Question 25 on yesterday's list was how big physics can continue to advance in an affordable way. Particle physicists agree that their top priority for their next major project after the LHC, now under construction at CERN, is a high-energy linear electron-positron collider. The next issues are how high an energy is desirable, attainable or affordable. An International Technology Recommendation Panel (ITRP) recommended last summer: http://www.interactions.org/cms/?pid=1014290 that, if the international linear collider (ILC) is to have an energy up to 1 TeV in the centre of mass, it should be based on superconducting technology. Such a machine would certainly have interesting physics at the top-antitop threshold, and very likely exciting Higgs physics.
However, before the start-up of the LHC, we probably will not be sure of the Higgs mass. Also, we will probably not know whether there is new physics beyond the Standard Model of particle physics, let alone what form it might take (supersymmetry or ...?), still less what its energy threshold might be, before data from the LHC are analyzed. For this reason, while supporting the effort towards a TeV linear collider, the major particle-physics funding agencies have decided to wait for the first results from the LHC before committing themselves to building it.
In parallel, the agencies have also endorsed plans by CERN to accelerate R&D on a project called CLIC: http://clic-study.web.cern.ch/CLIC-Study/, which aims at a much higher acceleration graident for the electrons and positrons, and would hence be capable of achieving a higher energy in the same length of tunnel. Whatever the LHC finds, it is likely that particle physicists would want to explore electron-positron collisions at multi-TeV energies. Therefore, if CLIC is feasible, it might well be a next step after a lower-energy machine. And, if the LHC finds no exciting new physics below the TeV scale, one might consider proceeding directly to CLIC.
Yesterday I attended a meeting at CERN of agencies and institutes (interested in) participating in the CLIC R&D effort, including representatives from China, Japan, Pakistan, Russia, Turkey and the US, as well as several CERN Member States. The plan is to demonstrate the basic feasibilty of the innovative CLIC technology by 2009, around the time when the LHC may be telling us about new physics. CLIC may turn out to be a useful insurance policy if the LHC finds that exciting new physics only lies beyond the TeV scale.
CLIC is based on a novel technology in which an intense low-energy electron beam is used to generate an electromagnetic wave that is used to push a lower-intensity beam to much higher energies in a relatively small distance. It seems to be the only realistic chance of colliding electrons and positrons at multi-TeV energies so, if it works, it will allay (at least for a while) some of David Gross's concerns about the prospects for future big physics projects.
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