In Lederman’s rather more quixotic book, The God Particle, published in the same year, he is rudely awoken from a dream in which he has been chatting amiably to the Greek philosopher Democritus:9
‘Shit.’ I was back home, groggily lifting my head off my papers. I noticed one photocopy of a news headline: Congessional Funding for the Super Collider in Doubt. My computer modem was beeping, and an E-mail message was ‘inviting’ me to Washington for a Senate hearing on the SSC.
Bill Clinton won the November 1992 presidential election, beating George Bush and Independent Texas businessman Ross Perot. The following June, SSC budget estimates had grown to $11 billion and the House of Representatives once again voted against the project. As Raphael Kasper, the SSC associate director, remarked: ‘Voting against the SSC became at some point a symbol of fiscal responsibility. Here was an expensive project that you could vote against.’10
Clinton was generally encouraging, but less committed to the project than Reagan and Bush had been. Competition now loomed in the form of a $25 billion programme to build the International Space Station, a project that would also be based in Texas, at NASA’s Johnson Space Center in Houston.
In September 1993 Weinberg, Richter, and Lederman made a last-ditch attempt to shore up support for the SSC. The British physicist Stephen Hawking sent a message of support on video. To no avail.
In October the House of Representatives voted narrowly (by one vote) in favour of the International Space Station. The next day the House voted two-to-one against the SSC. This time there would be no reprieve. Funding was allocated to mothball the facilities that had already been built. About 23 kilometres of tunnel had been excavated and $2 billion had been spent (see Figure 19), but no amount of Victorian optimism could now keep the project alive. The SSC was dead.
Pulitzer prize-winning author Herman Wouk wrote a novel based on the SSC experience. His author’s note at the beginning of A Hole in Texas says:11
Ever since coming up with the atomic and hydrogen bombs, [particle physicists] had been the pampered darlings of Congress. But all that suddenly and rudely ended. The sole residue of their miscarried quest for the Higgs boson was a hole in Texas, an enormous abandoned hole.
It’s still there.
FIGURE 19 When the SSC project was cancelled by Congress in October 1993, $2 billion had been spent and 23 kilometres of tunnel had been excavated beneath the Texas prairie.
Source: SSC Scientific and Technical Electronic Repository
On 16 December 1994, a little over a year after the SSC was cancelled, CERN’s member states voted to allocate $15 billion over twenty years to upgrade the LEP at the end of its useful life and turn it into a proton–proton collider. The idea for the Large Hadron Collider (LHC) had first been discussed over ten years previously, at a CERN workshop in Lausanne, Switzerland, in March 1984. It would produce collision energies up to 14 TeV, less than half the maximum energy of the SSC but more than enough to find the Higgs.
Rubbia declared that CERN would ‘pave the LEP tunnel with superconducting magnets.’12
9
A Fantastic Moment
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In which the Higgs boson is explained in terms that a British politician can understand, hints of the Higgs are found at CERN, the Large Hadron Collider is switched on, and then blows up
The SSC had been a huge gamble, and the physicists had lost. The rumblings of discontent that had led eventually to the cancellation of the American project had begun to surface in Europe. CERN benefitted from the fact that no one nation was responsible for its funding. But individual member states’ grumbling about the size of their subscription could yet translate to a decision to withdraw support. In April 1993, just six months before the House of Representatives decided finally to cancel the SSC, UK science minister William Waldegrave was issuing a challenge to the British community of high-energy physicists.
Waldegrave’s challenge presaged a significant shift in science policy by Prime Minister John Major’s Conservative government. A government white paper, which was to be published the following month, sought to shift the emphasis of science policy towards innovation, with the ultimate aim to improve wealth creation and the quality of life of British citizens. In other words, the purpose of British science was to serve the interests of the British economy, to the benefit of ‘UK plc’. The organization of government support for science and technology in Britain was to be completely overhauled.
The signs were ominous. Britain was still recovering from the global recession triggered by the stock market crash in October 1987, and could hardly afford its £55 million annual contribution to CERN. Whilst physicists could point to many spin-off developments from CERN, such as the project to join hypertext with the internet which led to the invention of the world wide web by Tim Berners-Lee in 1990, it was perhaps difficult to explain how the discovery of the Higgs boson would directly improve wealth creation and the quality of life of British people.
Fortunately, the physicists were not yet being asked to provide this kind of justification. But Waldegrave made it quite clear that they had to get an awful lot better at explaining precisely what it was they were trying to do.
Just what was this thing called the Higgs boson? Why was it so important that billions of dollars were needed just to find it? ‘If you can help me understand that, I stand a better chance of helping you to get the money to find it,’ Waldegrave told the audience at an annual conference of Britain’s Institute of Physics.1 He told them that if anyone could explain what all the fuss was about, in plain English, on one sheet of paper, then he would reward that person with a bottle of vintage champagne.
Of course, the fuss was all about the central role that the Higgs field had come to play in the structure of the Standard Model. Without the Higgs field there could be no electro-weak symmetry-breaking.* Without symmetry-breaking the W and Z particles would be massless, like the photon, and the electro-weak force would still be unified. Without interactions between elementary particles and the Higgs field, there would be no mass: no material substance, no stars, no planets, no life. And the direct evidence for the existence of this field could only come from finding its field particle, the Higgs boson. Find the Higgs boson, and suddenly we would understand a lot more about the true nature of the material world.
Explaining the Higgs mechanism in terms that a politician could understand demanded a simple analogy. David Miller, a professor of particle physics and astronomy at University College London, believed he had found just such an analogy. With a few cosmetic changes, he thought he could bring the explanation to life by trading on Waldegrave’s own experiences of a singular personality that had until recently dominated British politics: former Prime Minister Margaret Thatcher. He wrote:2
Imagine a cocktail party of political party workers who are uniformly distributed across the floor, all talking to their nearest neighbours. The ex-Prime Minister enters and crosses the room. All of the workers in her neighbourhood are strongly attracted to her and cluster round her. As she moves she attracts the people she comes close to, while the ones she has left return to their even spacing. Because of the knot of people always clustered around her she acquires a greater mass than normal, that is, she has more momentum for the same speed of movement across the room. Once moving she is harder to stop, and once stopped she is harder to get moving again because the clustering process has to be restarted. In three dimensions, and with the complications of relativity, this is the Higgs mechanism.
In order to give particles mass, a background field is invented which becomes locally distorted whenever a particle moves through it. The distortion – the clustering of the field around the particle – generates the particle’s mass. The idea comes directly from the physics of solids. Instead of a field spread throughout all space a solid contains a lattice of positively charged crystal atoms. When an electron moves through the lattice the atoms are attracted to it, causing the electron’s effective mass to be as much as 4
0 times bigger than the mass of a free electron.
The postulated Higgs field in the vacuum is a sort of hypothetical lattice which fills our universe. We need it because otherwise we cannot explain why the Z and W particles which carry the weak interactions are so heavy while the photon which carries electromagnetic forces is massless.
This describes the mechanism by which massless elementary particles (represented in the analogy by Thatcher) interact with the Higgs field (the uniform distribution of party workers) and so gain mass, as shown in Figure 20. To explain the Higgs boson, Miller continued:
Now consider a rumour passing through our room full of uniformly spread political workers. Those near the door hear of it first and cluster together to get the details, then they turn and move closer to their next neighbours who want to know about it too. A wave of clustering passes through the room. It may spread out to all the corners, or it may form a compact bunch which carries the news along a line of workers from the door to some dignitary at the other side of the room. Since the information is carried by clusters of people, and since it was clustering which gave extra mass to the ex-Prime Minister, then the rumourcarrying clusters also have mass.
The Higgs boson is predicted to be just such a clustering in the Higgs field. We will find it much easier to believe that the field exists, and that the mechanism for giving other particles mass is true, if we actually see the Higgs particle itself. Again, there are analogies in the physics of solids. A crystal lattice can carry waves of clustering without needing an electron to move and attract the atoms. These waves can behave as if they are particles. They are called phonons, and they too are bosons. There could be a Higgs mechanism, and a Higgs field throughout our universe, without there being a Higgs boson. The next generation of colliders will sort this out.
This is illustrated in Figure 21.
FIGURE 20 The explanation of the Higgs mechanism used by David Miller in his winning entry. As Margaret Thatcher makes her way through the ‘field’ of party workers, the field clusters around her and her progress is slowed. This is the equivalent of gaining mass.
Source: © copyright CERN
FIGURE 21 The Higgs boson is like a softly spoken rumour that makes its way through the ‘field’ of party workers. As the field clusters together to hear the rumour, a localised ‘particle’ is formed which then makes its way around the room.
Source: © copyright CERN
Waldegrave received 117 entries, in itself indicative of the importance of the physicists’ quest. Five winners were selected, but Miller’s entry was judged by the physics community to be the best. Miller duly collected his bottle of Veuve Clicquot, although it seems he did not get to taste it. ‘My wife, my sister-in-law and my son’s girlfriend drank the champagne,’ he explained.3
Despite its straightened circumstances, the British government continued to honour its commitment to CERN.*
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With the hunt for the Higgs boson temporarily suspended, there were still a few other Standard Model particles to be found. The discovery of the top quark was eventually announced at Fermilab on 2 March 1995, by two competing research teams each consisting of about 400 physicists. It was identified through its decay products. Energetic protons and anti-protons collide to produce a top–anti-top pair. Each of these particles decays into a bottom quark and a W particle. One W particle decays into a muon and a muon anti-neutrino. The other decays into an up- and a down-quark. The end-result is a collision which produces a muon, muon anti-neutrino, and four quark jets. The mass of the top quark was found to be an astonishing 175 GeV, almost 40 times larger than the mass of its third-generation partner, the bottom quark.
Aside from the Higgs boson, the only other particle that remained to be discovered was the tau neutrino. Its discovery was announced at Fermilab five years later, on 20 July 2000. It was now possible to map the sequence of weak-force interactions that change one flavour of quark into another, Figure 22.
There was still some hope that the Tevatron or the LEP might find the Higgs boson, and these machines were now pushed to their limits. The problem was that the mass of the Higgs boson could not be predicted with any accuracy. Unlike with the search for the W and Z particles, the physicists didn’t quite know where to look.
FIGURE 22 The dominant ‘flavour-changing’ weak force decays involving quarks are down→up, strange→up, charm→strange, bottom→charm and top→bottom. Two less probable decay paths are also shown (dashed lines): charm→down and bottom→up. Upward transitions involve the emission of a W− particle which decays into a lepton (such as an electron) and its corresponding anti-neutrino. Downward transitions involve the emission of a W+ particle which decays into an anti-lepton (such as a positron) and its corresponding neutrino.
The collective understanding was that it would have a mass of the order 100–250 GeV. It would be detected through its decay channels, thought to involve production of bottom–anti-bottom pairs in association with top and bottom quarks, two high-energy photons, pairs of Z particles that would in turn decay into four leptons (electrons, muons, and neutrinos), pairs of W particles, and pairs of tau leptons.
The LEP had proved to be a powerful and versatile machine but had come to the end of its useful life and was due to be de-commissioned in September 2000. In a last-ditch attempt to find the Higgs, CERN physicists now pushed the machine well beyond its limits. It had achieved its design beam energy of 45 GeV (producing electron–positron collision energies of 90 GeV) in August 1989. Various upgrades had lifted the collision energy to 170 GeV, giving the capability to generate pairs of W particles. In the summer of 2000, further modifications pushed collision energies above 200 GeV.
On 15 June 2000, CERN physicist Nikos Konstantinidis studied an event that had been recorded the previous day by the Aleph detector.* It featured four quark jets, two of which had come from the decay of a Z particle. The other two jets appeared to have come from the decay of a heavier particle, with a mass of the order of 114 GeV.
It looked to all the world like a Higgs boson.
Of course, a single event did not constitute a discovery, but it was quickly followed by two more events recorded by Aleph and two events recorded by a second detector collaboration, called Delphi.† This was still insufficient to claim a discovery, but enough to persuade CERN’s Director-General, Luciano Maiani, to stay LEP’s execution until 2 November. When a third detector, called L3, recorded a different kind of event, which appeared to involve the decay of a Higgs into a Z particle which then decays into two neutrinos, it seemed that CERN was on the threshold of one of the most important discoveries in high-energy physics since the invention of the Higgs boson in 1964.
The CERN physicists now bid to keep the LEP running for another six months. Maiani seemed to be inclined to agree to this request but, after much soul-searching in a series of meetings with his senior research scientists, he eventually concluded that the evidence was insufficient to justify a potential delay to the construction of the LHC. There could be no managed transition, no graceful switching from the LEP to the LHC over an extended period of time. To build the LHC, the tunnel housing the LEP would have to be completely gutted. Maiani felt he had no choice but to close the LEP down. The CERN community learnt of the decision through a press release.
Many physicists were convinced that they were close to making a momentous discovery, and the way that Maiani had handled the situation left a bitter taste. However, when the collision events had been subjected to further scrutiny, the likelihood that these were really tell-tale signals of the Higgs boson was even further reduced. ‘I understand the frustration and sadness of those who feel that they had the Higgs boson within their grasp,’ wrote Maiani in February 2001, ‘and fear that it may be years before their work can be confirmed.’4
All that could be concluded was that the Higgs boson must be heavier than 114.4 GeV, with a mass likely to be of the order of 115.6 GeV.
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With the discove
ries of the top quark and the tau neutrino, the collection of elementary particles that make up the Standard Model was virtually complete. Physicists faced the unprecedented situation that there were now no experimental data that did not conform to theoretical predictions. There was, nevertheless, much for the theorists to do.
The Standard Model’s deep flaws had been painfully apparent from the very moment of its inception. The model has to accommodate a rather alarming number of ‘fundamental’ or ‘elementary’ particles. These particles are connected together in a framework that requires 20 parameters that cannot be derived from theory but must be measured. Of these 20 parameters, twelve are required to specify the masses of the quarks and leptons and three are required to specify the strengths of the forces between them.
Then there is the problem with the mass of the Higgs boson itself. The Higgs acquires mass through so-called ‘loop corrections’, which take account of its interactions with virtual particles. Loop corrections involving heavier particles such as a virtual top quark give the Higgs much more mass than it can afford to have if it is to break the electro-weak symmetry in the way that is required. The upshot is that the weak force is predicted to be a lot weaker than it really is. This is known as the ‘hierarchy problem’.
And, despite Glashow, Weinberg, and Salam’s ultimately successful combination of the weak and electromagnetic forces, the SU(3)×SU(2)×U(1) structure of Yang–Mills field theories that makes up the Standard Model is far from being a fully unified theory of particle forces.
Lacking guidance from experiment, the theorists had no choice but to be guided by aesthetics, following their instincts in the search for theories that could transcend the Standard Model and explain the laws of nature at an even more fundamental level.
Higgs:The invention and discovery of the 'God Particle' Page 14
