Your job is now to do a series of measurements of the Z boson mass, and from these measurements create 2 histograms which at the end will be combined with those made by other groups. HYPATIA features both an invariant mass calculator and histograms. To see how to use these, follow this link. You will use these tools together with the particle identification information from HYPATIA in this exercise.

Your collision images (event-displays) contain a mixture of Z events into electron-positron or muon-antimuon, but also completely different types of collision products, such as jets stemming from quarks or gluons and W bosons. (If you don't remember how these look like, go back to Identifying events). You should use your knowledge about particle identification and the HYPATHIA invariant mass (rest-mass) calculator to investigate whether a Z boson was created or not.

1. For each collision, enter in the tally-sheet whether you see a Z decaying to an electron-positron pair, to a muon-antimuon pair or whether the event was a background event. Look at each collision and use the pointing tools and particle information to determine whether a Z boson was initially created or not.
2. If you believe you see a Z boson, decide whether the Z decayed to an electron-positron pair or a muon-antimuon pair.
   
   Once you have identified the correct tracks belonging to the Z boson, insert the pairs of electron tracks or pairs of muon tracks into the HYPATIA invariant mass table. After this punch in the invariant mass into the results-spreadsheet, and mark whether this is an electron pair or a muon pair. In order to do a proper scientific analysis, do not omit invariant masses even though they seems too high or too low according to your knowledge on the Z boson mass
3. If you believe the collision resulted in a background event, do not enter the invariant mass of the particles into any histogram, only register it in the tally-sheet.

1. Compare the histograms of the electron-positron and muon-antimuon pairs.
   • Can you point out differences/similarities?
   • How often does the Z boson decay into electron-positron pairs? How often does the decay result in muon-antimuon pairs?
   • What did you expect? Why?
2. What is the most probable mass of the Z boson?
   • How does this compare to the world-measurement of 91.2 GeV/c²?
3. What could be the possible explanations of why the distribution is so wide? Why is there not one exact value for the Z boson mass?
4. Have you discovered the Z' boson?
   • If you think so, what is the Z' boson's mass?
5. Why is it useful to combine your results with those obtained by other groups?

A videoconference takes place at the end of the International Masterclasses day. You will have the opportunity to interact with students from other cities and countries who have also analysed ATLAS (or CMS) data. You can compare their results with yours. To do this, you must save your invariant mass spreadsheet and upload it following instructions on this page.

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