CMS Masterclass 2014 Documentation

This document in PDF form: Download here. 

CMS WZH Measurement 

Created by K. Cecire, T. McCauley, QuarkNet LHC fellows 
The CMS WZH measurement is supported for International Masterclasses. 
Languages supported: Chinese, Dutch, English, French, German, Hebrew, Hungarian, Italian, Japanese, Polish, Portuguese, Spanish 
  • Students use event display of (mostly) leptonic decays to determine
    • lepton ID (electron, muon),
    • likely particle ID (W, Z, zoo, Higgs),
    • charge if W, using curvature of electron or muon tracks,
    • and masses if Z or Higgs candidates (taken from spreadsheet).
  • Students use particle counts to find e/µ and W+/W- ratios.
  • Students create mass plots. They find the mass of the Z boson and a possible Higgs signal but also other peaks in the mass plot to reveal additional particles.
  • Students use the iSpy-online event display (or iSpy-dvd if there is insufficient internet bandwidth) and a Google online spreadsheet. Instructional screencasts are available for iSpy-online and iSpy-dvd (on DVD).
Dataset: There are 3000 dilepton events from CMS, made up of W, Z, J/Ψ, upsilon, background, and few Higgs (H) candidates. Only W, Z, H, and background are intitially revealed to students; other particles are discovered by them and explained by mentor. 




for hardware and software

  • Reliable, high-speed internet connection
  • Up-to-date version of Firefox, Chrome, or Safari
  • Masterclass DVD 2014
  • Firefox 6.0 or higher only
  • Spreadsheet (e.g. MS Excel)
  • Work out a system for data transfer from student level to institute level and then for transmission of results to moderators
  • Note: Moderators cannot show institute mass plot results in the videoconference. The mentor should share the institute mass plot through shared desktop. (In the Fermilab-based masterclass, there is an Indico page for the videoconference and the mentor can upload material such as a mass plot by means of a modification key.)

Outline of the day

This is not a final schedule but an outline for most U.S. institutes.
  • Arrival/Registration (~30 min)
  • Cloud chamber, e/m apparatus, or other "gateway experience" apparatus setup for students to inspect
  • Start and Ice-breaker activity (~30 min)
    • Students in small groups create 1-2 good questions about particle physics and/or LHC. Where practical, groups should be made of students from different schools.
  • Mentor presentation
    • Template (~60 min, including Q&A)
  • Tour of facilities/labs/cool stuff (~45 min)
  • Analysis preparation (1-2 teachers facilitate; ~60 min, including Q&A)
    • Presentation
    • CMS data analysis slides (beta)
    • ATLAS data analysis slides (beta)
    • Guided practice with discussion
  • Lunch with a physicist (~60 min)
  • Data Analysis (~90 min)
  • Pre-conference (~30 min)
    • Students and mentor discuss meaning of analysis results.
    • Students and mentor discuss questions for other institutes.
    • Revisit questions from ice-breaker, discuss which to ask in video conference.
  • Designated IT expert preps video connection.
  • Videoconference (~30 min)
    • Greetings
    • Presentation of results
    • Discussion of results
    • Q&A
  • Summing up and evaluation (~15 min)
Student tasks
  • Each pair of students analyzes a set of 100 events
  • For each event, distinguish between electron and muon decay and between W+, W-, Z candidate, Higgs candidate, and zoo.
  • Record into Google spreadsheet
  • Be prepared to discuss in Masterclass Institute and in videoconference; prepare good questions.


Student procedure

  • Pairs of students are assigned sets of 100 events to analyze in iSpy-online or iSpy-dvd.
  • Event sets are found in the event display ( by choosing the folder icon (top left).
  • A window appears. On the right side, students first choose "masterclass-2014" and then their event set (labelled masterclass_1, masterclass_2, etc.).
  • Students also open the Google spreadsheet for their masterclass, find the tab for their Institute, and then scroll down to the area dedicated to their event set (also labelled masterclass_1, masterclass_2, etc.).
  • Note: All institutes are strongly encouraged to use the provided Google spreadsheet rather than a local spreadsheet. The Google spreadsheet provided online is designed to facilitate transfer of results from students to mentors to moderators near-automatically. These are linked at the CMS Library page (scroll down to Data Analysis).
Open iSpy-online event display. The "mc" in dataset labels shown have been replaced with "masterclass" for
greater clarity.
Open Google spreadsheet.
Students use event display of (mostly) leptonic decays to determine
  • Lepton ID (electron, muon). This is to characterize the event, not a individual particles.
    • If the event has one muon track (long, red) or two muon tracks (actually likely a muon-antimuon pair) it is a single muon event.
    • If the event has one electron track (short, red) or two electron track (actually likely an electron-positron pair) is is a single electron event.
  • Likely particle ID (W, Z, zoo, Higgs)
    • W candidate appears as a single electron track or muon track and a missing Et vector (yellow arrow, always transverse to beamline)
    • Z candidate appears as 2 muons or 2 electrons; it is not always a Z. It may or may not have missing Et in the event.
    • Suspected Higgs (H); these are rare but noteworthy. Events show as
      • H→ZZ: 2 electrons and 2 muons, or 4 electrons, or 4 muons.
      • H→γγ: no electron tracks but 2 large energy deposits (seen as towers) in ECal.
    • Zoo events are "none of the above" but there can be interesting events among these.
  • Charge if W, using curvature of electron or muon tracks (it helps to zoom in and to use a paper straight edge)
  • Mass if Z or H candidate (taken from spreadsheet)
Adjust event settings to get x-y view for track curvature in case of W candidates.
Candidate W+ → e+ + νe .
Candidate W- → μ- + anti-νμ .
Candidate Z → e+ + e- .
Students enter all data as "1's" into the online spreadsheet in the Data tab for their Masterclass Institute (e.g. Data Padova). Each line is for an event and has an event number corresponding to event numbers in the iSpy-online event display.
  • The student enters a "1" in the line to indicate an electron or a muon event.
  • She or he enters another "1" to discriminate between W+, W-, W (unknown charge), Z cand, ZZ cand, or zoo.
  • Students should only enter "1's", not other numbers.
  • Totals are made at the bottom of each section of 100 events in the spreadsheet tab. (Students should work in pairs, with each pair assigned a computer and 100 events to analyze.)
  • Totals for all students in the Masterclass Institute are automatically transferred to the Results tab for the Institute. Institute totals are then transferred automatically to the Moderators tab for all Institutes sharing a particular videconference.
  • Place "1's" to indicate parent particle data. Totals appear after last line of set. Next set (masterclass_2) follows, with a new background color. See below.
Data entered into spreadsheet.
  • When a student enters a "1" for "Z cand" or "H cand", an invariant mass will appear in the line. The student should then
    • Round the mass to the nearest odd number value in GeV (e.g. 89.4 rounds to 89, as above).
    • Go the the Massplot tab for his or her institute (e.g. Massplot Manhattan, as below) and find the rounded odd mass in the massplot data table. The student enters a "1" just above that mass. If there are any "1's" there already, the student adds his or hers just above the highest "1".
  • The total of number of events at each mass is then automatically transferred to the mass plot in the Results tab.
Enter masses in the "Trapp plot" on the Massplot page. They transfer to a clean mass plot (appears in Results tab).
  • These numbers are also transferred to the Moderators tab for the mass plot for all Institutes sharing the videoconference.
  • There should be more peaks in the mass plot than just the peak for the Z boson.
  • Students have 60-90 minutes to analyze data. The mentor must reserve time for a discussion of results prior to the videoconference. This is more important than finishing the data analysis: once it is time for the all-hands discussion, student analysis must stop and the group should use whatever results they have at that point.

Presentation of results

The mass plot and summary of results are automatically built in the Institute Mass Plot tab and the Institute Results tab of the spreadsheet. The mass plot will show not only a Z peak but also peaks due to other particles. The results will show the numbers of electron, muon, W+, W-, W (unknown charge), Z cand, H cand, and zoo events plus the key rations e/mu and W+/W-. The mentor should project the mass plot and the count/ratio results and discuss their signficance with students. The mentor should also help the students to generate questions for the videoconference or other later discussion. 

Sample questions

In discussion, the moderator might ask students
  • How many peaks are there in the mass plot?
  • Where is the Z peak? What is the mass of the Z boson?
  • What do the other peaks mean?
  • Is there anything interesting in the 120-130 GeV range? What is it?
  • What is the ratio of electrons to muons? Is it close to what we should expect?
  • What is the W+/W- ratio? What should it be?
In the videoconference, studenta might ask questions like
  • Why are the widths or heights or numbers of peaks different from one Institute to the next?
  • Why do different Institutes get different ratios? How did they identify electrons or muons or W candidates or Z candidates or zoo events? How did they measure charge for W candidates?
  • Now that the Tevatron is shut down, what do you do at Fermilab?
  • Is it boring at CERN when the LHC is not running?
  • Why did you become a physicist?
...and better questions which only students can create. 


Fermilab-based moderators should refer to the FNAL Masterclass 2014 Moderators page.
All moderators should note the following.
As long as all groups use the Google spreadsheets, moderators do not need to look for uploads from any of the masterclass institutes or make histograms on their own. There is one Google spreadsheet for each set of masterclass institutes participating in a single videconference. Each institute has 3 tabs in that spreadsheet:
  • Data - students record W or Z candidates, e or µ events, Z candidates, and "zoo" events. Choosing Z candidates results in masses popping up, which student round the the nearest odd number.
  • Results - numbers of different types of particles are automatically transferred here; e/µ and W+/W- are calculated for the whole institute.
  • Massplot - when a student gets a Z candidate mass and rounds it to the nearest odd integral value, he/she goes to this tab to place a "1" above that value. This builds up a mass plot.
The totals and ratios from each institute are automatically transferred to the Moderators tab in the spreadsheet. Totals and ratios for all the participating institutes (as represented in the spreadsheet) are automatically transferred to the Moderators tab; a mass plot automatically builds from institute results.
Moderators should discuss the results from each institute and the overall results with the students, making sure to stay within time constraints.
If an institute does not use the online spreadsheet, the mentor for that institute should show their mass plot and results using Vidyo desktop sharing.


for students and teachers: