Pileup

Pileup is a term used in several fields to describe the accumulation of multiple events occurring close together in time or space, leading to overlapping signals or data. The specific meaning depends on the discipline, but the core idea is that several instances are superimposed, complicating analysis and interpretation.

In particle physics, pileup refers to multiple proton-proton or heavy-ion collisions happening within the same detector readout window at a collider. These overlapping interactions produce a mixture of tracks and energy deposits that can obscure the signal from a single collision of interest. Pileup degrades measurements of momenta, energies, and event topology, and it increases background in searches for new phenomena. Mitigation methods include identifying the primary interaction vertex among many, subtracting estimated contributions from additional collisions, and employing per-particle weighting schemes to reduce the impact of pileup on reconstructed objects.

In genomics, a pileup describes the aggregation of sequencing reads that align to a single position in the reference genome. The pileup representation summarizes bases and quality metrics at each genomic site and is used in downstream analyses such as variant calling. Formats and tools around pileup, including mpileup, facilitate the evaluation of sequence variants and coverage.

In transportation and accident reporting, a pileup refers to a multi-vehicle collision or to a long queue of vehicles caused by an incident or congestion. Such pileups can propagate delays and require coordinated response to restore normal traffic flow.

Across contexts, pileup generally denotes overlapping events that are challenging to disentangle without specialized methods.