Events in DELPHI


This is an exercise in identifying particles in the DELPHI detector at the LEP collider at CERN, the European Laboratory for particle physics. If you would like to look first at a simpler "identification parade", look here.


The DELPHI detector

This is an exploded view of the DELPHI detector. Clicking on this image will give you a larger, clickable view. You can also see a larger image with detailed labelling here (as a gif file) or here (as a postscript file)

There are six major components. You can find out more about them by clicking on each title.

Tracking Detectors
Particle Identification
Electromagnetic Calorimeters
Magnet coil
Hadron Calorimeters
Muon Chambers

The following images show the computer-reconstructed "event display" with all the detector components in the same colour scheme, viewed from the end and from the side, but with no event data (tracks etc).

Detector - end view
Detector - side view

Now you can see how specific particles appear in the detector:

Electrons: leave tracks and large energy deposits in electromagnetic calorimeters, but usually continue no further (Electron - end view, side view)

Photons: leave a large deposit of energy in the electromagnetic calorimeter with no track pointing towards it. (Photon - end view, side view)

Muons: leave tracks, and small energy deposits in calorimeters; they penetrate all the way to muon chambers where they leave hits (Muon - end view, side view)

Hadrons: charged hadrons leave tracks; each hadron tends to leave only a small amount of energy in the hadron calorimeter as it only has a small fraction of the total energy in the event; the tracks can often be seen to curve (Single hadron - end view, side view)

Jets: hadrons typically appear in a jet containing several hadrons; the jet will appear as several tracks pointing towards a large (broad) deposit of energy in the calorimeters; a jet of hadrons like this is formed when a quark or antiquark is produced in the electron-positron annihilations (Hadron jet - end view, side view


Events in DELPHI

Now look at some typical events in which these kinds of particle occur. In each case the energy of the initial electron-positron annihilation is initially converted into a new particle-antiparticle pair.

e+e--> Z0->? The first set of events show decays of Z0s to different particle-antiparticle pairs. Can you identify them? (Notice that the beam energy is around 45 GeV - equivalent to half the mass of the Z0 .)

Event 1 - end view, side view
Event 2 - end view, side view
Event 3 - end view, side view
Event 4 - end view, side view

The next set contains some more complicated events: and different decays of pairs of W particles.

e+e--> W+W-: For these events the total energy must be sufficient to create the mass of the pair of W particles, so you will see that the beam energy, which is half the total, is about 86 GeV. The W+ and W- are extremely short lived, and so what you see in the detector are the decay products of the particles. Different decays for each W particle are possible, such as:

Any combination of pairs of these decays is possible, eg:

Now see if you can identify which sort of particles are present in the following pictures, and work out the kinds of event they are. To make them clearer, these images do not always show all the detector components, and you will find different colour schemes in some of them!

Event 5 - end view, side view
Event 6
Event 7-end view, side view
Event 8
Event 9
Event 10
Event 11
Event 12