Still early days in the 2012 data-taking run, which just started a couple of weeks ago, but already the Large Hadron Collider [LHC] accelerator wizards, operating the machine at 8 TeV of energy per proton-proton collision (compared to last year’s 7 TeV) have brought the collision rates back up nearly to where they were last year. This is very good news, in that it indicates there are no significant unexpected technical problems preventing the accelerator from operating at the high collision rates that are required this year. And the experiments are already starting to collect useful data at 8 TeV.
The challenges for the experiments of operating at 8 TeV and at the 2012 high collision rate are significant. One challenge is modeling. To understand how their experiments are working, well enough that they can tell the difference between a new physical phenomenon and a badly understood part of their detector, the experimenters have to run an enormous amount of computer simulation, modeling the beams, the collisions, and the detector itself. Well, 8 TeV isn’t 7 TeV; all of last year’s modeling was fine for last year’s data, but not for this year’s. So a lot of computers are running at full tilt right now, helping to ensure that all of the needed simulations for 8 TeV are finished before they’re needed for the first round of 2012 data analysis that will be taking place in the late spring and early summer.
Another challenge is “pile-up.” The LHC proton beams are not continuous; they consist of up to about 1300 bunches of protons, each bunch containing something like 100,000,000,000 protons. Collisions in each detector occur whenever two bunches pass through each other, every 50 nanoseconds (billionths of a second). With the beam settings that were seen late in 2011 and that will continue to intensify in 2012, every time two bunches cross at the center of the big experiments ATLAS and CMS, an average of 10 to 20 proton-proton collisions occur essentially simultaneously. That means that every proton-proton collision in which something interesting happens is doused in the debris from a dozen uninteresting ones. Moreover, some of the debris from all these collisions hangs around for a while, creating electronic noise that obscures measurements of future collisions. One of the questions for 2012 is how much of a nagging problem the increasing pile-up will pose for some of the more delicate measurements — especially study of Higgs particle decays, both expected ones and exotic ones, and searches for relatively light-weight new particles with low production rates, such as particles created only via the weak nuclear force (e.g. supersymmetric partners of the W, Z and Higgs particles.)
But I have a lot of confidence in my colleagues; barring a really nasty surprise, they’ll manage pretty well, as they did last year. And so far, so good!