Right now the LHC is about to start a short run with proton-proton collisions at a center of mass energy 2.76 TeV. This is lower than what we ran last year and is a special request from the heavy ion physicists. So you’ve heard a lot about why the particle physicists want to go to higher energy. But why do we heavy ion physicists want to go to lower energy?
We want a reference for our lead-lead collisions. If nucleus-nucleus collisions were nothing but a bunch of proton-proton collisions, what we measure in lead-lead collisions should be just some constant times what we measure in proton-proton collisions. This is a bit simplistic, but it’s a pretty good start. A lot of our measurements use proton-proton collisions as a reference and look for differences between proton-proton collisions and lead-lead collisions. For instance, in the paper I discussed here we looked at the distribution of particles as a function of their momenta in lead-lead collisions and compared that to what we observed in proton-proton collisions. For this paper we used the data from proton-proton collisions at 900 GeV and at 7 TeV to extrapolate to what we’d expect at 2.76 TeV, the same energy per nucleon as our lead-lead collisions. As discussed here our models for proton-proton collisions are pretty good but they get some of the details wrong – and miss some features like this. Since we depend on models to extrapolate to 2.76 TeV, we have greater uncertainty in our measurements than we would have if we had data at 2.76 TeV. The LHC can go down to 2.76 TeV and what we need 2.76 TeV proton-proton data doesn’t require as many statistics (as many total proton-proton collisions) as what the particle physicists need to look for things like the Higgs. So we’re having a short run with proton-proton collisions at a lower energy because it will significantly help the heavy ion physics program. (We’ll also get some core physics measurements out of the 2.76 TeV proton-proton data, but like the paper I discussed here, these will refine our understanding but not dramatically change our understanding of proton-proton collisions.) I hope you’re as excited as I am!