Identifying events with HYPATIA

Here, you'll learn how the events mentioned before look in the event display HYPATIA. You will use your newly acquired knowledge on identifying elementary particles. You'll learn to choose events with a Z boson 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 both views we see several tracks inside the tracking detector as well as some activity (yellow squares) in the first calorimeter shell (green part of the detector drawing). These are clear signs of electrons.
  • There are a lot of tracks visible in side view in the first picture above. In order to show only particles with a high transverse momentum, we can define so-called “cuts”. Thereby all 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 perpendicular momentum, the value 25 GeV is suited. This selects all particle tracks with a perpendicular momentum greater than 25 GeV. This applied cut can be seen in the second picture above.
  • If we zoom in on the end view we seem to have an electron-positron (note the charge difference) pair going back-to-back. The missing perpendicular momentum is zero, so no neutrino is produced. This is a typical Z→e- + e+ event.


  • In the end view as well as in the side view, a few particle tracks can be seen. This is typical for an event, which is observed with the ATLAS detector. Note that we have no missing perpendicular momentum, which means that no neutrino is produced. Inside side view one can clearly see two muons. The end view shows that the two muons goes in opposite direction to each other.
  • As you can see from this enlarged view into the direction of the proton the two muons are going back to back. Both particles might arise from one particle that decayed after its creation.
  • In this zoomed picture we clearly see the two muon tracks. If you look at the charge of the two muons we see that one of them has positive and the other negative charge. This means that we have detected a muon-antimuon pair. This is a clear sign that a Z boson was produced.




background processes


  • This event stand out from signal events in two ways: 1. you can see bundles of particles and 2. the value of the misssing transverse momentum is too small as that neutrino(s) could have been produced.
  • Both views enlarged and shown with the fish-eye option show the bundle of particles clearly.
  • This picture shows the enlarged side view of another event. Here you can recognize two collision vertices that occur approximately 60 centimeters away from each other. The vertices are shown by red circles. Now you got an impression how complicate event identification might be.


  • Note that the value of the missing perpendicular momentum is 38 GeV in this event. Which indicates that a neutrino has been produced. We also see a track in both side and end view
  • Inside end view one can clearly see a muon (or anti-muon). Its track inside the tracking detector runs in opposite direction to the red dashed line. This is a strong hint for a decay of a W boson into a muon (it runs to the left in this display) and a neutrino (it runs to the right).


  • In the end view you can see the signature of an electron with high transverse momentum and a neutrino (since the missing perpendicular momentum is 39 GeV) going in the opposite direction. In the side view you can also see the well-isolated electron.
  • The information from the lepton's track tell us that it is really an electron (and not a positron. Pay attention to the negative sign).