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Linear colliders target energy frontier

03 Sep 1999

Most particle physicists agree on the scientific case for a next-generation linear collider. It is just a question of persuading the politicians to foot the bill.

It is 10 years since the Large Electron Positron collider (LEP) smashed its first particles together at CERN, the European centre for particle physics near Geneva. Since then it has made valuable contributions to particle physics by confirming many of the predictions of the Standard Model. But to explore new science beyond this, physicists will need to build a successor to LEP, which at 200 GeV has just about reached its maximum energy.

LEP is one of a handful of electron-positron colliders in the world. Others include Tristan at the KEK lab in Japan, CESR at Cornell University in the US, and the Stanford Linear Collider (SLC), also in the US. (The HERA machine at the DESY lab in Germany collides electrons or positrons with protons.) With the exception of the SLC, the charged particles in these machines all radiate synchrotron radiation as they travel in a circle and this causes them to lose energy. The lighter the particle being accelerated, the more synchrotron radiation it generates.

New physics is most likely to occur in proton colliders, such as the Tevatron at Fermilab near Chicago, or the Large Hadron Collider (LHC), due to come on line at CERN in 2005. Protons are far heavier than electrons and therefore produce higher energies when they collide. But protons are composite particles, containing point-like quarks bound together by gluons. Their collisions are therefore messy and have no well-defined energy. To tease out the precise properties of any new particles produced in proton-proton collisions, physicists need to collide together point-like particles such as electrons and positrons.

So what kind of machine can physicists build that will collide point-like particles at the energies needed to observe new physics? One possibility is to build a muon collider. Muons are much heavier than electrons so the synchrotron-radiation losses are lower. They can consequently be collided at high energies. But muons decay into lighter particles, having a half-life of only about 63 µs, which would make it difficult to generate a sufficient collision flux. It would therefore be decades before a muon collider could be built, and a full feasibility study has still to be carried out (Physics World May 1997 p8).

Indeed, for more than a decade there has been a consensus that the next major particle accelerator should be a linear collider in which synchrotron-radiation losses would not be a problem. The SLC, at the Stanford Linear Accelerator Center (SLAC) in California, is the world’s only linear collider. It is about 3 km long and accelerates electrons and positrons to energies of 50 GeV in the same tunnel, before separating the beams and bringing them back together in order to collide at 100 GeV. In contrast, a future linear collider would accelerate electrons and positrons along separate tunnels that faced each other head on. Such a machine would measure some 30 km in length and produce centre-of-mass collision energies of the order of 1 TeV (1012eV).

According to current estimates, this energy would be high enough to create the Higgs boson, the so-far elusive particle that is thought by many physicists to endow all fundamental particles with mass. The Higgs is predicted to have a mass of about 100 GeV (see Net tightens round the Higgs boson). If LEP or the Tevatron do not see it first, the LHC will get the first glimpse of the Higgs, but an electron-positron machine will be needed to measure the most basic properties of the Higgs particle in detail, such as its decay width.

Finding the Higgs would plug a gap in the Standard Model and might point to new physics such as supersymmetry. In this theory, every known fundamental particle and force carrier would have a much heavier supersymmetric partner. Quarks would be partnered by “squarks” for instance, while W and Z bosons, the particles that carry the weak force, would be partnered by winos and zinos. Again the LHC and a linear collider could work hand-in-hand to put this theory to the test, between them having the potential to identify the supersymmetric partners of all the particles within the Standard Model.

There are currently three linear collider designs on the drawing board: one from a partnership between SLAC and KEK called the Next Linear Collider / Japanese Linear Collider (NLC/JLC); one from the TESLA collaboration, which is based at DESY; and one based at CERN called CLIC. Each of these has been, or is about to be, bench tested. However, since each proposal is likely to run to billions of dollars, the world is likely to fund only one design, if any. Like all major physics projects, the criteria for the successful design will be science and money. To do good science the machine will have to produce lots of high-energy collisions. And to be cheap the machine will have to be as short as possible.

All the designs rely on radio waves generated in cavities. The NLC/JLC will accelerate its particle beams to 1 TeV in conventional copper cavities. TESLA would operate at a slightly lower energy of 0.8 TeV, but would produce about four times the rate of collisions per unit area (luminosity). Part of the reason for this superior luminosity is TESLA’s use of superconducting cavities, which drain less power from the electric fields used to accelerate the particles within the cavities. But each superconducting cavity will be longer than its copper counterpart because of the longer-wavelength radio waves produced within these cavities. The result: a longer, and therefore more expensive, machine.

“We are convinced that TESLA has the highest performance potential for a future linear collider,” says Reinhard Brinkmann of DESY, who is in charge of TESLA’s collider design. “But there is a challenge too. Based on superconducting technology available about 10 years ago, the machine would be too expensive. The TESLA collaboration has set an ambitious goal of drastically reducing this cost figure.” Brinkmann says that a technical report, including cost and construction schedule, will be presented to the German government in 2001, at which point it will be evaluated by an independent body, the national science council. If the outcome from the council is positive, Brinkmann says that the TESLA collaboration would then seek international contributions to funding. If all goes smoothly construction will begin in 2003 and the machine will be built by around 2010.

The Americans already have a price for the NLC/JLC – and it frightens them. Last month the Department of Energy (DOE), which funds the bulk of high-energy physics research in the US, estimated that the NLC would cost $7.9bn, a figure which alarmed Congress. As a result, the Senate committee that funds the DOE voted to cut the NLC research budget from $12m to $6m next year. The outgoing director of SLAC, Burton Richter, says that if this cut goes ahead this will make at least a two-year hole in the NLC programme.

The memory of a costly failure still lingers – in 1993 the Superconducting Super Collider was cancelled in mid-construction after its costs soared to $11bn (Physics World November 1993 p5). A spokesman for the DOE says the US would be taking a major step backwards if it did not participate in a future linear collider. Senators will meet their opposite numbers from the House of Representatives this month to agree the budget. The House has already passed a bill that did not cut research funds.

Gregory Loew of SLAC says the Americans are working hard to make savings, estimating that costs may fall by 20%. He then hopes that a formal proposal for the NLC/JLC can be submitted to participating governments in about three years’ time. The plan is that construction would begin shortly after this submission and, like TESLA, the NLC/JLC should be completed in about 2010.

CLIC, on the other hand, is not so far down the road – construction would not start until about 2009. Ian Wilson, deputy study leader of the project at CERN, says CLIC takes a more futuristic approach. It would operate at about 3 TeV by extracting the power for the main beam from a secondary beam. “We believe that the CLIC two-beam scheme is the only way to go to multi-TeV linear colliders,” says Wilson. Because the design involves radio frequencies almost three times as high as those in the NLC/JLC, Wilson says that the accelerator would be considerably shorter than the other proposals. “CLIC could end up cheaper than the other designs, but until we have the figures to back this up we cannot say for definite.”

In addition to the design decision, there is also the question of where to site the machine. In the US, the two possible sites are SLAC or Fermilab, says Richter. In Europe, DESY is putting a strong case together, and local authorities are “very supportive”, according to Brinkmann. A central site at DESY could accommodate the experiments of both the collider and the free-electron laser that is part of TESLA. Japan is also certain to offer to host any global linear collider. In the end it may be the country that stumps up most of the cost that will host the machine.

Choice of design and site will involve complicated political negotiations, as Loew fully appreciates, and the state of the world economy will have a significant bearing on decisions. But he hopes that the physics remains at the focus of the debate. “If the Higgs is indeed found in the next two or three years, I believe the motivation for a linear collider somewhere will be very strong,” he says. “My personal hope is that at that point, an international linear collider will then be agreed upon to be built with the best and the ripest technology.”

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