Pileup
From Particle Wiki
In collider physics, pileup (or pile-up, PU) occurs when multiple independent collisions overlap in the detector.
One sometimes distinguishes in-time pileup, where the overlapping collisions occur in the same bunch crossing, and out-of-time pileup, where they occur in different bunch crossings.
Pileup affects event reconstruction and often needs to be taken into account and corrected for. This is called pileup mitigation.
