It’s Labor Day weekend here in the US, but over at CERN it’s the end of the August technical stop for the LHC. To rework a common saying, this is the first day of the rest of the 2011 run. We have two months left of proton-proton collisions, followed by one month of lead-lead collisions, and then in December we’ll have the holiday “extended technical stop” that will probably extend to the spring.
We’re expecting an important change in running conditions once we return from the technical stop, and that is a change in how the beams are focused. This will lead to an increased rate of collisions. Remember that the proton beams are “bunched”; the beam is not a continuous stream of particles but bunches with a large separation between them. The change in the focusing will help make the bunches more compact, and that in turn will mean that there will be more proton collisions every time a pair of bunches pass through each other. When our detectors record data, they record an entire bunch crossing as a single event. Thus, each individual event will be busier, with more collisions and more particles produced.
This is good news from a physics perspective — the more collisions happen, the greater the chance that there will be something interesting coming out. But it’s a challenge from an operational perspective. We try to record as many “interesting” events as possible, but we’re ultimately limited by how quickly we can read out the detector and how much space we have to store the data. Given that we’re going to have more data coming into fixed resources, we’re going to have to limit our definition of “interesting” a little further. The busier events are also a greater strain on the software and computing for the experiments (which I focus on). Each event takes more CPU time to process and requires more RAM. Previous experience and simulations give us some guidance as to how all of this will scale up from what we’ve seen so far, but we can’t know for sure without actually doing it. (The original plan for the machine development studies period before the technical stop was supposed to include a small-scale test of this, so that we could put the computing and everything else through its paces. But that got cancelled. I had originally planned to blog about that. Oh well.)
However, all of this will be worth the trouble. Remember all of the excitement of the EPS conference? That was at the end of July, just a little more than a month ago. There is now about twice as much data that can be analyzed. With the increases in collision rate, we might well be able to double the dataset once again just in these next two months. Or, we might do even better. This will have a critical impact on our searches for new phenomena, and could allow the LHC experiments to discover or rule out the standard-model Higgs boson by the end of this year. Coming soon, to a theater near you.
Tags: computing, Higgs, LHC, luminosity
