On Monday at 6 AM CERN time, the LHC ended its collisions of protons for 2012, and in fact until 2015, when the “long shutdown” for energy upgrades is completed. [There will be heavy-ion collisions in early 2013, but the details are beyond my expertise.] Here’s what appeared on the LHC status screen:
It’s the end of an era for the LHC, the end of what we might someday call “Run 1”, the period of first beams at (relatively) low energies, when we got our first glimpse of a new energy scale, and gathered just enough data to see the first glimmerings of (maybe) the Higgs boson. Considering how long we waited for the start of Run 1 — nearly twenty years from the first concepts for the the LHC and its detectors — it is rather amazing that we’re now at the end of the run, after a mere three years.
Still, it’s been a great three years. Here is the plot that captures the whole story:
This is the integrated luminosity recorded by CMS, essentially the number of collisions that the experiment observed, year by year. Remember all the excitement of the first data in 2010? That turned out to be a tiny amount of data compared to what we have recorded since; while we made very good use of it at the time, it was just hinting at the future success of the LHC. And even after the great advances in 2011, by the start of June 2012 we had recorded more data this year than we had in all of last year. Once again, all the experimenters thank the LHC team for the excellent performance of this still new machine.
The last few days of the proton run were spent looking towards the future. Since there won’t be any more proton collisions for two years, it’s important to do some tests that can guide our thinking about how to operate the LHC in 2015. So far, the LHC has run with “50 ns bunch spacing”; that is, the minimum time between bunch crossing is 50 nanoseconds. (Remember, the LHC beam is not continuous, but “bunched”, with a large number of protons close together in the beam, followed by a 50-foot gap before the next bunch in the beam.) This week, the LHC experimented with 25 ns bunch spacing, and even allowed the experiments to take a little bit of data in this mode on Saturday night and Sunday morning. Obviously, with the shorter bunch spacing, you can have beam collisions happening twice as often, and that means that you could potentially achieve the same total number of collisions with fewer protons per bunch. That’s good for the experiments, as each event that we record will have fewer collisions in it, making it easier for us to reconstruct what went on. With 25 ns spacing, we’d probably need less computing capacity and calibration and the like would be easier. But from the accelerator perspective, it is easier to operate the LHC with 50 ns spacing, and the machine operators can’t guarantee that they could provide as much integrated luminosity at 25 ns spacing as the could with 50 ns. Thus, it was important to take some time to understand how to operate the LHC this way. It’s ultimately up to the LHC managers to decide what the best mode for operations is. From the experiment side, it would be easier for us to have 25 ns spacing, but we wouldn’t want to do that at the cost of less data, and perhaps missing a chance of a discovery as a results.
Meanwhile, what does a 3000-member collaboration do with itself when there is no data to record? (Besides sending and reading email.) Quite a lot. First, there are a number of upgrades, repairs and improvements to be made on the detector in the next two years. There is a carefully choreographed dance to be performed in the collision hall, where the CMS detector must be opened up for access to the different components, and the schedule for all the work to be done could be pretty tight. There are also preparations to be made for how we analyze the data in 2015. The environment will be a lot like in 2010: we’ll be at a new beam energy, and in a physics environment that we’ve never seen before, so we’ll have to be ready for anything that might appear in the data. And we will continue our studies of the fabulous three years of data already recorded. During the past three years, the collaborations have released multiple papers on particular topics, with increases in the amounts of data analyzed each time and improvements in analysis techniques. But the next round of papers will use the full dataset, and there won’t be any “next” papers. The analysis techniques then must be the best possible; there won’t be another shot for improvements, as the next word will be the final word, at least until 2015. This too will take a lot of effort from the scientists.
Congratulations to everyone on a successful Run 1, and let’s look ahead to a busy shutdown and an exciting Run 2 beyond!