publish their findings simultaneously. Lagarrigue politely declined. The CERN physicists had identified genuine muonless events in the collisions of muon neutrinos with nucleons and had estimated the ratio of neutral to charged current events to be 0.21. For collisions involving muon anti-neutrinos the ratio was 0.45. The physicists now moved to declare that weak neutral currents had at last been found, and submitted a paper to the journal Physics Letters. It was published in September.<
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The NAL group had found the combined ratio of neutral to charged currents for both muon neutrino and anti-neutrino collisions to be 0.29, in good agreement with the CERN results.*
At this critical juncture Rubbia’s visa expired and, despite holding a professorship at Harvard, he was threatened with deportation. During his appeal hearing at the offices of the Immigation and Naturalization Service in Boston he lost his temper. He was on a plane out of the country within 24 hours.
With Rubbia out of the picture, the NAL collaborators began to back-track. Their paper, which they had submitted to the journal Physical Review Letters in August, was rejected by peer reviewers concerned that the problems of eliminating erroneous muonless events had not been properly addressed. Cline and Mann now rebuilt their detector, intending to settle the matter, one way or the other.
The genuine muonless events promptly disappeared, with ratios of neutral to charged current events falling as low as 0.05. The NAL physicists became convinced that they had been misled by their earlier results.
Rubbia was also a prominent figure at CERN and decided to stir up trouble. He advised CERN Director-General Willibald Jentschke that the Gargamelle collaboration had made a big mistake. CERN was still very much in the shadow of its more prestigious American rivals and its international reputation had suffered some setbacks from previous errors. Many European physicists were inclined to think that the Gargamelle result must be wrong and one senior CERN physicist staked half the contents of his wine cellar on a bet against the result. Appalled at the thought that CERN’s reputation was about to suffer another blow, Jentschke summoned the Gargamelle physicists to a meeting. It seemed like an inquisition.
But, although the Gargamelle physicists were shaken by these developments, they were resolute. They chose to stand by their conclusions. Encountering Jentschke in the lift at CERN, Perkins offered reassurance. ‘I knew the group had gone through the event analysis many times and for almost a year we had searched intensively for some other explanation for the effects observed, without success,’ Perkins explained. ‘So I thought the result was absolutely solid, and [Jentschke] should just ignore rumours from across the Atlantic. I don’t know if my words reassured him, but he got out of the lift with a smile on his face.’6
Rubbia returned to NAL in early November and together the NAL physicists began to draft a rather different paper, declaring that weak neutral currents had not been found in contradiction with the recent report from CERN and the predictions of the electro-weak theory.
There following a rather embarrassing volte face. By mid-December 1973 the NAL physicists had realized that pions creeping in from other neutrino collisions had been misidentified in their detector as muons. This effect had virtually eliminated the count of muonless events. Weak neutral currents were back. Cline now had to admit ‘the distinct possibility that a muonless signal of order ten per cent is showing up in the data’.7 He could not find a way to make these events disappear. The NAL team decided to resubmit their original paper, with suitable modification. It was published in Physical Review Letters in April 1974.
Some in the physics community referred jokingly to the discovery of ‘alternating neutral currents’.
By mid-1974, other laboratories had confirmed the result and the confusion had cleared. Weak neutral currents were an established experimental fact.
But the implications of this discovery were of even greater significance. Weak neutral currents implied the existence of ‘heavy photons’ responsible for carrying the weak force. And if no neutral currents could be found in strange-particle decays, this must be because they are suppressed by the GIM mechanism.
In other words, there must also be a fourth quark.
7
They Must Be Ws
____________
In which quantum chromodynamics is formulated, the charm-quark is discovered, and the W and Z particles are found, precisely where they were predicted to be
The pieces of the jigsaw were now falling into place. The puzzle of the existence of point-particles moving freely about inside nucleons, revealed in the experiments on deep inelastic scattering at SLAC, was shown not to be a puzzle at all. It was a direct consequence of the nature of the strong nuclear force, which behaves rather counterintuitively.
When imagining the nature of an interaction governed by a force between two particles, we tend to think of examples such as gravity or electromagnetism, in which the force grows stronger as the particles get closer together.* But the strong nuclear force doesn’t behave in this way. The force exhibits what is known as asymptotic freedom. In the asymptotic limit of zero separation between two quarks the particles feel no force and are completely ‘free’. As the separation between them increases beyond the boundary of the nucleon, however, the strong force tightens its grip and holds them in check.
It is as if the quarks were fastened to the ends of strong elastic. When the quarks are close together inside a nucleon, the elastic is relaxed and there is little or no force between them. The force is experienced only when we try to pull the quarks apart and so stretch the elastic (see Figure 17).
In late 1972 Princeton theorist David Gross had set out to show that asymptotic freedom was simply impossible in a quantum field theory. With the help of his student Frank Wilczek he managed instead to prove precisely the opposite. Quantum field theories based on local gauge symmetries can accommodate asymptotic freedom. A young Harvard graduate student called David Politzer independently made the same discovery. Their papers were published back-to-back in the June 1973 issue of Physical Review Letters.*
FIGURE 17 (a) The electromagnetic force of attraction between two electrically charged particles increases as the particles move closer together. But the colour force that binds quarks inside hadrons behaves rather differently, (b). In the limit of zero separation between a quark and an anti-quark (for example), the force falls to zero. The force increases as the quarks are separated.
Gell-Mann retreated once again to the Aspen Center that June, clutching preprints of the Gross–Wilczek and Politzer papers. He was joined by Fritzsch and Heinrich Leutwyler, a Swiss theorist from the University of Bern on study leave at Caltech. Together they developed a Yang–Mills quantum field theory of three coloured quarks and eight coloured, massless gluons.† To account for asymptotic freedom, the gluons were now required to carry colour charge. No tricks involving a Higgs-like mechanism were required.
The new theory needed a name. In 1973 Gell-Mann and Fritzsch had been referring to it as quantum hadron dynamics, but the following summer Gell-Mann thought he had come up with a better name. ‘The theory had many virtues and no known vices,’ he explained. ‘It was during a subsequent summer at Aspen that I invented the name quantum chromodynamics, or QCD, for the theory and urged it upon Heinz Pagels and others.’1
A great synthesis, combining the theories of the strong and electro-weak forces in a single SU(3)×SU(2)×U(1) structure, seemed at last to be at hand.
But whilst asymptotic freedom could explain why quarks interact only very weakly inside hadrons, it does not explain why the quarks are confined. Various picturesque models were devised. In one of these, the gluon fields surrounding the quarks are imagined to form narrow tubes or ‘strings’ of colour charge between the quarks as they separate. As the quarks are pulled apart, the string first tenses and then stretches, the resistance to further stretching increasing with increasing separation.
Eventually the string breaks, but at energies sufficient to conjure quark–anti-quark pairs spontaneously from the vacuum. So, pulling a quark from the interior of a nucleon, for example, cannot be done without creating an anti-quark which will immediately pair with it to form a meson, and another quark which will take its place inside the nucleon. The end result is that energy is channelled into the spontaneous creation of a meson, and no individual quarks can be observed. Quarks are not so much confined as never, but never, seen without a chaperone.*
The cost, in energy terms
, of an isolated or ‘naked’ colour charge is vast. In principle, the energy of a single, isolated quark is infinite. The quark rapidly accumulates a covering of virtual gluons in an attempt to mask the colour charge, and the energy increases. It costs a lot less energy to mask the charge either by pairing with an anti-quark of the same colour or combining with two other quarks of different colour such that the net colour charge is zero and the resulting host particle is ‘white’.
However, the quark charge cannot be masked completely. To do this we would need somehow to pile the quarks right on top of each other. But quarks are just like electrons – they are quantum particles with wave as well as particle properties. According to Heisenberg’s uncertainty principle, pinning the positions of the quarks down in this manner would lead to an infinite uncertainty in their momenta. This implies the possibility of infinite momentum, which is just as costly.
Nature settles for a compromise. The colour charge cannot be completely masked but the energy, manifested in the associated gluon fields, can be reduced to manageable proportions. This energy is nevertheless substantial. It turns out that the (hypothetical) masses of the up- and down-quarks are quite small, between 1.5 and 3.3 MeV and between 3.5 and 6.0 MeV, respectively.* The measured mass of a proton is 938 MeV, the neutron mass is about 940 MeV. The combined mass of two up-quarks and a down-quark would be something like 4.5–9.9 MeV. So where does the rest of the proton mass come from? It comes from the energy of the gluon fields inside the proton.
‘Does the inertia of a body depend on its energy content?’ Einstein had asked in 1905. The answer is yes. About 99 per cent of the mass of protons and neutrons is energy carried by the massless gluons that hold the quarks together. ‘Mass, a seemingly irreducible property of matter, and a byword for its resistance to change and sluggishness,’ wrote Wilczek, ‘turns out to reflect a harmonious interplay of symmetry, uncertainty, and energy.’2
Higgs:The invention and discovery of the 'God Particle' Page 11
