﻿ International Physics Masterclasses
Identifying Events with MINERVA

Here, you'll learn how the events mentioned before look in the event display MINERVA. 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 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 the value of the missing transverse momentum (38 GeV). In this event, a neutrino has been produced. Therefore this event is very interesting! Let's see whether we can identify other particles as well.
• In the 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 particle into a muon (going to the left), and a muon anti-neutrino (going to the right).
• 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 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. The applied cut can be seen in the second picture above.

• In the end view you can see the signature of an electron with high transverse momentum and a neutrino (Missing ET = 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 (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.
• Both views are enlarged and thanks to the fish-eye option show the bundle of particles clearly.
• 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 (green). There is a stray track that may come from a different vertex that has less then 3 tracks and is therefore not shown.

• 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 enlarged 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. Furthermore there is no missing transverse momentum at all, meaning: A neutrino was probably not produced.
• Cross-checks are always better in order to proof the assumption that a Z particle was produced. Both muons have distinct electric charges.