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
Tags: commissioning, LHC, startup
Seth,
Overbye got it from Physics Today, which attributed it to CERN, but actually got it from my blog. See
http://www.math.columbia.edu/~woit/wordpress/?p=2441
2kA gives 1.18 TeV, variously described as 1.1 or 1.2 TeV by different people…
In a compromise between physics and politics, we should clearly run at 1.97 TeV.
Everything I hear from within the CMS collaboration is that they will try to do 2.2 TeV center-of-mass collisions before the end of the year. Then, they’ll shut down most of January, and begin trying ramp up to higher-energy collisions.
7 TeV center-of-mass next year (2010) is optimistic – it will probably be lower.
But everyone has their own guess, and that’s all they really can be with something this complicated. Just guesses.
Perchance the full power test for LHC is not set for 21 December 2012, is it?
;-D
You can find the information in the minutes from the run meetings (at least in the case of CMS).
There is one important measurement needs to be done at collision energies lower than the planned 7 TeV and higher. It is measurement of the inclusive proton-proton cross-section.
The last few days I have some interesting discussions on how to measure luminosity at the LHC. While it is possible to use the LHC beam parameters to estimate the luminosity, this method has a large uncertainty on the order 10 to 20 percent. A better method which has been used in SPS and Tevatron is to use the counting rate from inclusive proton-proton collisions. The key ingredient to that is a precise measurement of the inclusive proton-proton cross-section, to be done at lower instantaneous luminosity. It is also preferred to do the measurement at a few values of collision energies (say at 2 TeV, 3 TeV, …) below the planned operational collision energy.
It will be a difficult task and while to some it may not appear as interesting or exciting as searches for new physics, new physics searches can’t proceed very well without precise determination of luminosity. A senior professor who was both at SPS and Tevatron even said that SPS had one experiment dedicated principally for this task.
A question that has been bothering me for a month:
There are two magnets in the ring that do not seem to want to go down to 1.8K
http://lhc.web.cern.ch/lhc/Cooldown_status.htm
One in Sector 7-8 and one in Sector 8-1. Will this have to be addressed? Can the beam circulate the whole ring with a magnet “stuck” at 10K?
Actually, to set the record straight, I got it from a source at CERN. For strange bureaucratic reasons, however, the item was originally posted on our website where no one could see it, and then several days later reposted on the blog referenced in Peter’s comment.