LHC Update: Chamonix

This week the operations groups of the Large Hadron Collider are holding their annual workshop in Chamonix to determine how to run the collider during 2012 and beyond.   Many technical slides are up giving us a good indication of the status of the machine with most of the winter maintenance completed.

Last year they were expected to produce 1/fb of integrated luminosity and they gave the experiments 5/fb, so this year we reward them with an expectation of at least 15/fb, no pressure. According to Lamont, the tentative running parameters include a beta* of 60cm in ATLAS and CMS compared to 1m last year. This means a tighter squeeze at the collision points and potentially a 60% increase in luminosity, so they are already half way to the target. The rest depends on stability and the time they can keep it in stable beams. Last year 50% of fills lasted less than 3 hours and turnaround times were dominated by machine availability. Improving the efficiency will be key to getting more luminosity.

Will they stick with 50ns switch to 25ns to double the number of bunches? This wont double the luminosity as you might expect because the injector has to split the bunches resulting in lower intensity. They will also be limited by beam induced heating and emittance. Overall the smaller bunch spacing may decrease the luminosity but it would also provide some much wanted relief from the effects of pile-up in the experiments. There are other downsides to consider. The increased effects of the e-cloud at 25ns means lower luminosity lifetimes. To mitigate this problem they will need a lot of scrubbing runs taking about two weeks of machine running time compared to 2 or 3 days if they stick with 50ns. Will the extra uncertainty that this implies rule out 25ns operations for this year? See the talk by Rumolo for more details.

Another decision concerns the beam energy. Last year they ran at 3.5TeV per beam, but this year they tentatively hope to increase this to 4 TeV. That would be great news for physics because it increases the discovery potential at the higher masses where new particles may be waiting to revolutionize our knowledge. Whether they can run safely at the higher energy this depends largely on the results of tests on defective busbar joints. This will be reported tomorrow so look out here for an update.

In July the biannual ICHEP conference is to be held in Melbourne. It is the biggest particle physics conference on the planet and it would make a big splash if they can produce some good results by then. The best hope is for a successful update on the Higgs search which would require doubling the total luminosity to add another 5/fb by mid-June.  It’s not impossible, but they have already lost a week of runs due to a problem with RF in CMS as reported in the CERN Bulletin. The start of beam operations is now scheduled for 21st March.

Update 7-Feb-2012: Today’s talks indicate that 4TeV per beam will not present risks any larger than those accepted for 2011. this is due to the reduced number of quenches and results of checks on the splices. See this talk in particular. Final decisions are not yet in and as we saw last year nothing is settled until then.

3 Responses to LHC Update: Chamonix

  1. carla says:

    I’m betting on 8Tev, 25ns, 10/fb for 2012 😉

  2. Chris Austin says:

    Could you explain in a nutshell what beta* is, and why it is measured in metres or cm? I take it that a better squeeze means a reduced beam cross section at the interaction point, but if so, why not just state the beam width at the IP?

    • Philip Gibbs says:

      beta* is defined as the distance away from the interaction point where the beam is twice as wide as it is at the interaction point. More useful is the formula w^2 = \epsilon \beta^{*} where \epsilon is the beam emittance and w is the beam width at the interaction point. beta* and emittance are controlled separately so it makes sense to talk about them rather than the beam width, but luminosity is inversely proportional to the width squared.

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