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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.