Exploring the Z Particle in 3-D

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Left: Large Electron Positron detector ....................Right: Z decay into three jets.
(Images courtesy of CERN)

This is a resource that is available on CERN’s website at http://keyhole.web.cern.ch/keyhole/ It allows students to analyze real 3-D data from the LEP (Large Electron Positron Collider). It examines the decay of the Z particle, one of the three carriers of the weak force, which was discovered at CERN in 1983. The students will learn about the signatures of different particles as they pass through the different layers of the detector. The details of its detector (DELPHI) are very similar to that of ATLAS, which is being used for the Large Hadron Collider that is starting up in September 2008, so this information is very topical. If you want a copy of the resource on a CD-ROM, you should email Antonella.Del.Rosso@cern.ch.

Interactive Lecture (20 minutes)
Push the SKIP button and going straight to index -> projects. I reccomend that you go through the first page with your students using a data projector. This will give them a brief overview of the DELPHI detector and explain how to recognize the presence of photons, electrons/positrons, charged hadrons, neutral hadrons, muons and neutrinos. Then they learn about WIRED (World Wide Web Interactive Remote Event Displays) the system that displays the data from the detector. Next, they see an animation of each decay type and then real data displayed on WIRED. There are five types of decay and they need how to recognize each type.

Small Group Work with Computer (20 minutes)
Have them go to Projects in the main menu. They will have to do a twenty-question quiz first in the Briefing Room. Then they will see the page that have already done as a class. They can review this or skip through to the second page where they will find the data sets. Give each student a spreadsheet for the first set of one hundred events with answers. Have them follow the link to this set so they can practise their skills. When they are ready, you will give them one of the following spreadsheets without answers and have them analyze it.
second third fourth fifth sixth seventh eighth ninth tenth

Interactive Lecture (15 minutes)
The third page looks at what the results mean. However, you should probably do this with the class yourself. Each set of 100 events will give rise to different Z branching ratios because the samples are so small. The percentage error is calculated by sqrt(1/N + 1/Np ). The results should agree within the stated errors. If all 1000 of the results are compiled, the error will get smaller and the results will be closer to what CERN had:
Electron: 3.366 +/- 0.008 %
Muon: 3.367 +/- 0.013 %
Tau (taon): 3.360 +/- 0.015 %
Quark: 69.9%

However, the class’s results still won’t be close to the above values because we failed to detect the neutrinos. By conservation of energy and momentum, the physicists at CERN know that 20% of the decays went into neutrinos. That is why CERN’s results only add to 80%. Therefore, in order to compare results, the class needs to multiply their results by 0.8. These will still be somewhat different because CERN analysed millions of decays, not just a thousand. However, they should agree within the stated error.

What do these results tell us? Decay events, like many things in quantum physics, are intrinsically random. However, the behaviour of large numbers of events can be very predictable. These results show that the Z is equally likely to decay into any of the three charged leptons (electrons, muons and tauns). This is called Lepton Universality. Also, all uncharged leptons (the three neutrino types) are equally likely to decay and they are twice as likely as the uncharged ones. These statistics confirmed that there are three and only three generations of particles.

Homework (20 minutes)
Send the students to watch the ten minutes of animation showing the ATLAS detector which is one of two main detectors for the Large Hadron Collider at CERN. Ask them to be prepared to describe the parts and how they are similar to and different from the DELPI detector that was used with the LEP. The short answer is that the ATLAS is much bigger. The components are basically the same, though they will probably be more sensitive etc.

CERN has other resources for teachers, some of which are reviewed here.

Last updated July 2010