PARTICLE COLLISIONS.

WW-exercise: Identify W-decays

Collisions with WW decays are much more difficult to analyse than Z decays. These collisions have much higher energy, around 200 GeV, and since it is two decaying vector bosons, there are many more particle tracks in the detector. The goal of the exercise with the WW decays is to measure the branching ratios of the W particle, much in the same way as was done for the Z particle.

First read the chapter "Picture analysis" and the pages that describe how to calculate the branching ratios for the Z particle ("Z-exercise 1"), that information is partly relevant also for the WW decays.

A W particle has four decay possibilities, it can decay into:

• an electron and an electron-neutrino
• a muon and a muon-neutrino
• a tau particle and a tau-neutrino
• a quark and an antiquark

The three first possibilities are together called leptonic decays, and the last is called hadronic decay. We can calculate the branching ratio of the W particle by analysing pictures of WW decays and counting the number of times the different decay possibilities occur.

A WW decay were one of the W's has decayed into an electron and an electron-neutrino, and the second has decayed into a muon and a muon-neutrino. The green track (the electron) has deposited much energy in the EM-calorimeter (the inner circle). The muon, the read track, has been registered in the muon detector further out. Note that the two tracks are not at 180 degree from each other. This is due to the momenta of the two neutrinos that haven't been registered in the detector.

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