Beam Day at the LHC via Cosmic Variance by John on 8/25/08

September 10 is looking more and more like Beam Day for the LHC at CERN. The entire ring is now at superconducting temperatures, which means all magnets can in principle be energized.

Today CERN announced that the final "synchronization test" was a success, injecting beam from the older Super Proton Synchrotron into the LHC, where it was guided a few kilometers through the LHC vacuum beam pipe. (I also heard a story at Fermilab last week that on at least one occasion, while performing controlled beam oscillation tests, they oscillated a bit too much, causing some beam to enter one of the magnets, causing it to quench, that is, go from the superconducting to normal conducting state. This causes a great mechanical stress on the magnet, for which it is designed, but which you'd like to minimize. It won't be the last [WINDOWS-1252?]time…)

So then what is Beam Day? It is foreseen as the day on which they will attempt to run the entire LHC and injection complex, and get beam to circulate stably in the accelerator. My understanding is that they will attempt to circulate in both directions (the LHC is really two accelerators in one) at the energy with which the protons are injected, 450 GeV. If successful, there will ensue a several week period of studies, finding all the idiosyncrasies of the machine. The goal is to make sure that when, hopefully in October, they crank the energy up, the proton beam bunches will remain stably orbiting on their nominal axis. During this period there may be brief periods when the beam bunches collide. This will give a much needed first glimpse of actual collision data to the experiments (but not a glimpse of any kin of new physics) and help us start to shake down the detectors.

I believe that the plan is still to accelerate in October to 5 TeV and collide with a center-of-mass energy of 10 TeV, five times that of the Tevatron. If things go really well, and we get a reasonably significant amount of collision data at those energies, and the experiments work at a basic level, we'll get a great start on getting the detector alignment and calibrations done.

Could we see new physics with 10 TeV data? A safe answer is "probably not" but, to me, that means there is at least a tiny chance that if nature has something really striking in store for us at high energies, we might see it. For example, even with poorly calibrated and poorly aligned detectors, if there is a new resonance at very high mass which decays to pairs of quarks, then we might see a "bump" (oh no, not bump hunting again!) in the mass spectrum.

In fact it's not really even possible to say whether such a thing is "likely" or not (Sean's earlier musings notwithstanding) since it will either be there or not.

If it's there, though, we will see it, and we never would have before. With more energy and more data next year we can look for more and subtler effects, any of which could profoundly change our view of space and time, energy and matter. That's what makes this such an exciting time, after two decades of planning and building and preparing we're finally going to get to see what we never could before.

If we re going to mortgage our children's future, let's mortgage it on things like the LHC.