ATLAS Data Express

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ATLAS Data Express is a short particle physics masterclass investigation that can be used as part of a workshop or as a short class project. Participants examine static displays of a limited number of events. The main goal is to separate Z candidate events other events by visual inspection and then create 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 we will study are "dimuon"  events; the Z can decay into a muon-antimuon pair. Z candidates are identified by 2 long muon tracks; each will appear as a combination of a short blue track in the inner detector (inner black ring) and a longer orange track in the outer muon chambers (blue rings). Participants will search for Z candidates in the data.


Individual or pair:

  • Participate in analysis prep seminar (slides) (ATLAS animation)
  • Open the event display file
  • Go to set of events assigned
  • Categorize and record each event as
    • Z → μ+μ- candidate (2 distinct muon tracks),
    • background (anything else).
  • For each Z candidate, note and record
    • the mass, rounded to nearest odd number (found at upper right of event) 
    • whether it is an electron or a muon event.
  • When finished, count how many instances of each odd number you have recorded.


Use your own resources to

  • Combine numbers of "odd masses" in all groups.
  • Create a histogram for whole group to observe.
  • Analyze other aspects of the data (optional).

Help: Use the Google spreadsheet.


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.