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Seth Zenz | Imperial College London | UK

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What Now?

Good morning! I’m back at work here at CERN, and I can assure you that there is no pall of doom over the laboratory. Yes, it’s a bummer that collisions won’t happen for a while, but everyone I know still has plenty of work to do to get ready — heck, the only reason I even have time to blog is that I’m waiting for code to compile!

There are plenty of sources for what exactly went wrong, and how long it will officially take to repair; you can see some links in the updates of my last entry. The bottom line is that the needed repair is not a huge one, but it will be very time consuming because of the necessity of warming up the magnets to do it. Why do we need to warm the magnets up? Well, because they’re filled with liquid helium, and you can’t do much work on the magnets while the helium inside. And, as someone asked in a comment, why does it take so long anyway? Didn’t the magnets warm up by a hundred degrees rather quickly during Friday’s malfunction? Yes, they did, but they did it by venting a large amount of helium into the tunnel — and, although helium isn’t dangerous unless there’s so much of it that it crowds out the air, it sure is expensive. The accelerator experts need to slowly warm up, remove, and store the helium; this will save it for future use and prevent damage to the magnets.

So what are we going to do with the next few months? Well, no high-level decisions have been made, and obviously graduate students don’t get to vote on them anyway, but I doubt that there will be collisions in 2008. The old schedule was to slowly get the machine working, and hopefully achieve 5 TeV on 5 TeV collissions sometime in October. If everything went well, this would have allowed maybe a month of physics running before the winter shutdown. (The winter shutdown is CERN’s typical time to do maintenance because electricity is more expensive due to everyone using it for heating; accelerators in places with a lot of air conditioning often shut down in the summer for similar reasons.) After that, the plan was to have a long shutdown during which the machine would be prepared for full energy 7 TeV on 7 TeV collisions, after which it would come online again in Spring 2009. It doesn’t make any sense to shut down the accelerator for repairs, run it for a short while, and then shut it down again for upgrades — so I expect the planned work for the shutdown will begin in parallel with the repairs. Perhaps that means that the LHC will come online at full energy even a bit sooner than it would have otherwise, but bear in mind that that’s speculation based more on my hopes and guesses than on my (non-existent) accelerator-commissioning expertise.

For me and my colleagues working on the ATLAS pixel detector, there is a lot of work still to be done. Our sub-detector is now taking data, but we have a long list of things still to be achieved before it’s operating at its best. We have been doing our utmost to get things ready, but realistically, if the first full energy LHC collisions had been in October, there would have been more work to do: there would still have been a few pieces of our detector shut down because of electronics problems, and the accuracy of our measurements would have been reduced because we didn’t yet know the alignment between different parts of the detector very well. Obviously we would have welcomed that collision data, and used it to continue our improvements, but there was plenty more calibration and commissioning work to do over the winter shutdown. Now we’ll just do that work before we see first collisions instead of after, and hopefully we’ll be in great shape by the time the accelerator is back.

For me personally, the news is not a big setback. I had already decided (by coincidence, last week) that it would be better to stay at CERN and help with the pixel comissioning work in the winter and early spring, even if it meant forgoing the chance to use 2008 data to write my thesis. The downside of this decision was that it committed me to probably being in graduate school until 2011, for a total of seven years — but the upside was that I would learn more about the detector, and be able to do a more thorough job on my thesis as well. Because of the incident last Friday, it turns out that I didn’t really have a choice after all; but since I had already made the decision, it doesn’t feel like much of a loss.

But certainly this is bad news for a lot of people. Many graduate students and postdocs were counting on 2008 data, and they will now be spending quite a bit longer in their present positions than they had hoped, or making other difficult decisions. And everyone working in particle physics, or interested in particle physics, will now have to wait a few months longer to see what the LHC has in store.

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