3. Average event multiplicity and track selection

In this exercise we are going to study the particles that are produced in each event. First:

• plot the $$\eta$$ and $$\varphi$$ distributions of the particles for all accepted events in a two-dimensional histogram
• make sure that the axis have proper ranges (how large is the detector?) and a reasonable amount of bins

If you've added this histogram, run the analysis, and look at the two dimensional histogram. Does the distribution make sense?

Just as some events are not of high enough quality to do analysis with, some tracks are also not good for looking at certain observables. In addition to 'event cuts', we therefore have to apply 'track cuts' as well. In ALICE, we use a system of predefined track selection criteria, that are checked by reading the 'filterbit' of a track.

• Check in the code where the filterbit selection is done
• Change the filterbit selection from value 128 to 512

How does your $$\eta$$ and $$\varphi$$ distribution change, if you run your task again with lterbit 512? What do you think is the cause of the difference? Please ask one of us to explain this in a bit more detail.