An international group of particle physicists based at the DESY laboratory in Hamburg, Germany, has staked its claim to host the world's next major accelerator by unveiling plans for a Euro 3.136bn (about $2.8bn) linear collider. Last month the TESLA collaboration revealed its design for a 33 km long collider that would smash electrons and positrons together at energies of 500 billion electron volts (0.5 TeV) to probe physics beyond the Standard Model. The machine would also incorporate a powerful X-ray laser to carry out research in condensed-matter physics, chemistry, biology and materials science.

TESLA is one of three proposals for a next-generation linear collider, but is the first to be fully costed and made public. The Stanford Linear Accelerator Centre in the US and the KEK laboratory in Japan are collaborating on R&D for two similar designs - the Next Linear Collider (NLC) and the Japanese Linear Collider (JLC), while CERN is working on a design called CLIC (Physics World September 1999 pp8-9).

Different approaches

Most particle physicists believe that a linear collider is the obvious machine to complement the 14 TeV Large Hadron Collider (LHC), which is due to come on line at CERN in 2006. By colliding point-like particles, a linear collider would be able to make precise measurements on new particles discovered in the higher-energy collisions between protons, which are composite particles, at the LHC. These new particles could include the Higgs boson and more exotic "supersymmetric" particles not predicted by the Standard Model.

But at several billion dollars each, the world will almost certainly pay for only one linear collider. CLIC would operate at the highest energies - at about 3 TeV - but it would not be ready to be built until well into the next decade. That leaves TESLA and the NLC or the JLC as the most likely candidates, since all could be constructed by about 2010.

The German Science Council, Germany's most senior scientific advisory body, will now review the TESLA technical-design report and is expected to present its conclusions to the federal government by 2002. Albrecht Wagner, director of the DESY lab and chairman of the board of the TESLA collaboration, says the government has "expressed an interest in hosting the collider, provided that the international community is behind the project and the technology is in place".

Unlike the other designs, which would use conventional accelerator technology, the TESLA machine would be made from superconducting cavities. At 33 km long this limits the collider to a maximum energy of 0.8 TeV - less than the 1 TeV that the NLC or the JLC could reach in theory - but the collision at TESLA rate would be higher.

The TESLA design report states that the "advantage of superconducting technology, combined with the high efficiency to convert electrical energy to beam energy, has been acknowledged from the very beginning of the R&D on linear colliders". Superconducting cavities, however, have always been considered prohibitively expensive. But the TESLA collaboration has succeeded in reducing the cost significantly over the past decade, and the cost of the collider is now comparable to the other designs. The NLC would cost about $4bn, but it would not include a free-electron laser. TESLA's detector would cost Euro 210m, with the X-ray laser laboratory adding an extra Euro 531m.

Researchers at DESY have operated a 300 m prototype of TESLA for more than 8600 hours. "TESLA has worked in a test environment, so we can put forward a proposal where we can be confident that we can build a machine tomorrow if we were given the money," says Wagner.

Dave Burke, the NLC project leader, believes that both the TESLA and the NLC/JLC designs are "very strong", and says that the American particle-physics community "must now put its cards on the table" by endorsing the concept of a linear collider. This backing will probably come in the Autumn when a high-level panel of particle physicists presents its conclusions on the future of the discipline (Physics World March p9, print version only).

Burke says that until now US particle physicists have maintained that any linear collider should operate at 1-1.5 TeV in order to fully complement the LHC, and they have not given their support to a 0.5 TeV machine. "But the panel may not take this view in the light of the low Higgs mass tentatively discovered at CERN," he says. He adds that there are still technical problems to be overcome if the NLC is to reach energies of 1 TeV or more.

The International Committee for Future Accelerators is carrying out a comparative study of the different designs and is due to publish a report by the end of the year. Both camps are confident that the ICFA will endorse their designs, but the decision about which proposal to fund and where to build any linear collider may simply come down to politics. In the end, says Burke, it could be the government that puts up the most money that hosts the machine.

A global venture

Wagner believes that some of the political heat about where to build the machine, if such a machine is to be built, could be removed by modelling it on the so-called "Global Accelerator Network". Devised by the ICFA, this is a blueprint for any future large scientific facility. "Projects of the size and complexity of TESLA should be truly international," says Wagner. "The idea behind the network is to ensure that all the key players have a stake in any future accelerator. Each country or lab would be responsible for building and running a part of the machine and so the site selection should become less of an issue." He adds that there could be three control rooms for a linear collider: one in the US, one in Europe and one in Japan. "This would mean that each region is involved in the day-to-day running of the machine - and they would have no more night shifts to cope with."

Burke underlines the importance of building a linear collider, of whatever variety, rather than choosing a particular design. But he admits that American particle physics would suffer if the US could not host a major facility in the future: "Scientists thrive on excitement. Personally speaking, in order to attract high-energy physicists we need to have a frontier machine on our soil."