Shortly after the August Quark Matter conference, the Large Hadron Collider geared up for a test run of proton on lead (pPb) collisions. Proton nucleus collisions are the intermediate stage between the short length scale proton proton physics and finite sized physics of lead lead collisions. These types of collisions are going to be the main focus of the January running period and this short test was meant to get us ready for what’s to come. One of the test fills, a few hour continuous running period from midnight to 6am on September 13, gave us about two million good proton lead collisions.
It was very exciting as this is the first look at these collisions at such a high energy, 5TeV center of mass, producing never-before-seen particle densities in proton nucleus collisions. We were curious not only to see what reference measurements we can obtain from these collisions, but also if there is any new undiscovered physics lurking around. Indeed last time we saw unprecedented particle densities in proton proton collisions when the LHC turned on two years ago our group discovered a surprise in the form of a “ridge” in two particle correlations. This “ridge” indicates that something is driving particles produced from these collisions which have very little time to talk to each other, i.e. they are causally disconnected shortly after the collision, to have some slight preference in which plane they are produced. To see this effect in proton proton we had to look at extreme rare collisions in which high enough particle densities were produced, picking out the top 300K of over 150 billion events.
So while we expected to see some kind of ridge since it’s there both in pp and PbPb collisions we didn’t quite expect to see a strong signal in just the two million events we recorded. However when we looked at pPb collisions with similar particle densities as those which showed a ridge in pp collisions, we saw a huge ridge correlation! The effect is the same as described above for the pp ridge. We have something driving produced particles to align to some sort of plane, except in a manner much stronger than observed in the high multiplicity pp collisions, so strong it resembles PbPb collisions more than pp. This generated quite a bit of excitement, the unveiling of the result at the Hot Quarks conference was met with applause. CMS posted the following article on this result, which was the top headline in the physicsworld magazine a few days ago.
This exciting news may be just the tip of the iceberg of what’s to come from the full proton lead run in January. The entire statistics from the test run that yielded this result, two million good collisions, is just one second of running time in 2013!