How can one calculate background rejection from a background sample applying cuts ??
Compare the number of sampled background events which pass the cuts to the total number of sampled background events.
So what is background efficiency? Is background efficiency and background rejection are the same ?
They are different ways to describe the same thing. If 5% of your background events pass the selection (1 out of 20), you have a background efficiency of 5% and a background rejection of 20.
Separate names with a comma.