The Linear Collider Will Be Cold

For many years now, the highest priority of experimental particle physicists for a next-generation accelerator project has been a new electron-positron linear accelerator. The last high energy electron-positron collider, LEP, reached a total energy of 209 Gev before being shutdown in 2000. To get to higher energies than LEP, a ring isn’t viable because synchrotron radiation losses go as the fourth power of energy, so a ring with much higher energies than LEP would use an intolerable amount of electric power.

A linear collider, where you build two linear accelerators and collide their beams together, doesn’t have the synchrotron radiation problems (although the electric power demands are still a problem since the beams you accelerate only can collide once, not many times like in a storage ring). There have been several competing designs for a linear collider, with one of the main difference in the designs being whether the RF accelerating structures are superconducting (“cold”) or room temperature (“warm”). These designs all are for a machine that would start out with a total energy of 500 Gev and ultimately reach 1 Tev. A committee was formed called the “International Technology Recommendation Panel” (ITRP), and it has issued a press release announcing its decision today. The ITRP came down on the superconducting side; this is a design mainly developed at DESY in Hamburg as part of the TESLA project.

The German government has decided to use the TESLA technology to construct a free-electron X-ray laser at DESY called XFEL. They have done this in a way that would allow XFEL to ultimately be upgraded to a linear collider at DESY, but have put off any decision about whether to actually fund and build such a machine.

The ITRP decision will allow work on a final design for the linear collider to begin, but the trickiest questions still lie ahead. Where will the thing be built and who is going to pay for it? The order of magnitude of the cost is $5 billion and the general assumption is that this will be an international collaboration. Besides the possibility of siting it at DESY in Germany, sites that have been discussed in the US mainly are at Fermilab in Illinois (being pushed by Fermilab), or somewhere in California (being pushed by SLAC). Even the most optimistic time scales for designing, funding and building a linear collider don’t have it running until late in the next decade. More realistic might be the mid 2020’s. The question of where the machine is located is crucial to the long-term future of the SLAC and Fermilab laboratories. If it is at their site or nearby they have an assured future, if not their future becomes much more problematic. CERN has its own design for an even higher energy linear collider called “CLIC”, but CERN’s funds for the forseeable future are committed to constructing and funding the LHC, as well as possible future upgrades of that machine.

There’s a bewildering array of web-sites with information about this, including the new International Linear Collider Communication and linearcollider.org ones, another one at SLAC, one at Fermilab (which seems kind of out of date..) and Michael Peskin’s home page. This last one contains links to many talks by Peskin about the physics to be done by a linear collider as well as a web page of links to other information about the linear collider.

The ITRP decision was announced at one of the year’s biggest high energy physics conferences, the ICHEP being held now in Beijing. The web site for that conference contains many talks giving the latest results from experimental groups around the world. Unfortunately, as far as I can tell there’s nothing very earth-shattering being reported.