CMS Data Super Express

Study the W bosons produced in the Large Hadron Collider.


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Protons collide with incredible energy in the Large Hadron Collider. Sometimes the result of one of these collisions is a W boson - the particle which mediates the weak nuclear interaction. W bosons can be positive (+) or negative (-) in charge. But...

  • How often are W+ bosons made compared to W- bosons in the the LHC?
  • Should it be equal because nature has no preference?
  • Should we see all W+ because the proton is positive?
  • Can we find out?

Let's try.



  1. Learn a little about LHC, the CMS detector, and finding W candidate events. Look at the introductory slides.
  2. Make teams of two and divide the available CMS events among the teams. Each team also gets a report page.
  3. Characterize your events and count them:
    • W+ candidates have a single long red muon track which curves clockwise in the magnetic field of CMS.
    • W- candidates have a single long red muon track which curves anti-clockwise in the magnetic field of CMS.
    • W? events are too hard to measure so we cannot tell if they are W+ or W-.
    • Background events have more than one muon or are altogether different.
  4. Add them up on the report page. Then collect all the reports from all the teams and add up the number of W+ candidates and the number of W- candidates. Calculate W+/W-. If you like, you can use this spreadsheet.
  5. Find what the result was for CMS. How close was your group? Why do you think we get the answer we get? (Hint: it has to do with the structure of the proton,)


Next steps

  • Did you notice the mass in GeV on each event? We use these to find the mass of the Z boson in the standard CMS Data Express. Try it with your students!
  • Your students can do even more with a CMS or ATLAS masterclass. Talk with a physicist near you about International Masterclasses.
  • For a more intense data experience, try the CMS e-Lab.
  • Have questions about this activity? Send an e-mail and ask.