När smäller LHC? (När startar de upp (igen) egentligen?)

Today at 14:07
It was reported last week by Dennis Overbye at the New York Times that the LHC is only going to reach a center-of-mass energy of 2.2 TeV (i.e. an energy of 1.1 TeV per beam) before the winter shutdown. I was asked about this previously, and at the time I thought it was “in a schedule somewhere.” After looking around, though, it’s much less clear to me where the information actually come from — maybe I heard it from people who had read the New York Times blog, and maybe Overbye originally learned it from magical time-traveling Higgs Bosons! So we might have to demote the whole thing to the category of rumor — but it’s a rumor that appeared in the news, so I can certainly say what I’d think about it if it were true.

If indeed the LHC only achieves an energy of 2.2 TeV by the time it’s shut down in mid-December, some might be tempted to characterize it as a serious setback or defeat; in fact, it’s nothing of the kind. Here’s what’s really going on: the LHC is in the middle of an ongoing start-up process, and has to take a quick break in December and January, but will then pick up where it left off. That means that the point the accelerator startup happens to reach before shutdown doesn’t mean anything special at all — the really important thing is where it gets to when the process continues next year.

What we do know from CERN is that there are three stages of LHC magnet commisionning: to 2000, 4000, and 6000 amps of current. We also know that reaching 2000 amps “allow[s] the passage and guidance of beams at about 1.2 TeV” (which sounds close enough to the 1.1 TeV figure to be rounding uncertainty). So if there were only time to commission to 2000 amps before the end of the year, that could certainly explain the limited beam energy.

Is running at 2.2 TeV good for the physics program? Oddly enough, if we only have a few days of running, there are two lower energies that would be more fun for physicists. The best option might be to continue at the energy at which the center-of-mass energy achieved by the previous accelerator stage, 0.9 TeV. This is the energy that the first collisions will happen at, and longer running at a single energy would let those of us who work on the detectors get a better handle on how our data looks. (For example, I could attempt to do an early, quick version of my track jet analysis. In fact, I’ll try to do that no matter what; it will be good practice, if nothing else.) Another option might be to run at exactly 1.96 TeV, which is the energy of the Tevatron accelerator at Fermilab; that would give us a rare chance to look at the differences between proton-proton and proton-antiproton collisions at the same energy.

But the physics program isn’t the top priority this year, it’s getting the LHC fully up and running. Whatever the rumors say, we don’t yet know how far accelerator commissioning will get this year. Even 2.2 TeV would be enough to make the LHC the highest-energy collider in the world, which is an accomplishment to be proud of. No matter what, there will be much more to do next year, and we can start making discoveries! — Seth

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