Introduction to the projects

For seven years LEP was set up as a Z particle factory allowing physicists to learn about the weak force. The Z particles produced in the electron-positron collisions existed for only a fleeting moment before decaying into other particles that were then recorded by particle detectors. The different particles leave distinctive tracks behind them, allowing them to be identified. You are about to participate in making some of the most important measurements in recent years, to put our model of the Universe to the test. The first experiment is the easiest to perform.

As a tool you will be using a program, WIRED, to visually scan and manipulate Z particle decays that were recorded during 1998 by the DELPHI detector at LEP. WIRED is a computer program developed at CERN in the Java language that allows physicists to study computer reconstructions of collisions in 3-D.

If you want to understand in more detail how the DELPHI detector works, it might be a good idea to look at the chapter "DELPHI in depth" now. But the picture below should give you enough information to move on to the projects right away. It shows how different kinds of particle interact with different kinds of detector components. All charged particles leave tracks behind in the tracking chambers, while neutral particles do not. Photons, electrons and positrons lose all their energy in the electromagnetic calorimeter.

Particles containing quarks, collectively known as hadrons, lose their energy in the hadronic calorimeter. Muons leave tracks behind in the tracking chambers, lose a little energy in the calorimeters, and are also detected by the muon chambers, the outermost layer of the detector. Neutrinos do not interact with any of the detector components and escape unseen.

Listed below are examples of all the different visible decays of the Z particle. Have a look at each of them, taking care to read the associated text, to familiarize yourself with the different types of event. Clicking on an event will load a simple version of the WIRED program that allows you to zoom and rotate. The electromagnetic calorimeter is displayed for orientation. Be patient! The first loading of WIRED can take some time.

Special case of e+e- -> Z -> 2 jets

The version of WIRED that you'll be using for the projects is more sophisticated. The feature it contains that will be of the most help to you in the interpretation of events is the ability to switch on and off the outlines of each detector. By doing so you can find out, for example, if a large energy deposit in the calorimeter section belongs to the electromagnetic calorimeter or the hadron calorimeter. This allows you to identify the particle that made the deposit and hence classify the event. It's a good idea to view the outline of only one detector at a time. When several are shown at once the large number of overlapping lines makes it difficult to interpret the event.

The names of the buttons in the display are a bit cryptic but you'll quickly learn what they mean and how to use them. Below is a list explaining what they do. Bear in mind that the DELPHI detector looks like a barrel plugged at each end by an end-cap.

Turn to next page for
"Z branching ratios" project "Strong coupling constant" project


Particle Physics Education CD-ROM 1999 CERN