Last week Fermilab hosted two workshops on the so-called Project X proposal for building a linac designed to produce a high-intensity proton beam. The first workshop dealt with issues surrounding the proposed accelerator itself, the second with the physics that it might be able to investigate. Project X is being discussed in the context of an increasing realization that prospects for the ILC getting approved and built anytime soon are slim, so the US particle physics community in general, and Fermilab in particular, need to have a viable plan B for what they will be doing during the next decade. DOE secretary Ohrbach, in a recent talk at Fermilab made it clear that he thinks the ILC project is still at the stage of an R and D project, not yet near the point where a decision about it can be made and a full engineering design developed. For commentary about this from Barry Barish, director of the ILC project, see here.
One argument for Project X is that it would help develop some of the linac technology needed for the ILC, but the main arguments for the machine revolve around a striking change of direction for US particle physics, from the use of colliders to do experiments at the energy frontier to fixed-target physics at lower-energies. In some ways this would be a return to the older style of particle physics experiments that was the norm before the era of colliders. The point of Project X would be to produce a beam capable of being used to generate more intense beams of neutrinos that could contribute to neutrino physics, and to do what is now often called “flavor physics”. This is the study of phenomena involving heavy quarks and/or rare decays, with the hope of seeing beyond the standard model effects that occur not in lowest order approximation, but in higher order contributions to decay rates. There are quite a few decays that one can look for that either can’t occur at all in the standard model, or only can occur at unobservably small rates. An observation of such a decay and measurement of its rate would provide evidence of new physics. Many such studies already conducted provide strong bounds on quite a few possibilities, so one can imagine competing with colliders such as the LHC to either rule out or find new TeV-scale physics by doing this sort of experiment.
One interesting document to read about this is the account of a panel discussion on charm physics that occurred this past August. A participant emphasized how history has recently been running against the people working on flavor physics, telling the following story:
… over lunch we were talking about the future of the field, and I was drifting off, and ended up in a fantasy world where things were done the right way. And in this world the LHC was in fact built and came on the air, and found the Higgs, and found many new events that we couldn’t explain with the Standard Model. And people had realised that in order to interpret these possible signals of new physics, we would also have to have flavour physics studies of rare phenomena, so that we could start to see patterns emerging… and working symbiotically together, the LHC and the flavor sector would get to the root of what was happening, something that would be very difficult if not impossible to do with the LHC alone.
But then I woke up. And I thought about a colloquium I’d given recently, where one of the chief experimentalists there took me into his office and shut the door and said to my face, “Flavor physics is dead!” and apparently he’s not the only one who said it: some pretty important people have said it. And when something like that is said over and over it begins to have a truth of itself.
Deciding whether Project X makes sense will require figuring out exactly what kinds of experimental results it will make possible that would not be possible using existing or currently planned facilities. For more about this, see the introductory and wrap-up talks by Joe Lykken and a talk by Jon Bagger that summarizes the issues well. The workshop also featured an excellent talk by Michelangelo Mangano summarizing the current situation of particle physics, emphasizing what it might be possible to learn through other means than the LHC, which is what is getting almost all the attention these days. He pointed to the activities of the CERN Working Group on the Interplay Between Collider and Flavour Physics that are documented at this web-site.
Update: Alexey Petrov was at the Project X workshop, and has a very interesting posting about it.