The new year brought a new type of collision at the LHC: the accelerator smashed protons and lead nuclei together. Although we already caught a glimpse of these asymmetric proton-lead (pPb) collisions during a pilot run last September, the data collected early this year was the first sustained pPb run at LHC energies.
The high luminosity pp data taking aimed at precision measurements addressing the properties of the recently discovered higgs boson ended in mid december 2012 with a total integrated luminosity of 23fb-1. After three weeks of downtime the recommissioning of the accelerator and the CMS detector for pPb data taking started on January 4th during the annual CERN Christmas break. Commissioning the detector involves making sure all of the subdetectors of the CMS experiment are operating within their expected parameters. On 7 January, when everyone returned to CERN, cooling and power were up and running. Since the collision rate in the pPb run was lower than that in the pp one, two forward subdetectors that couldn’t withstand the radiation under the harsher conditions of proton collisions have been reinstalled in CMS: CASTOR and ZDC. These subdetectors will help measure the collision remnants that travel very close to the beampipe, crucial to the study of pPb data.
The biggest challenge in preparing the CMS experiment for pPb collisions was to configure the trigger system that selects the collisions to be recorded to mass storage. Compared to proton proton or lead-lead collisions we are looking at different physics processes happening at different beam intensities. This requires dedicated trigger menus to be developed by the CMS Heavy Ion group, which tell the data aquisition system how many events of which type (e.g events containing high pT jets or pairs of muons) to keep and which events to reject. When colliding lead nuclei, there are around 4000 interactions each second, of which around 200 are selected to be recorded to tape. The proton-lead collisions, on the other hand, happened at around 2,000,000 interactions each second, and CMS recorded around 1000 of these. This of course requires a carefull selection of events to ensure all future physics analysis will have a sufficient number of events to work with, while making sure not to exceed the available output bandwidth.
For this run, CMS also joined forces with the TOTEM experiment to cover a greater range of collision data. The two are essentially separate entities — independent experiments that use different analysis software — and they are fully complementary. CMS measures in the central region and TOTEM exclusively measures in the very forward region. Combining information from both allows us to perform a lot of physics studies that previously were impossible to do by correlating proton remnants seen in TOTEM with objects such as jets and Upsilon particles observed in the central part of CMS.
During the 2013 Ion beam period the LHC delivered 31.3 nb-1 of pPb collisions to CMS and also provided a short run of pp collisions at the center of mass energy of the PbPb collisions collected in 2011. The pPb data sample corresponds to about 60 billion collisions sampled by the CMS experiment and will now serve as a reference to the PbPb collision data.
In PbPb collisions a hot and dense system of strongly interacting matter is expected to be produced, which in turn we study by means of hard interactions produced inside the hot medium. Fast partons produced in these hard interactions traverse the hot medium and are modified by it, e.g. they lose energy due to the interaction with the medium. To perform precision studies to quantify these medium modifications it is essential to disentangle the effects on the production of these probes due to the initial state, i.e the parton distribution function of a highly Lorentz contracted lead nucleus, and the final state effect due to the presence of the medium. At this stage the pPb data comes into play, since colliding a proton with a lead ion will expose all initial state effects while the system size of the interaction in expected to be too small to form an extened volume of a strongly interacting medium. Apart from serving as a reference sample to PbPb collisions the analysis of pPb data is already starting to yield results that also make the study of these collisions a very interesting topic in its own right, like the recent analysis of two particle correlations that shows an unexpectedly large ridge like structure.
Going into the first long shutdown period of the LHC lasting until end of 2014 the CMS Heavy Ion group is now well equipped with a large PbPb collision data set and matching statistics pPb and pp reference samples. The quiet time during the LHC shutdown is eagerly awaited by all group members to dig into the wealth of data, produce exciting new physis results and further our understanding of nuclear matter at extreme conditions.
[Note about all event displays: The red and blue boxes show energy deposits from particles produced in the collisions in the electromagnetic and hadronic calorimeters, respectively. As the inner tracker was not activated during this run, charged particles could not be reconstructed as charged tracks.]