Beam Day at the LHC

By John Conway | August 25, 2008 1:40 pm

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 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.

  • Martin

    “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.”

    I see the frequentists haven’t completely died out yet, then…

  • John

    I guess I demand that anything we’d call a probability (or, colloquially a likelihood) is something that we can at least in principle measure. Is that reasonable? I now wonder whether we don’t all assign mental probabilities to things that we can never measure…

  • graviton383

    I see Fermilab is having a `pajama’ party on Sept 10 & similar other LHCfests are being held at Brookhaven and in San Francisco (by SLAC and Berkeley)…looks like fun.

  • Jason Dick

    I would tend to think that the primary problem with detecting new physics during this commissioning stage at 10TeV would be primarily an issue of luminosity, not necessarily an issue of the energy level reached. It’s still quite a bit above current levels, and also quite a bit above the levels that should be required to produce the Higgs. But won’t the primary obstacle simply be that the beam luminosity probably won’t get to the point where we’re capable of distinguishing new physics until around the time that they similarly ramp the energy up?

  • Martin

    John — yes, and the point about ‘mental probabilities’ is that if you want them to be consistent they actually have to obey the rules that govern probabilities, so it makes sense to talk about them that way.


  • Count Iblis

    If you see some weak signal that could be the signature of some particle predicted by some theory and you want to translate that signal into a probablity that this particle has indeed been detected, you need to apply Bayes’s theorem and then you do need the prior probability that this theory is correct.

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