Higgs:The invention and discovery of the 'God Particle'
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The discovery of the Higgs mechanism in 1964 had shown how massless bosons of this kind could acquire mass. Weinberg and Salam had gone on to apply the Higgs mechanism to electro-weak symmetry-breaking in 1967–68. The resulting theory had been shown to be renormalizable in 1971. And now the carriers of the weak force had been found, precisely where they had been expected.
The very existence of the W and Z particles with the predicted masses provided rather compelling evidence that the SU(2)×U(1) electro-weak theory was basically right. And if the theory was right, then interactions with an all-pervasive energy field (the Higgs field) were responsible for endowing the weak-force carriers with mass. And if the Higgs field exists, so too must the Higgs boson.
But finding the Higgs boson was going to require an even bigger collider.
8
Throw Deep
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In which Ronald Reagan throws his weight behind the Superconducting Supercollider, but when the project is cancelled by Congress six years later all that remains is a hole in Texas
What the physicists had learned from their experience of electro-weak unification could be applied again to a larger problem. The electro-weak theory implied that at some time shortly after the big bang, the temperature of the universe would have been so high that the weak nuclear force and the electromagnetic force would have been indistinguishable. There was instead a single electro-weak force carried by massless bosons.
This is known as the ‘electro-weak epoch’. As the universe cooled, the background Higgs field ‘crystallized’ and the higher gauge symmetry of the electro-weak force was broken (or, more correctly, ‘hidden’). The massless bosons of electromagnetism (photons) continued unimpeded, but the weak-force bosons interacted with the Higgs field and gained mass to become the W and Z particles. The upshot is that in terms of interaction strengths and scales, the weak and electromagnetic forces now look very different.
In 1974 Weinberg, American theorist Howard Georgi, and Australian-born physicist Helen Quinn showed that the strengths of the interactions of all three particle forces become near-equal at energies between a hundred billion and a hundred million billion GeV.* These energies, corresponding to temperatures of around ten billion billion billion (1028) degrees, would have been prevalent at about a hundred million billion billion billionth (10–35) of a second after the big bang.
It seems reasonable to suppose that in this ‘grand unification epoch’, the strong nuclear force and electro-weak force would have been likewise indistinguishable, collapsing into a single ‘electro-nuclear’ force. All force carriers would have been identical and there would have been no mass, no electrical charge, no quark flavour (up, down) or colour (red, green, blue). Breaking this, even higher, symmetry required more Higgs fields, crystallizing at higher temperatures and so forcing a divide between quarks, electrons, and neutrinos and between the strong and electro-weak forces.
One of the first examples of such a grand unified theory (GUT) was developed by Glashow and Georgi in 1974.† This was based on the SU(5) symmetry group, which they declared to be the ‘gauge group of the world’.1 One consequence of the higher symmetry was that all elementary particles had simply become facets of each other. In Glashow and Georgi’s theory, transformations between quarks and leptons had now become possible. This meant that a quark inside a proton could transform into a lepton. ‘And then I realized that this made the proton, the basic building block of the atom, unstable,’ Georgi said. ‘At that point I became very depressed and went to bed.’2
As grand unification occurs only at energies that can never be realized in any earth-bound collider, it might be tempting to question the value of such theories. However, GUTs predict the existence of new particles which can in principle be revealed in collision experiments. And, although the grand unification epoch may have ended billions of years ago, it left a lasting imprint on the universe that we can observe today.
At least, this was the logic followed by young American postdoctoral physicist Alan Guth. He had confirmed that among the new particles predicted by GUTs was the magnetic monopole, a single unit of magnetic ‘charge’ equivalent to an isolated north or south pole. In May 1979 he had begun work with a fellow postdoc, Chinese-American Henry Tye, to determine the number of magnetic monopoles likely to have been produced in the big bang. Their mission was to explain why, if magnetic monopoles were indeed formed in the early universe, none are visible today.
Guth and Tye realized that they could suppress the formation of monopoles by changing the nature of the phase transition from grand unified to electro-weak epochs. This was a matter of tinkering with the properties of the Higgs fields involved. They discovered that the monopoles disappeared if, instead of a smooth phase transition or ‘crystallization’ at the transition temperature, the universe had instead undergone supercooling. In this scenario, the temperature falls so rapidly that the universe persists in its grand unified state well below the transition temperature.*
When in December 1979 Guth explored the wider effects of the onset of supercooling, he discovered that it predicted a period of extraordinary exponential expansion of space-time. Initially rather nonplussed by this result, he quickly realized that this explosive expansion could explain important features of the observable universe, in ways that the prevailing big bang cosmology could not. ‘I do not remember ever trying to invent a name for this extraordinary phenomenon of exponential expansion,’ Guth later wrote, ‘but my diary shows that by the end of December I had begun to call it inflation.’3
Inflationary cosmology underwent some modifications largely as a result of further tinkering with the properties of the Higgs fields used to break the symmetry at the end of the grand unification epoch. These early theories predicted too much uniformity, implying a rather bland universe with no structure – no stars, planets, or galaxies. Cosmologists began to realize that the seeds of this observable structure had to come from quantum fluctuations in the early universe, amplified by inflation. But the properties of the Higgs fields required for this were incompatible with the Higgs fields of the Glashow–Georgi GUT.
By the early 1980s, experimental results were in any case confirming that the proton is more stable than Georgi and Glashow’s theory implied.† No longer constrained by theories derived from particle physics, cosmologists were free to fit the observable universe by further tweaking the Higgs fields, which became collectively known as the inflaton field to emphasize its significance. Their predictions were borne out in spectacular fashion in April 1992 by results from the Cosmic Background Explorer (COBE) satellite, which mapped tiny variations in the temperature of the cosmic background radiation, the cold remnant of the hot radiation that had disengaged from matter about four hundred thousand years after the big bang.*
Brout and Englert, Higgs, Guralnik, Hagen, and Kibble had invented the Higgs field as a means to break the symmetries involved in Yang–Mills quantum field theories. Weinberg and Salam showed how the trick could be applied to electro-weak symmetry-breaking, and the technique was used to predict correctly the masses of the W and Z particles. The same trick had subsequently been used to break the symmetry of the electro-nuclear force. This trick had some surprising consequences, leading to the discovery of inflationary cosmology and the precise prediction of the large-scale structure of the universe.
The entirely theoretical notions of Higgs fields and the false vacuums they imply had become central to both the Standard Model of particle physics and what would emerge as a Standard Model of big bang cosmology. Did these Higgs fields exist? There was only one way to find out.
The Higgs bosons of the grand unified Higgs fields possess huge masses and are simply out of reach of terrestrial colliders. However, although the mass of the Higgs boson of the original electro-weak Higgs field had proved hard to predict with any certainty, in the mid-1980s it was believed to be well within the grasp of the next generation of colliders.
American particle physicists were stil
l smarting from being beaten to the discovery of the W and Z particles by their European rivals. A June 1983 New York Times editorial had declared ‘Europe 3, US not even Z zero’, and claimed that European physicists had now taken the lead in the race to discover the ultimate building blocks of nature.4 The American physicists sought revenge. They were determined that the Higgs boson would be discovered at an American facility.
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On 3 July 1983 Fermilab’s Tevatron accelerator was turned on. Its six-kilometre ring reached its design energy of 512 GeV just twelve hours later. By colliding protons and anti-protons, it promised collision energies of 1 TeV. It had cost $120 million. ISABELLE, a new 400 GeV proton–proton collider under construction at Brookhaven, was now judged to be already obsolete. In July the project was cancelled by the US Department of Energy’s High Energy Physics Advisory Panel.
Construction work was about to begin on CERN’s LEP collider, to be housed in a 27-kilometre ring almost 600 feet beneath the French–Swiss border, which it would cross in four places. This would become the largest civil engineering project in Europe. But the LEP was intended as a W and Z particle factory, to be used for refining our understanding of the new particles and searching for the missing top quark. It was not a Higgs-hunter.
The Tevatron might provide opportunities to glimpse the Higgs boson, but there could be no guarantees. It was time to think big. Lederman had earlier proposed a giant leap forward – a super-massive proton–proton collider based on the use of superconducting magnets and capable of collision energies up to 40 TeV. He had called it the ‘Desertron’, because it would need to be built in a flat expanse of desert, and because it would be the only machine capable of crossing the ‘energy desert’, the energy gulf predicted by GUTs to be devoid of interesting new physics. The Desertron became the Very Big Accelerator (VBA). Having cancelled ISABELLE, the Advisory Panel now urged priority for the VBA, which was swiftly renamed the Superconducting Supercollider (SSC).
The design for the SSC was completed by the end of 1986. It came with a price tag of $4.4 billion, firmly propelling it into the big league of American science projects and requiring presidential approval. Lederman was asked to provide a short, 10-minute video about the project for President Ronald Reagan to review. He used the opportunity to appeal to Reagan’s frontier spirit, drawing a direct analogy between the exploration of the unchartered areas of particle physics with the exploration of the American West.
The formal case for the SSC was put before Reagan and his Cabinet during a presentation at the White House in January 1987. Arguments for and against the investment bounced back and forth. Reagan’s budget director argued that approval would achieve little more than make a bunch of physicists very happy. Reagan replied that this was something he probably should consider, as he had made his own physics schoolteacher very unhappy.
As the arguments subsided, attention turned to Reagan for a final decision. Reagan read out a passage by American writer Jack London: ‘I would rather that my spark should burn out in a brilliant blaze than it should be stifled in dry rot.’5 He explained that these words had once been quoted to the quarterback Ken ‘Snake’ Stabler. Stabler had steered the Oakland Raiders to a Super Bowl victory in 1977 and was famous for his passing accuracy and his ‘Ghost to the Post’, a 42-yard pass to Dave Casper (the ‘Ghost’) which set up an equalizing field goal in the dying seconds of an AFC playoff game against Baltimore Colts. The goal sent the game into overtime and the Raiders ran out eventual winners.
Stabler had interpreted the Jack London quote in the context of his own approach to American football. ‘Throw deep,’ Stabler had said.6 In the face of adversity, it is better to adopt the riskier strategy and burn out in a brilliant blaze.
Reagan, a stalwart of American B-movies before entering politics in 1964, had acquired the nickname ‘the Gipper’ after appearing as college footballer George Gipp in the 1940 film Knute Rockne, All American. Gipp had died of a throat infection at the age of 25, and the film contains the famous quote: ‘And the last thing George said to me, “Rock,” he said, “sometime when the team is up against it and the breaks are beating the boys, tell them to go out there with all they’ve got and win just one for the Gipper.”’7
There can be little doubt that Reagan found deep psychological resonance with the concept of the SSC. Already bedazzled by the promise that science could provide America with a last line of defence in the form of the Strategic Defense Initiative (SDI, also known as ‘Star Wars’), he was now more than willing to go out there with all they’ve got in the interests of American scientific leadership. The Gipper was ready to throw deep.
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The project had won approval, but was nevertheless beset by doubts. In its pitch the Department of Energy had explained how the SSC would become an international endeavour, supported by financial contributions from other countries. But the rhetoric from American physicists undermined this intention. Why would other countries support a project that was overtly designed to restore American leadership in high-energy physics? Europe was in any case firmly committed to CERN. Not surprisingly, the SSC attracted little interest from overseas.
Resentment had also built up within the American physics community, and this now spilled over into confrontation. With such a high cost, just what exactly was being sacrificed in the search for the Higgs boson? There were many other, individually much less costly projects that were much more likely to provide potentially valuable technological advances. American physics budgets couldn’t fund all these and the SSC, and these projects now appeared to be at considerable risk. Was high-energy physics really a thousand times more valuable than other scientific fields?
‘Big science’ became a pejorative term.
Congressional and Senate support for the SSC was maintained for as long as the location of the new collider was unknown. The National Academies of Science and Engineering received 43 proposals from 25 different states. The Texas government established a commission, and it promised $1 billion funding if the SSC would be built in its territory. It might have made more sense to build the new collider at Fermilab, where much of the infrastructure and many of the physicists who would be needed were already established. But, in November 1988, the National Academies decided that the SSC would be built in a Cretaceous period geological formation called the Austin Chalk, deep beneath the Texan prairie in former cotton-rich Ellis County.
Reagan’s vice-president, Texan George Bush, had succeeded him as president just two days before the announcement. There was no suggestion of bias in the National Academies’ decision, but Bush became a strong supporter. However, now that the site location was known, support from other congressmen and senators began to evaporate.
The physicists had to battle continuously to wrestle funds from Congress, and were called to testify for the project every time it came up for review. In the meantime, budget estimates mushroomed as engineers began to understand more clearly the implications of constructing a huge ring of superconducting magnets. By the time funds were released to begin construction in 1990, budget estimates had almost doubled to $8 billion.
Test holes were drilled into the Austin Chalk and some of the infrastructure was built near Waxahachie, on part of a 17-thousand-acre site that had been reserved for the project by the Texan government. Laboratories were constructed for the development and testing of the magnets. Large structures were assembled to house the refrigeration units required to produce and circulate the liquid helium needed to keep the magnets at their superconducting temperature.
Two detector collaborations were formed. The Solenoidal Detector Collaboration (SDC) would consist of a thousand physicists and engineers from more than a hundred different institutions around the world. This would be a general-purpose detector costing $500 million. It was hoped that this would begin logging data before the end of 1999. The Gammas, Electrons, and Muons (GEM) group would be similar in size and would compete with the SDC.
Ma
ny physicists took a gamble and either organized a period of leave from their current jobs or quit their jobs altogether and relocated to join the SSC project. In all, about two thousand people gathered in and around Waxahachie. To an outsider unfamiliar with SSC politics, all this activity must have appeared rather reassuring. Laboratories were being built, holes were being drilled, and people were gathering in large numbers.
But there were other signs that were rather more ominous. The American administration was struggling with an already large and growing budget deficit. President Bush returned from a visit to Japan in January 1992 empty-handed: the Japanese insisted that the SSC was not an international project and, as such, they wouldn’t support it. The noise about ‘big science’ was rising to a crescendo. In June the House of Representatives voted in favour of an amendment to the federal budget that would have shut the SSC project down. The project survived through the intervention of the Senate.
The gloom that was starting to gather around the project was reflected in Weinberg’s popular book Dreams of a Final Theory, which was published in 1993. He wrote:8
Despite all the building and drilling, I knew that funding for the project might yet be stopped. I could imagine that the test holes might be filled in and the Magnet Building left empty, with only a few farmers’ fading memories to testify that a great scientific laboratory had ever been planned for Ellis County. Perhaps I was under the spell of [Thomas] Huxley’s Victorian optimism, but I could not believe that this would happen, or that in our time the search for the final laws of nature would be abandoned.