Identifying Events with Tracer

Here, you'll learn how the events mentioned before look in Tracer 3D Display. You will get to use your knowledge about identifying elementary particles. You'll learn to choose events with a W particle as signal events and to distinguish them from the background events. Again, you'll find explanations about this in the form of a picture gallery.



Signal processes
  • In this view, several particle tracks are visible with tracks extending into the muon segment. Note the missing transverse momentum value (~58 GeV), indicating the presence of a neutrino. This makes the event especially interesting! Let’s see if we can identify other particles as well.
  • In this event, two muons (or anti-muons) are clearly visible. One of the tracks inside the tracking detector runs in the opposite direction to the red dashed line, which strongly suggests the decay of a W particle into a muon and a muon anti-neutrino. You can select multiple tracks by pressing Ctrl and clicking on them. Below, in the information window, you can see details about these two tracks. The charges are +1 and -1, indicating they are a muon and an anti-muon.
  • There are a lot of tracks visible in the event. In order to show only particles with a high transverse momentum, we can define so-called “cuts”/“filters”. Thereby only particles with a minimal value for the transverse momentum are chosen to be seen. The minimal value has to be specified. To set a minimal value for the transverse momentum, the value 20 GeV is suitable. This selects all particle tracks with a transverse momentum greater than 20 GeV.


  • In the end view you can see the signature of an electron with high transverse momentum and a neutrino (Missing ET = 27 GeV) going in the opposite direction.
  • The information from the lepton's track tell us that it is really an electron (pay attention to the negative sign of the electric charge.).


Background processes
  • This event stands out from signal events because one can see more than one bundle of particles.
  • This picture shows an even more enlarged view of the event. You can see very well how the bundles of particles originate from the interaction vertex (red).


  • This event display shows the decay of a Z particle, which is the electric neutral messenger particle of the weak force. The Z decayed immediately after creation into a muon and an anti-muon.
  • As you can see from this view into the direction of the proton beam muon and anti-muon going back to back. Both particles might arise from one particle that decayed after its creation. Cross-checks are always better in order to proof the assumption that a Z particle was produced. Both muons have distinct electric charges.
  • Furthermore there is no missing transverse momentum at all, meaning: A neutrino was probably not produced.