The machine will stretch for 50 kilometres, a giant string of supercooled, superconducting cavities that will pump energy into two colliding beams of matter and antimatter. Physicists have already spent years designing this amazing machine, called the International Linear Collider. If it ever gets built, it will be the most ambitious accelerator of its kind, costing many billions of dollars.
And yet, the linear collider will be a mere baby in terms of the energy levels it can reach, compared with another high-profile accelerator, the Large Hadron Collider now nearing completion at the CERN laboratory near Geneva. From next year the LHC will be the world’s most powerful particle accelerator. So why are physicists so keen to build the linear collider?
Part of the answer is that it is likely to be built in the US, and without it American particle physicists face a bleak future, with no big accelerators to play with on home ground and no fresh data to work on. Are several thousand physicists clutching at one very long straw – or is there really something special about this machine?
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The ILC’s supporters have a tough sell ahead. On the face of it, it sounds like yet another expensive hole in the ground, uncomfortably reminiscent of the superconducting supercollider (SSC), which was scrapped in 1993 after $2 billion had already been spent on the preliminary stages of construction – and without the SSC’s selling point of reaching unexplored realms of energy. But look a little closer, and the potential rewards from the ILC are dazzling. It could be the tool to finally take us beyond the standard model of particle physics, not merely opening up a new world of exotic particles, but telling us why those particles exist and behave as they do. The ILC could examine the origins of mass, dissect dark matter, reveal secret symmetries of the universe, perhaps even discover extra dimensions of space.
“The potential rewards from the linear collider are dazzling. It could finally take us beyond the standard model”
To do all that, the linear collider will use the traditional technique of high-energy physics – bang two things together hard and see what comes out (see “Go with a bang”).
Enter the terascale
The same method is used at the most powerful accelerator in operation today, the Tevatron at Fermilab in Chicago, which can inject a total of 2 teraelectronvolts (TeV, or 1012 eV) into collisions between protons and antiprotons. That’s 2000 times the energy locked up in the mass of a proton, or about a million times the punch in a particle of radioactivity. But the Tevatron is due to shut down in 2010, and in the US, particle physics is looking queasy. “From an American perspective it’s a little frightening: when you turn off the Tevatron, you don’t get data coming in for a long time,” says James Rosenzweig of the University of California at Los Angeles, who works on future accelerator technologies.
The LHC should power up at CERN next year, eventually reaching collision energies of 14 TeV and giving Europe the edge in the high-energy physics stakes. Researchers hope that such energies will be enough to start making particles from the next layer of reality. They are confident that something new will emerge because at these energies the standard model of particle physics breaks down.
The standard model has been a great success over the past 30 years, building up the universe out of six quarks, six leptons (three electron-like particles and three types of neutrino) and four particles carrying the electromagnetic, weak and strong forces that glue particles together. It has matched the results of almost every experiment going, but the model is not perfect. It does not describe all forces in the same framework, and gravity is missing altogether. In the standard model neutrinos have zero mass; in nature, they don’t. And worst of all, when you use the equations and particles of the standard model to work out what happens in high-energy collisions above about 1 TeV, it starts predicting nonsense, saying that some outcomes occur with more than 100 per cent probability.
One solution to this problem is the notorious Higgs boson, a hypothetical particle that could simply slot into the standard model. The Higgs is a manifestation of an energy field that would give mass to most subatomic particles, and explain why the electromagnetic and the weak nuclear force are so different, even though they spring from the same source. Higgses would also clear up the high-energy problem, making the equations balance and produce sensible answers, so that things don’t happen more than all of the time.
The Higgs is something of a smokescreen, however. The simplest version of the theory, with only one kind of Higgs particle, has serious problems of its own: the Higgs mass ought to be infinite, an obvious impossibility. “Nobody believes that it’s just the standard Higgs,” says ILC theorist JoAnne Hewett, of the Stanford Linear Accelerator Center in Menlo Park, California.
Many physicists are instead pinning their hopes on the theory called supersymmetry, which posits many new particles, heavy siblings of the known quarks and leptons, with several Higgs-like particles among them. Then there are other theories in which space has more than the three dimensions we see, with new heavy particles inhabiting these extra dimensions.
Evidence from deep space suggests that other new particles are within reach of the LHC. The rapid motions of stars and galaxies point to some kind of exotic dark matter scattered throughout the universe. Some cosmologists believe that this is made of weakly interacting massive particles, known as WIMPs, and they calculate that WIMPs should have masses of a few hundred gigaelectronvolts (GeV, 109 eV). In any case, say the theorists, something will turn up – and probably at a low enough energy for the LHC to reach.
So why do we still need the ILC? Its maximum collision energy will be 1 TeV, only a few per cent of the mighty LHC and less even than the Tevatron. But brute strength isn’t everything. The Tevatron and the LHC collide protons, each made of three quarks floating in a bag of force-carrying gluons and short-lived quark pairs. And protons make blunt instruments. “It’s like banging together two grand pianos – you get all sorts of complicated stuff flying out,” says Brian Foster of the University of Oxford and head of the ILC’s European design team. That can make it hard to tell what new particles have been produced.
Instead, the ILC would collide electrons with their antiparticles, positrons. These are much sharper instruments, single point-like particles. They carry the full advertised energy of the machine, whereas at a proton collider, the energy available for making new particles is only the small share carried by two individual quarks or gluons.
A hatful of particles
Even more important, while the state of the colliding quarks and gluons in the LHC is a mystery, in the ILC the exact energy and other properties of each electron and positron will be known in advance. This makes it possible to work out the precise properties of whatever exotica are flung from the collision. So the hope is that the LHC will discover a hatful of new particles, and then the ILC will examine them closely so that physicists can uncover the principles of nature that lie beneath.
For example, heavy particles inhabiting extra dimensions should be versions of familiar particles, boosted to high enough energy to oscillate in the tight confines of the extra dimensions. These “Kaluza-Klein” states, named after two theoretical physicists of the early 20th century who proposed extra dimensions to space-time, would have a distinctive spectrum of energy and other properties. The ILC could measure them so precisely that we would find out not just that extra dimensions exist, but how many there are, how large they are, and how they are curved and connected.
Supersymmetry too has its signature. And although the LHC might be able to reveal it, the ILC would be far better suited to the task. Even if there did only seem to be a single Higgs, the ILC could zero in on it and find out if it is indeed alone, or is influenced by much more massive cousins.
And the ILC would be unrivalled as a dark-matter laboratory. The LHC might create WIMPs, and other experiments around the world might detect them passing through the Earth, but that won’t prove that WIMPs form a significant fraction of dark matter. The ILC could measure the exact interaction strength of WIMPs, which would tell cosmologists how much of the stuff should have been created in the big bang.
Particle physicists generally agree that building the linear collider is the next logical step in advancing our understanding of the universe, a view endorsed earlier this year by the US National Academies. Even so, some physicists have reservations about the ILC design. Last December, the 60-strong design team agreed that the best approach would be to accelerate the electrons and positrons using about 16,000 superconducting cavities made from niobium.
The trouble is that this design has inherent limits. Turning up the power too high destroys the superconducting nature of the niobium and the cavity becomes useless, no matter how cunningly designed. “It’s an inelegant solution,” says Rosenzweig. “You can rely on this stuff, but you’re not going to get more than a factor of two beyond current accelerating gradient.”
Will that matter? It depends on what’s out there. If the LHC does indeed discover a set of mysterious particles with energies of just a few hundred gigaelectronvolts, the ILC will be well suited to examining them. But what if all the interesting stuff is at higher energies? “Then there’s a question of whether you’d want the ILC at all,” says CERN’s chief theorist, John Ellis.
It’s not that the ILC would be useless, even then. Its precision would give the accelerator a surprisingly long reach, being able to sniff out hints of particles far beyond its energy range. A more powerful lepton collider might make more sense, however. Another machine is on the table, called the compact linear collider (CLIC), which could reach up to 5 TeV using a less well tested technology. “As an insurance policy, CERN is trying to complete the research and development on CLIC,” says Ellis.
“The accelerator’s precision would give it a surprisingly long reach, sniffing out new particles far beyond its energy range”
Then again, the current design for the ILC might turn out to be too powerful. “It could be that at lowish energy there’s some amazingly interesting thing – then you might be able to build a cheaper machine,” says CERN physicist John Swain.
So why are physicists designing the ILC now, before we know what to expect? “We don’t have the time to wait,” says Rosenzweig. “What you build is very expensive and takes a long time to complete. Waiting longer would harm high-energy physics. You would lose a lot of scientific momentum and intellectual infrastructure.” If the accelerator designers do lose interest and find other jobs, it will be very difficult to rebuild their expertise. “You can’t put those brains on the shelf,” says Swain.
Cagey about costs
So for now it’s full steam ahead with the ILC design project. If President Bush’s 2007 budget request to Congress is approved, the ILC’s funding will double to $60 million a year. That’s just for the research and development, and only the US part of it; European and Asian teams are also at work.
The design team is anxious to avoid the ballooning budget of the SSC, which rose from $4 to $10 billion before the thing was scrapped, so it is being cagey about cost estimates at the moment. By the end of this year, however, it will need to publish a firm figure. An earlier estimate put a tag of $5 billion on the first phase, and it’s likely to turn out more expensive, putting the ILC at the high-cost end of big science.
The ILC will almost certainly have to be truly international if it is to scrape together enough cash. And that raises a tricky question: where to put it? The US government will probably be crucial in funding it, and might not warm to such an expensive project unless it is sited on US soil.
The ILC team are understandably bullish about their machine. “There is a strong case to build the linear collider, no matter what you find at the LHC – even if it is nothing,” says Foster. If the LHC sees no new particles at all, then high-energy particle interactions must be somehow subtly different from the standard model’s impossible predictions. The ILC might be able to find out exactly how.
No actual decision will be made until the LHC provides a sketch map of the new territory. Realistically, the ILC’s prospects depend on what’s out there. “We need input from the LHC to tell us what new physics appears at what energy scale,” says Ellis. “I’m sure the politicians will need it too before they decide whether to entrust us with another few billion.”
Go with a bang
Hit it hard and see what comes out. The deceptively simple essence of particle physics is not as destructive as it sounds. The forces between colliding particles actually create new matter, as long as there is enough energy available.
According to that most famous of equations, E = mc2, energy and mass are equivalent, and it takes more energy to create a heavy particle than a light one. In fact, particle physicists measure mass by a unit of energy: the electronvolt (eV) is the kinetic energy that a single electron would gain by pinging from the negative to the positive terminal of a 1-volt battery. A proton has a rest mass of 938 million electronvolts; the lightweight electron merely 511,000 eV.
The Z particle, which helps to carry the weak nuclear force, has a mass of 91.2 billion electronvolts (GeV), with a single particle being heavier than five molecules of water. To make Z particles at the Stanford Linear Accelerator Center, beams of electrons and positrons were given exactly 46.6 GeV of kinetic energy. When they collided head on, there was just enough energy available to make a Z.
Other collisions can be much messier, with hundreds of particles flying out, so particle accelerators have huge and complex detectors wrapped around the collision zone. These trace the paths of all the shrapnel and reconstruct the explosions, performing a forensic investigation to spot any interesting interactions or short-lived new objects that can only be recognised from the particles they leave behind when they decay.
One aim of the linear collider will be to spot particles of dark matter. They would slip out without registering in the detector, but should betray themselves by stealing a lot of energy from the collision.
End of the rings
The International Linear Collider (ILC) would be the biggest electron gun ever built. Colliding the electrons head-on with positrons, both accelerated to 500 GeV, would give a total collision energy of 1 TeV. That’s five times what was achieved at the Large Electron-Positron collider (LEP), the accelerator that once occupied the 27-kilometre tunnel at CERN that is now being used for the LHC. Electrons and positrons circled LEP, gradually being boosted to energies as high as 100 GeV.
To reach higher energies still, a ring is impractical, because circling electrons rapidly lose energy by emitting photons in a process called synchrotron radiation. The faster they go, the faster their energy leaks away. That’s where the “linear” bit of the ILC comes in – make the path straight and there are no losses. So the plan is to have two separate accelerators pointed head to head.
That raises its own problems, however. In a ring, you can make the counter-rotating electron and positron beams cross as often as you want, so the particles have many chances to hit each other. In a linear collider, they’ve only got one chance. The solution is to focus the beams down to just a few nanometres across. That way, each electron is running into a dense mass of positrons (and vice versa) so plenty of collisions should take place. It’s difficult, but the necessary techniques have been developed at the Stanford Linear Accelerator Center and at the KEK laboratory in Tsukuba, Japan.
