This CD-ROM gives students an insight into :
We suggest you read the Introduction to the CD-ROM carefully.
You may find it useful to print the Help page and keep it close at hand for reference.
It is important for every student to read the "Introduction to the CD-ROM" and to take a quick look at the "Help page" to see the how the CD-ROM is organized.
The first time students use the CD-ROM, they should follow the "Fast Track" to the analysis projects. If time allows they may go off the beaten track and follow red links to more detailed information. The Fast Track, however, contains all they need to know to get started on an analysis project. In subsequent sessions, they may use the Menu bar to go directly to the analysis projects, or they may browse more detailed information by following the red links.
All the projects use the same program, called WIRED for World Wide Web Interactive Remote Event Display, and the same technique: visual inspection of computer reconstructed particle collisions, otherwise known as events. The introductory page teaches students how to identify different events by using a few examples.
It would be very useful to the authors in improving this CD-ROM to see the results of your students' analyses. We would be grateful if you would send them to one of us using the contact details below.
The purpose of this project is to study how the Z particle decays and in particular, to measure the fraction of decays into particles of different types. These fractions are called Z branching ratios. They allow us to learn what the Universe is made of.
The Z particle is neutral. This means that it can decay into pairs of oppositely charged particles like an electron-positron pair, or a muon-antimuon pair, or a tau-antitau pair, or a quark and an antiquark. It can also decay into pairs of neutral particles, neutrinos and antineutrinos. Neutrinos don't leave any traces in the detector, meaning that the Z branching ratio into neutrinos cannot be measured directly, but it is these "invisible" decays of the Z that allow us to work out what the Universe is made of. We'll come back to them later.
To measure the Z branching ratios into visible particles, students will look at reconstructed Z particle decays. Using what they have learned from the introduction about what different decays look like, they will identify the decays and add up the number of times the Z decays into an electron and a positron, a muon and an antimuon, a tau and an antitau, or a quark and an antiquark.
The more events your students study, the more precise their result will be. Consequently the best way to approach the task is to split the class up into teams, each with responsibility for analyzing one file of 100 events. Each group should prepare a table, like the one shown below, in which they can record their observations. An event can only be one of the four possibilities. When they have finished, they can calculate the Z branching ratios into each particle type by dividing the number of each kind of event by the total number of events they looked at.
When each analysis team has finished you can ask them to present their branching ratios. If you plot the results from each group on the board you will find that they are not all the same, but they scatter around a central value. The next step of the analysis will be to combine all the results in a statistically meaningful way to arrive at your class' final answer, which you can then compare with CERN's published results.
After the project, there is a discussion of the significance of the branching ratio measurement, and its implications for the composition of matter in the Universe.
[Click here to open the "Z branching ratio" project]
This analysis project takes a closer look at the events identified as quark-antiquark pairs. Most of these will be so-called two-jet events, where two jets of particles emerge from the interaction point. A smaller number, however, will have three jets, and an even smaller number will have four or even more.
The reason for this is that quarks and antiquarks can radiate gluons, the carriers of the strong force, which go on to generate jets of their own. The probability that a quark or antiquark will radiate a gluon is directly related to the strength of the strong force. This means that by counting up the numbers of two, three, and four or more jet events, we can measure the strength of the strong force. That is the goal of this project, which is followed by a discussion of the relative strengths of all the forces of nature.
The events are the same as for the previous project, but the tables that your students should keep are slightly different:
Since the data sets are the same for projects 1 and 2, if the students have enough time there is nothing to stop them from doing both projects at once.
[Click here to open the "Strong coupling constant" project]
The following general tips should help you get the most out of this CD-ROM:
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