The Detector

We are going to study particle tracks that have been seen by an experiment called OPAL, at CERN, near to Geneva. This is one of the four experiments that runs at LEP (the Large Electron-Positron collider), which is the largest particle accelerator in the world.

The OPAL experiment is approximately cylindrical in shape. The particles are produced on the axis of the cylinder at the centre and then travel outwards. As they do so, the particles pass through several different types of detectors, which are arranged in concentric cylinders about the axis. Different types of particle leave different signals in the various detectors and this allows us to distinguish them. You can find a diagram of the experiment and a lot more details here.

Let's have a look at an example picture of a particle interaction. This shows the cylindrical detector viewed from one end. Therefore the concentric cylinders corresponding to the different dectectors show up as concentric circles.

The incoming beams of electrons and positrons travel in opposite directions along the axis of the cylindrical detector. (In this picture this corresponds to a line that is perpendicular to your computer screen and through the centre of the concentric circles.)

End-On View of Event


Travelling from the inside outwards, the first detector sees the tracks produced by electrically charged particles. (A charged particle track is shown as the purple line in the picture.) You may just be able to see that the track is very slightly curved. This is because a magnetic field is applied. (In the picture the direction of the magnetic field is perpendicular to the screen.) By measuring very accurately the degree of curvature we can determine the momentum of each charged particle track.

Next on its outward journey the particle passes through a detector called the "electromagnetic calorimeter". As it interacts with the atoms in the detector the particle loses energy. The detector measures the total amount of energy the particle deposits; that is why it is called a calorimeter. (In the picture the energy deposited is shown as the yellow rectangle.)

In the picture a colour code is used to indicate the momentum of charged particle tracks and the energy deposited in the calorimeters. A scale at the bottom of the picture shows which colour corresponds to which range of energy/momentum. Thus, red corresponds to energy/momentum of between 0 and 0.5 GeV, yellow to between 0.5 and 1.0 GeV, green to between 1.0 and 2.0 GeV, and so on. Tracks and clusters drawn in white correspond to energy/momentum of greater than 16 GeV. (You should be able to see that the charged track in the picture corresponds to a momentum of between 4 and 8 GeV and that the cluster in the electromagnetic calorimeter corresponds to an energy of between 0.5 and 1.0 GeV.)

Many particles lose all their energy in the electromagnetic calorimeter and therefore progress no further. However, the particle in the picture passes through the electromagnetic calorimeter and into the next detector, which is called the "hadronic calorimeter". The energy the particle deposits in the hadronic calorimeter is shown in the picture as the purple rectangle and crosses.

Almost all particles lose all of their remaining energy in the hadronic calorimeter and therefore progress no further. However, the particle in the picture passes through the hadronic calorimeter and into the outermost detectors, which are called the "muon chambers". The signals the particle produces in the muon chambers are shown as the yellow crosses and the red arrow. A muon is the only type of particle that is likely to pass through all the calorimeters and leave signals in the muon chambers.

Sometimes it's useful to look at the detector and the particle signals from the side. Here is a picture of the same "event" we have been looking at already, but from side-on.

Side View of Event


By looking at the different signals they produce in the various detectors we will try to distinguish between three different types of particles: muons, electrons and hadrons. Click on the items below to see how these particles differ from one another.

some more example events containing a muon
example events containing an electron
example events containing a hadron

Once you've understood the way we identify the tracks produced by the different types of particles, then please:
(challenge) click here to proceed to the next part of the programme, which is "Challenge Part 1".

(home) Back to Home Page with table of contents.

Terry Wyatt. March 1997.