CMS J/Ψ Measurement
This page uses the "old" iSpy-online. For iSpy-webgl, please refer to CMS Masterclass Documentation.

You will study decays containing muon pairs to determine which events are J/Ψ (charm-anticharm meson) candidates and then create a histogram of the mass calculations for the events that pass the "student trigger."


When a J/Ψ particle decays, it typically produces a pair of muons. Look CAREFULLY at the two events below. Is there evidence of muon pairs (red tracks) in either or both events? Could either be a candidate for J/Ψ? Is the evidence weak or strong? Are you confident of your conclusions?
An event must pass two tests before it can be considered a J/Ψ candidate. You will use a rating system to keep track of how confident you are in your conclusion. You and a partner will study 100 events. It will be up to students in collaboration with mentors and teachers to determine what weight to give each of the criteria and ratings. You will record your ratings in a data spreadsheet.

Test 1: Opposite charges

To be a possible J/Ψ candidate, the event must have two muon tracks of opposite electric charge.
  • If the two tracks curve the same direction inside the solenoidal magnet (nested just inside HCAL Outer), they have the same sign and thus do not come from a J/Ψ decay. (Based on this fact, what must the charge of J/Ψ be?)
Note: The x-y (end) view reliably displays particle charge and other views can be deceptive for this.

If the event does not pass the charge test, the event almost certainly is something other than a J/Ψ decay: rate it "0" (see below).
Test 2: Muon track quality

If two muons pass the "charge test," then rate the likelihood of a J/Ψ candidate by rating each muon track. If there are more than two, rate the two best of opposite charge.
  • Each muon shows a "global muon" track. (Note: "tracker muon" and "stand-alone muon" refer to tracks of a muon in different parts of CMS while "global muon" is a track to cover the whole path of that muon, if it can be constructed accurately.)
  • Each muon track shows multiple hits in the Drift Tubes (DT), Resistive Plate Chambers (RPC), or Cathode Strip Chambers (CSC). These are all elements of the CMS muon detector, and tracks outside the superconducting magnet are calculated from signals in these systems.
  • The track is not closely associated with a particle jet (which looks like a cone in the event display).
Note that you will need to manipulate the display and turn components such as CSC hits on and off to best understand the muon tracks. Be sure to open the iSpy Online window wide enough to see the buttons on the left panel.

Elements of a J/Ψ Candidate Event

Before You Start

If you are not familiar with iSpy Online:
Rate the J/Ψ Candidates

Use the guidelines above to rate the candidate events. Apply what you learned as consistently and rigorously as you can. The rating of the J/Ψ candidate on a 0-3 scale is based on your study of the muons in the event display:
1 - POOR
2 - FAIR
3 - GOOD
Enter the rating for each event in the Data Spreadsheet. Mass calculation—the second part of the masterclass exercise—is enabled in the spreadsheet only when a muon ratings are given.

In cases of anomalous or difficult-to-rate events, consult with other students, teachers, and mentors to apply as consistent a rating as possible.

General Consistency Check

Count up how many events of each rating you have in your set. Compare this with other groups. If the numbers are very different, discuss your criteria and see if someone needs to make an adjustment.

More Data

If you finish all 100 events, start a new set, preferably one that no one has done. There are 2000 events and the more you can work through, the better your overall results will be.


The whole group does this part of the CMS Masterclass under the leadership of the mentor.
  1. Decide which ratings to include in the mass plot, for example events rated 3 only, or events rated 2 or 3, etc.
  2. Use the Data Spreadsheet to extract the masses calculated for the selected events.
  3. Plot the masses the selected candidates in a histogram.
  4. If practical, revisit the decisions made prior to creating the histogram to see if the mass plot can be improved.
  5. Record the best mass plot for the J/Ψ particle.
  6. Discuss the interpretation of the overall result as well as the contribution of each group.