# Pile-up in particle physics

Regarding pile-up, I came across two definitions. One dealing with electronics of detector where more than one event get recorded at the same time and other regarding all the pp collisions in the bunch which do not lead to hard scatter that we are looking for our analysis. My question is regarding the second type of pile-up I mentioned.

Below is slide(from HEP presentation) about the same where they latter tell how to re-weight MC to data to match rho of data. Here, in the slide - If I have "N" primary vertex then does that mean "N" interactions as well? In that case, I can't understand the plot in the slide...

Thanks a lot!

$\mu$ is the average expected number of interactions. $\left<n_V\right>$ is the number of vertices found by the experiment in the data. Ideally they would be the same, of course. But in the real world some interactions are so subtle they are not detected (maybe the tracks go down the beam pipe not into the detector), and sometimes extra (spurious) vertices are found, so they don't agree. Correct understanding of this difference is important - hence the slide.
• In this context $\mu$ is the usual measure. – RogerJBarlow Aug 14 '18 at 5:58
• I guess you mean $\nu$ here?.. As $\mu$ is same for both data and MC – kbg Aug 14 '18 at 9:19