At the start of each calendar year, the CERN management holds a workshop in Chamonix to discuss the LHC run plan for the coming year and beyond. This year’s meeting was held two weeks ago, and this past week CERN announced the outcomes. Now, after last year’s Chamonix, the plan came out differently than many of us had been expecting. But this year’s workshop results were consistent with this year’s rumors.
There is a clear physics goal for this year: both CMS and ATLAS should each individually be in a position to discover the standard-model Higgs boson, if it exists. There are two ways that the LHC will try to make this possible. The first is to deliver as many collisions (i.e. as much integrated luminosity) as the LHC can manage. The integrated luminosity target for this year is fifteen inverse femtobarns for each experiment, three times as much as was delivered last year. It will still be a challenge to discover the Higgs with that much data; the experiments will have to run efficiently and the experiments will have to be as clever as ever. But it is possible. CERN is also prepared to extend the LHC run if necessary to meet this luminosity target. This is important, as the LHC will enter a long shutdown after 2012, so this year is our last shot for a while at making a discovery, of a Higgs or anything else. We should remember that last year’s target was a mere one inverse femtobarn, yet we got five times that. Can we hope that the LHC will outperform expectations again this year?
The second way to improve our chances of discovery is to raise the energy of the beams. The production rate for the Higgs and many other hypothetical particles increases with beam energy. Thus the LHC will run with 4 TeV per beam rather than the 3.5 TeV of last year. The operational experience of the past two years gives the LHC physicists confidence that this beam energy will be safe for the machine. This means that the LHC will probably never run at 3.5 TeV/beam again; the data we have recorded will now be unique in human history. It means that we’ll have to think about how to juggle resources so that people can look at both the old and the new data, and how to properly archive it for future use. Also, all sorts of measurements that we have made before at the LHC become new again: we can ask how does the production rate for phenomenon X change as you change the beam energy from 3.5 TeV to 4 TeV.
One change that the experiments had hoped for, but will not come to pass, was a change in the time interval between collisions. It was 50 ns during 2011, and it will stay that way. That means that we are now expecting an average of 30 simultaneous proton-proton collisions per beam crossing. Had the bunch spacing been reduced to 25 ns, we could have hoped to record a similar amount of data, but with much simpler events. However, the LHC experts weren’t sure that they could provide as much integrated luminosity at 25 ns spacing as at 50 ns; it is a very different way to operate the machine. Integrating data is the need for the year, so 50 ns it is. The experiments have shown that they can handle the complex events, although it would be a stretch to call it a pleasant experience.
Finally, the plan for the longer term was sketched out. The LHC will enter a “long technical stop” (as CERN likes to put it) at the end of the year, which will go on for twenty months. Given that we’ll need some time afterwards to re-commission the accelerator and the detectors, it’s probably two years from “physics to physics.” This will give the machine and the experiments time to implement some needed and useful upgrades and repairs. On the machine side, this includes the preparations to run the LHC at much closer to the design energy. That is 7 TeV per beam, although it is sounding like 6.5 TeV/beam is much more likely to be the safe operating point. At this point, we can only guess what the particle physics landscape will look like, but a higher-energy LHC will allow us to explore it thoroughly.
That’s the plan — let’s get ready to re-start the search for new physics in under two months!
