Identifying particles

To analyse the proton-proton collisions that the program displays, you should know how you can identify electrons (as well as positrons), muons (and anti-muons), neutrinos, and hadronic particles and jets in the event display. You will find elucidation of this topic on this page in form as a photo gallery again.



  • This signature is generated by an electron. The particle has left a track (red) in the inner detector (therefore it carries electric charge) and has released all its energy within the electromagnetic calorimeter since it is the only one in which you can find deposits. This can be seen from the yellow little boxes inside the light-green structures that represent the electromagnetic calorimeter. Since there are neither entries in the hadronic calorimeter nor the muon chamber it is an electron or positron.
  • The same event in the end view. The track through all three inner detectors and the yellow little boxes of the deposits in the electromagnetic calorimeter can be easily recognised.
  • The side view shows the same. If you combine the side and end view you will get a spatial impression of the event. This is excellent training for your mind.
  • How can you decide whether it is an electron or a positron? In the toolbar of MINERVA you find a hand with a pointed forefinger. By clicking on this option you can chose a track of a particle in the event display (again by clicking on it). You will then see information in the lower-right window. This information contains, for example, measured values of space-dependent components of the momentum (Px, Py, Pz) and the transverse momentum (PT). In the following picture you see the information of this window.
  • The last row contains information about the nature of the electric charge of the particle belonging to the chosen track (Charge: here -1). “-1” means that the particle is electric negatively charged. “1” means the opposite: electric positively charged. We can identify our chosen particle as electron.


  • In this event display one sees a track (orange) in the inner detector, small energy depositions in both the electromagnetic as well the hadronic calorimeter (both displayed by yellow little boxes inside the light-green and red structures), and small tracks (orange) in the muon chambers. It is a muon (or an anti-muon) which is the only particle that goes through the whole detector and thereby leaves signals in all shells.
  • In this enlarged view, you can see the orange tracks in the muon chambers very clearly.
  • In side view, the individual entries in the muon chambers are represented as orange crosses. All of these crosses inside one chamber are connected by an orange line, which symbolizes the track of the muon in this chamber. Connecting all orange tracks in mind will show you the path of the muon through the outer layers of the ATLAS detector.
  • Muon or anti-muon? The same procedure that we described with the electron/positron provides the result: In this event display, a muon is pictured (Charge: -1).


  • How does one recognize a neutrino? Neutrinos don't interact with even a single component of the ATLAS-detector. They neither interact with the tracking detector, nor the calorimeters, nor the muon chambers. How can one therefore detect something which we cannot see? Since all quarks and gluons move along the beam axis before the proton-proton collision all of their velocity components at right angles to the beam (perpendicular) and therefore the so called transverse total momentum is zero. Due to momentum conservation the transverse total momentum (the vectorial sum of the transverse momentum of all particles) has to be zero after the collision as well. If the measurements contradict this, either particles that are invisible to the detector are produced (e.g. one or more neutrinos which have got exactly this missing transverse momentum) or particles carrying tranverse momentum leave ATLAS without beeing detected or ATLAS do not certainly measure.
  • In the ATLAS-Detector, the missing transverse momentum is determined by the energy deposited in the calorimeters. When there is an imbalance within this energy distribution – which is called missing transverse energy (Missing ET) – this suggests a neutrino which was produced during the collision. There are two ways that this is shown with MINERVA: 1. By the Missing ET-value in the upper right event display that has a grey frame, and 2. by the red dashed line in end view. This line makes the direction of the energy imbalance clear on the one side. On the other side is the thickness of this line a measure for the value of the missing transverse energy.
  • In this event, an electron and neutrino were produced nearly exclusively. Since these two particles are kind of the only ones that have been produced the transverse total momentum is split between these two because of conservation of momentum. That is why the neutrino with its share of the transverse momentum flies away from the electron in the nearly opposite direction. The related missing energy is determined with the help of the event display and plotted in the direction of its transverse momentum. A thick dashed red line therefore always indicates the existence of one or more invisible particles, e.g. neutrinos. Smaller missing transverse momentums of about 10-20 GeV (thin red dashed lines) can also be caused by measurement uncertainties of the detector.


  • In this event display so called jets are shown. Each jet consists of a bundle of several particles. The electric charged particles cause tracks in the inner detector whereas the neutral ones don't. If you extrapolate the tracks you will find many entries in the calorimeters. Other depositions nearby cannot assign to a track because they were caused by electric neutral particles. Especially the hadronic calorimeter contains many entries. That is because every jet is the result of a gluon, quark, or antiquark that is ejected from the proton during the collision. Thereby big amounts of energy are at work in order to overcome the huge binding forces. A part of this energy is used to create new quark-antiquark pairs which move in nearly the same direction and bind each other to form new particles – so called hadrons. These generate the shown jets that have a grey background in this picture in order to recognise it more easily.
  • Keep in mind: Particles that fan out, cause tracks in the inner detector and have entries in the electromagnetic and especially the hadronic calorimeter can be put down to quarks, antiquarks, and gluon, and are called jets.