The life of a particle physicist is lonely and hard. After midnight, the dimly lit and seemingly endless corridors of Europe’s biggest particle physics laboratory, CERN, in Geneva, are still punctuated with shafts of light from under doors behind which young, mostly male physicists sit hunched over computer screens. CERN seems as busy by night as by day.
There are about 4000 scientists at CERN at any one time. Most come from the 16 European member states who pay for CERN, although Americans, Canadians and other non-Europeans are participating in increasing numbers. Many researchers come just for a few weeks at a time to work on a particular experiment, and so grab every available waking moment. CERN’s hostel is on the same site, so you can travel from its spartan bedrooms to the work areas without ever seeing a Swiss alp.
For most of the physicists, the peaks on a computer printout hold more excitement than the mountains outside. Over the past decade, CERN has become the pre-eminent experimental centre for studying the fundamental structure of matter. It has managed to wrest from America its lead in building the big machines needed to study the fundamental particles and forces that appear to make the Universe what it is. CERN has considerable expertise in building synchrotrons – the gigantic ring-shaped accelerators fitted with powerful magnets that send beams of particles such as electrons and protons smashing into each other at incredibly high energies. Detectors, the size of houses, collect the debris from these truly cosmic collisions and send the information back to banks of computers for analysis.
Advertisement
Now CERN wants to build on its reputation with a major new project, the Large Hadron Collider, or LHC. The decision on whether to go ahead is controversial because the US has also started to build a large collider, the Superconducting Super Collider (SSC) to do a similar job. Some people see the projects as complementary – machines with different specifications that will crosscheck results. But others see the duplication as a pointless, costly exercise in scientific and technological chauvinism.
European particle physics surged ahead at the end of the 1970s as the result of the vision of Carlo Rubbia, now director-general of CERN. He rejigged CERN’s existing proton collider, the Super Proton Synchrotron (SPS), so that it could collide protons with antiprotons. This novel machine led to the momentous discovery in 1983 of the W and Z particles which were predicted by the so-called Standard Model of particle physics.
The Standard Model is a rich but, as yet, incomplete description of nature which attempts to reveal the relationships between matter and energy in terms of fundamental building blocks – particles – and their underlying symmetry. It has the even more ambitious goal of describing how the early Universe evolved from the primal cosmic fireball of the big bang. Studying the interactions of these particles at the extremely high energies that colliders can reach provides clues about conditions in the early Universe.
The model predicts two sets of matter particles. The first set consists of six leptons which include the electron, muon and tau particle and their neutrino partners. Making up the second set are the six quarks called up, down, strange, charm, bottom and top. Quarks are the building blocks of nuclear particles such as protons, neutrons and mesons, although the top quark has yet to be detected. All charged particles feel the electromagnetic force which is mediated by photons. Quarks also feel the strong force mediated by gluons and the weak nuclear force which is mediated by W and Z particles.
In order to study the Standard Model in more detail, physicists and engineers at CERN have gone on to build what is currently the world’s largest particle accelerator, the Large Electron Positron Collider. This collides electrons and positrons at more than a 100 gigaelectronvolts in an underground ring 27 kilometres in circumference. The collisions produce a multitude of Z particles, which allows physicists to examine the decays of Zs in great detail. In 1992 LEP will be upgraded to nearly 200 gigaelectronvolts so that it will be able to produce W particles as well.
But physicists would like to go much further in energy. Not only does the top quark remain elusive at the scales of energies already probed but also the Standard Model requires the existence of another particle called the Higgs which gives the other particles mass. To find out whether the Higgs exists requires much higher energies, probably of the order of 1 to 2 teraelectronvolts (1000 to 2000 gigaelectronvolts). What is more, theorists suspect that nature possesses a deeper symmetry than that described by the Standard Model. This is called supersymmetry. Its theoretical framework predicts a whole new set of fundamental entities called supersymmetric particles. Experiments at much higher energies might find them.
The new collider, the LHC, would operate at 10 times that amount of energy so it is suited to hunting the Higgs. It would use superconducting magnets to accelerate counter-rotating beams of protons at very high intensities, or luminosities, to give collisions with energies of 16 teraelectronvolts. Physicists hope that by building the new collider on top of LEP in the same tunnel they will keep down the cost to about the same as that of LEP itself – about a billion dollars. The LHC would also be able to take advantage of the facilities and accelerators that already exist. The old SPS, for example, would become the main part of the injector complex, providing an intense source of protons.
At the end of last year, European physicists effectively sanctioned the project when the European Committee for Future Accelerators agreed that the LHC was the next logical step in the European accelerator programme. Last month CERN’s technical review committee published its report on the design of the LHC. The CERN council of member states is expected to come to a decision towards the end of 1992 on whether to go ahead. The installation could then be completed by 1997 and its commissioning would start by 1998.
The technical report is particularly important because some incredibly ambitious engineering will be required for the project. Careful planning is needed to avoid disrupting LEP’s existing experimental programme. But more important, the LHC must be seen as offering maximum value for money. This involves an economical and flexible design strategy to obtain particle beams with the highest energies and intensities possible within the limited configuration of the LEP tunnel.
The LHC designers are planning to develop a new generation of advanced bending magnets and detectors. The design of the 1800 superconducting magnets is benefiting from experience obtained at HERA, the new machine being built at the DESY laboratory in Hamburg to collide protons and electrons. The magnets for the LHC are, however, designed to provide a magnetic field of 10 tesla, which is twice as strong as that for HERA. Indeed, 10 tesla represents the maximum field that today’s technology could achieve for such magnets.
Magnets made from coils of superconducting niobium-titanium cables cooled by liquid helium have become the norm in accelerator technology. But the magnets for the LHC will be cooled by superfluid helium so that they can operate at a temeperature of 1.9 K, instead of the usual 4.2 K obtainable using normal liquid helium. Although untried in collider technology this approach has been successfully used in the French fusion reactor Tore Supra. The lower temperature allows the cable to carry the much higher current needed to make a high-field magnet compact enough to fit into the accelerator tunnel. Superfluid helium also has the advantage of efficiently carrying away any heat produced by stray protons diffusing out of the beams. (There is no heat from power losses in the magnets to contend with because they are superconducting.)
Another feature is also designed to save both space in the tunnel and money. The magnets will contain two apertures in a single yoke, to house the two magnetic channels for the oppositely circulating beams of particles. This avoids having a separate set of magnets, with its own bulky cryogenic system, for each beam. CERN has already ordered 10 prototype magnets which will be ready by early 1992.
Engineers have also been using a computer to study the conditions necessary to produce beams with the maximum luminosity that is technically feasible, which is as high as 3 x 10 34 particles per square centimetre per second. On a Cray supercomputer they simulate one million turns of the beams around the collider ring in the equivalent of 20 different machine environments. But this is a costly and time-consuming business when 60 hours of time on a Cray represents only a couple of minutes of real beam time.
The beams consist of bunches of particles which are strongly focused by quadrupole magnets made of tin-niobium alloy. To obtain the highest possible luminosity, bunches with the maximum number of particles must first be injected into the beam channels; and secondly, injecting the bunches in the counter-rotating beams so that they cross each other as often as possible which is once every 15 nanoseconds, or about 1000 times as frequently as the bunch crossing in LEP.
This fast collision rate will pose a tough challenge for the designers of the detectors. The time between bunch crossings is shorter than the read-out time of the detectors, which means that the data have to be stored with precise synchronisation in order to decide whether or not a particular event is important. Each bunch crossing may result in tens of results, with each collision producing a large number of highly ionised particles. So any detector will have to deal with this formidable amount of complicated information while remaining unaffected by intense doses of hard radiation.
The designers envisage having two detectors, a general-purpose unit similar to those of LEP but 10 times as large, and a detector designed specially to deal with the complex interactions resulting from high luminosities. Each detector might have as many as a million channels for collecting data.
Next year engineers hope to test the injectors and a small section of the beam channel with 10 magnets. Giorgio Brianti, head of new accelerator projects, is confident that the new technology will work. But the CERN council will not approve the LHC unless the feasibility studies are successful.
Just as important to the success of the LHC is the future financial and political management of CERN. Here CERN can count itself lucky in having a canny and energetic ringmaster. Rubbia has all the skills of a showman experienced in the international circus of big science. He has had to convince CERN member states that the LHC will not cause an increase in the CERN budget. To this end, Rubbia has committed himself to providing 20 per cent of the budget from countries that are not currently members of CERN.
Recently, Rubbia has been pursuing a punishing jet-setting schedule of trying to persuade governments around the world to participate in the laboratory. He is proud of his success so far. ‘CERN is rapidly going from the stage of being a European agency to becoming a world laboratory,’ he says. Poland became a member of CERN this month and Hungary and Czechoslovakia are in the process of negotiating entry. There is now a cooperation agreement with Israel which will contribute financially to CERN at a level of about 30 per cent of that given by full members. The Soviet Union, Canada, Australia, Japan, India and China are all sending physicists to CERN, and one in four users is from a nonmember state.
The CERN party line also emphasises that the kind of technology involved in the LHC design will produce commercial spin-offs. In particular, the niobium-titanium cable could carry the entire output of a nuclear power station for a 1000 kilometres with power losses of less than 1 per cent. Superconducting coils could be used for storing energy and for transport. One application being tested in Japan uses superconducting dipole magnets to drive a propellerless ship.
Rubbia hopes that such strategies will convince member states that the LHC must go ahead. Like most of his colleagues, he believes that since CERN is in the business of making fundamental discoveries it has no choice. Nevertheless, over the past few years, Britain, in particular, has wavered in its commitment to CERN because of the high cost of belonging to this exclusive club. This year’s contribution now stands at £50 million and accounts for an uncomfortably large part of Britains’s science budget.
Much to the embarrassment and annoyance of the British physicists who use CERN, at least one senior British scientist has questioned the value of the LHC. David Phillips who advises the government on the funding of science through the Advisory Board for the Research Councils recently showed a tactless lack of sympathy for big science. Last March he was quoted as saying privately that: ‘Physicists are useless, they just like to play with their toys and when they grow tired with them want more money for new ones.’ Despite the fact that Phillips has played a major role in the decision making for British science, he has never been to CERN. Phillips has, however, visited the site of the much bigger ‘toy’ across the Atlantic – the SSC. He came away, suggesting that Britain should contribute to the American project instead of the LHC. This irritated many physicists at CERN. ‘I should like Mr Phillips to come to CERN,’ Rubbia has said.
The Americans, anxious not to be left behind in the high energy race, began construction of the SSC in 1989. The collider will stretch around 85 kilometres of Ellis county in the state of Texas. Like the LHC, the SSC will collide protons but at an energy roughly three times greater, at 40 tera-electronvolts. Although the machine represents an engineering project of unprecedented scope, it will use more conventional superconducting magnets with a magnetic field strength of 6.59 tesla and will attain a luminosity an order of magnitude less. The SSC should be ready by the end of the decade and will initially explore the same physics as the LHC.
Although Rubbia and Roy Schwitters, the director of the SSC, are both careful not to adopt the role of rivals too publicly, there is clearly a sense of what Schwitters calls ‘healthy competition’. Rubbia says he sees the two colliders as ‘complementary machines studying the most advanced subjects in physics’, while also pointing out the LHC’s advantages in terms of scientific aptness, timing and cost. ‘The LHC is the correct machine to finish the physics programmes that were started with the SPS and LEP. Rather than build the biggest machine, you should try to build the right machine directly keyed to specific issues.’ Rubbia somewhat slyly suggests that the Americans should have waited before building the SSC so as to benefit from the ‘groundwork done with the LHC’.
But particle physics, like space science, is an area of frontier research where continental pride thrives at the highest political level. And the groundwork that Rubbia mentions is the kind that wins Nobel prizes – the discovery of the mechanism of mass generation, the top quark and supersymmetric particles. American researchers are keen to regain the world lead in fundamental science. ‘The US has not built a major laboratory in many years,’ says Schwitters. This may be why the US is reluctant to acknowledge how many Americans work at CERN, says Rubbia.
Nevertheless, many physicists are worried by the escalating cost of the SSC, which has risen from an estimated $5.9 billion in 1986 to $8.25 billion. ‘If CERN did not exist they (the Americans) would never have chosen to launch such a colossal project,’ says Brianti. Funds for building the laboratory now have to be approved by Congress and the Senate on a year-to-year basis, making the timing of the schedule for SSC less certain. Schwitters is guardedly optimistic that other countries such as the Soviet Union, Japan and India, can be persuaded to participate in the project. But some may find the suggested entry fee of a billion dollars too high. It represents the entire cost of the LHC, as Rubbia in his salesman’s patter is quick to point out.
Schwitters is also having problems selling the SSC at home. American physicists are reluctant to leave the more cultured surroundings of SLAC, Brookhaven or Fermilab to head for the bland open stretches of Texas. ‘When you go around the site you see very few faces you recognise. There is perhaps a lack of experienced accelerator people’, says Brianti.
What the Americans dread the most is the LHC scooping the important results ahead of the SSC. The accelerator has been sold to the American public as the only machine with an energy range capable of discovering the fundamental secrets of the Universe. There now seems a real possibility that SSC could lose out. The top quark could well be found at Fermilab near Chicago when the Tevatron reaches maximum energy in 1995. Recent results at LEP already hint that supersymmetry would lie well within the energy range of the LHC.
Much depends on which machine starts to work first. Rubbia is anxious to point out that CERN already has the infrastructure and experience necessary to forge ahead. According to Rubbia, ‘If the existing drive can be maintained by the European physics community and formal approval of the project can be gained from the CERN council in 1992 then there is no doubt that LHC will be up and running some years before its American counterpart.’ But Schwitters is not so sure. He points out that LHC’s novel magnets could cause problems. The LHC is necessarily limited in scope, and being situated in the same tunnel as LEP could impede access. ‘The timescale is similar; we should not make the assumption that the LHC will be first,’ he says.
If the LHC does clean up all the new physics results, could the SSC be a 10-billion-dollar disaster? Rubbia concedes that is an unlikely scenario. Ultimately, sheer brute energy will win over all the subtle refinements of European high technology. Eventually, the SSC will come into its own. As well as being able to study the Higgs mechanism and other fundamental processes that happened just after the big bang, the SSC could discover a totally new regime of physics. When this happens, the LHC will switch over to other types of collisions. Because it is in the same tunnel as LEP, it will be able to collide electrons and protons but at higher energies than HERA. Another scheme is to use the beam channels for collisions between heavy ions. Such experiments would reveal a great deal about the interactions of quarks.
Although both the European and American collaborations are keen to race ahead, Rubbia points out that the differences in the long-term programmes of the LHC and the SSC means that both machines are needed. ‘Science will be the winner,’ he says. The laws of the Universe will be the final judge.
