munich

The European Laboratory for Particle Physics (CERN) in Geneva, Switzerland, has been urged to begin technical design work for the successor to its Large Hadron Collider (LHC), now being built and due to come into operation in 2005.

The SFr2.6 billion (US$1.7 billion) LHC stretches the limits of accelerator technology, and has an expected lifetime of up to 20 years. Its high energy of 14 TeV should allow it to detect the Higgs boson and supersymmetry. An internal report points out that its successor would need to operate at even higher energies. This would require the development of more powerful accelerating systems, or longer tunnels — both very expensive options.

The report was commissioned last summer by CERN's director general, Christopher Llewellyn Smith, and prepared by a small group of CERN particle physicists. It has now been presented to CERN's science programme committee. It suggests that future design work should focus on the development of cost-effective high-field magnets.

It identifies three possibilities for future accelerator concepts. The most studied, in the United States and Japan as well as at CERN, is a 5 TeV electron collider. A second option, a muon collider, is particularly challenging technically because of the rapid rate of decay of muons.

A third option would be a Future Large Hadron Collider (FLHC), a design concept for which has already been developed in the United States (see Nature 385, 471; 1997). This would use a large-circumference, narrow tunnel, built with technology used for laying sewerage pipes. The narrow bore of the tunnel would require the ring to be controlled robotically, rather than by technicians. An electron-positron collider could conceivably share the same tunnel, adding to the possible scientific payoff.

The report says that it is important for CERN to study these concepts now because the technologies will take decades to develop. It will also be many years before particle physicists know what questions remain to be answered after the LHC and other relatively high-energy colliders now being planned have yielded their secrets, and which new type of collider could best answer them.

Tunnel requirements would influence the decision. CERN could conceivably build a tunnel for a muon accelerator at its site, as the characteristics of muons mean that it would be relatively compact. A 30 km tunnel suitable for a linear electron collider could be located along the nearby Jura mountain range. But the report admits that CERN might find it hard to find a site for a tunnel to meet the requirements of an FLHC, which would need to be at least 120 km in circumference.