2. Event properties

If you managed to run the task, congratulations! This is step one towards becoming a professional physicist (no turning back from here ... ) . In this exercise we will add code to our analysis class to obtain more information about the collisions. First, create additional output histograms in which you'll store

• the total number of events, named "NEvents". This will be extended later is this exercise
• the primary vertex positions along the beam line for all events, named "VtxPos"

Try to run the code again, after you've added these histograms.

2.1 All set?

Not all collisions in the detector are actually usable for physics analysis. Events which happen very far from the center of the detector, for example, are usually not used for analysis (any idea why?). To make sure that we only look at high quality data, we do 'event selection'. Let's add some event selection criteria to your task.

• modify the code such that only primary vertices are accepted within 10 cm of detector center;$$|z_{vtx}| < 10$$ cm
• modify NEvents to store the information on how many events are accepted and declined due to the vertex cut. Tip: use a histogram as a simple 'bookkeeping' device, fill it with a 0 if an event is rejected, and a 1 when the event is accepted. More creative solutions are always welcome!