CMS Data Express


CMS Data Express is a short particle physics masterclass investigation that can be used as part of a workshop or as a short class project. The main goal is to separate Z candidate events from other events by visual inspection and then create a mass plot for the Z boson.

The Z boson is important in LHC discovery science and as a marker for calibration of LHC detectors: it is a well-known particle, so the location and width of the mass plot give physicists a good idea of how the detector is performing. The Z candidate events are "dimuon" events; the Z can decay into a muon pair. Z candidates are identified by 2 long muon tracks. Participants will search for Z candidates in the data.

The W candidate events consist of decays into single muons and neutrinos. However, the neutrinos do not interact with the detector and hence leave no tracks or energy deposits; their momenta are estimated by a process of summing all the momenta in the event to determine what is "missing". Thus a W candidate appears as a single long muon track in the detector.


Individual or pair:

  • Participate in analysis prep seminar
  • Open the event display file
  • Go to set of events assigned
  • Categorize and record each event as
    • W+ candidate (one muon track deflected clockwise) 
    • W- candidate (muon track defelcted counterclockwise) 
    • W candidate (muon track not clearly deflected; charge cannot be determined)
    • Z candidate (2 distinct muon tracks)
    • Unknown 
  • For each Z candidate, note the mass, round to nearest odd number, record
  • When finished, count
    • how many instances of each odd number you have recorded.
    • how many W+ candidates you have.
    • how many W- candidates you have.


Use your own resources or the data combination spreadsheet (Download xls) to

  • Combine numbers of "odd masses" in all groups
  • Create a histogram for whole group to observe
  • Add numbers of all W+ candidates and all W- candidates; find ratio W+/W-


The histogram created by the group is a mass plot. Since the mass of any one type of particle is uncertain by nature and due to experimental uncertainty, it will have a distribution the peak of which is the experimental determination of the mass. Creation of mass plots and other histograms are the central measurements made in the CMS e-Lab but with many more events than used in this exercise.

The ratio W+/W- is a probe of the proton structure and a comparison to the performance of CMS over a much larger data set.