When you analyse the events you have selected, you should write down how many events of the different types of decays you find. If you are doing this exercise alone it is enough to analyse 2-300 pictures with collisions to get a quite good estimate of the branching ratios of the Z particle. If a whole class makes this exercise the students can be divided into groups which analyse the ten links with 100 decays in each link. In the end the results of the groups are put together to get a good final result with small statistical errors. The following types of decays should be counted:
When it is time to analyse the results we will have problems with the decays into two neutrinos. Since these cannot be detected we have to trust the Standard Model which tells us that 20% of the Z-decays goes to two neutrinos. This means that if you have analysed 100 events, these will only correspond to 80% of the total number of Z-decays.
Now it is easy to calculate the branching ratios for the events you have analysed. If you for example has found 5 electron events (out of 100 analysed), this means that the Z particle decays into electrons in 5/(100/0,80) = 4% of the cases.
When you have calculated your branching ratios you can compare with the theoretical predictions of the Standard Model, see the link below.