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• Identifying Events
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Visualizing events in HYPATIA

It is time to see how the events we have talked about look in the event display HYPATIA. You will use your newly acquired knowledge on characterizing events and identifying elementary particles. You will learn to select signal events and to distinguish them from the background events. The picture gallery will guide you through this task.

Di-lepton and 4-lepton events (electrons and muons) are, in general, easy to identify. Di-photon events are sometimes trickier, especially when tracks point to the energy cluster deposited in the electromagnetic calorimeter (ECAL).

The picture gallery will guide you through this task.

signal processes

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• 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. The missing perpendicular momentum is very small, so no neutrino is produced. This is a typical Z→e^- + e⁺ event.

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• 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 there is very little missing perpendicular momentum, which means that no neutrino is produced. Inside side view one can clearly see two muons.
• From this enlarged view into the direction of the proton, you can clearly see the two muons. 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.

• 
• In both views we see several tracks inside the tracking detector as well as clusters (in yellow) in the electromagnetic calorimeter (green part of the detector drawing). The two towers in the lego plot correspond to the 2 important clusters in the calorimeter. As there are no tracks pointing to these clusters, these are not electrons but photons, as the next picture will confirm.
• Changing the minimal value of the transverse momentum (Control Window) from 1 GeV to 5 GeV ("pT-cut") all tracks disappear. In the track momentum window, we switch from the "Track" view to the "Physics object" view. In this case the physics (calorimeter) objects are in fact photons, as there are no tracks pointing to them. When choosing the "physics object" view, the event display features additional markers pointing to the calorimeter clusters, if any. The resulting picture above shows that we are in the presence of a gamma gamma event.
• The two photons can now be entered into the invariant mass window by clicking on "Photon" in the Track momentum window. This pair of photons has an invariant mass of 110.1 GeV.

• 
• The event above features two calorimeter clusters (one per view, in yellow) with 2 energy towers in the lego plot.
• After a pT cut of 5 GeV, we see the 2 calorimeter clusters, one without any track pointing to it and one with a double track pointing to it (see zoom above). The double track pointing to a calorimeter cluster (with numbers 45-46) has an invariant mass M(e+e-) =0.2GeV, compatible with a converted photon. In fact this pair disappears when requiring at least 2 pixel hits (next picture).
• The e+e- pair pointing to one of the clusters disappears when requiring at least 2 pixel hits (picture before). What about the other electron pair? It has nearly no corresponding energy deposited in the calorimeter and both tracks have the same sign. These can be considered as random tracks and can therefore be ignored. In fact requiring pT larger than 10 GeV, they would disappear. The photon calorimeter objects have an invariant mass of M(gamma gamma) = 100.1 GeV.

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• This event has many tracks, 2 of which are clear muons with hits in the muon spectrometer (check both views). It has also 2 energetic clusters (big energy towers in the lego plot) with tracks probably pointing to the clusters. Let's proceed further.
• Increasing the pT cut to 12 GeV (Control Window), only 5 tracks are left, the 2 opposite-charge muons and 2 opposite-charge electrons. The latter clearly show a match between the tracks and the calorimeter clusters. Each of the pairs stems from a Z (M(ee)=89.6 GeV and M(mumu)=91.1 geV). This is a ZZ event leading to 4 leptons. The 4-lepton invariant mass is M(4l)= 291 GeV.

• 
• This event, after a pT cut of 5 GeV, is left with 4 tracks, each pointing to a calorimeter cluster. Each of the pairs stems from a Z (M(ee) = 89.9 GeV and M(ee) = 87.6 GeV). This is a ZZ event leading to 4 electrons. The 4-electron invariant mass is M(4l) = 229.7 GeV.

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).

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• 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).