underlyingevent

Underlying event (UE) is the term used in collider physics to describe all activity in a hadron-hadron collision that is not part of the primary hard-scattering process. In proton–proton collisions at the LHC and similar facilities, the UE consists mainly of multiple parton interactions, initial-state radiation, final-state radiation, beam remnants, and their subsequent hadronization. The hard scatter produces the high-pT objects of interest, such as jets or leptons, while the UE adds additional soft particle production that accompanies it.

Experimentally, the UE cannot be cleanly separated from the hard process. Measurements use regions in phase space designed to be sensitive to UE, often defined by the azimuthal angle relative to the leading object. The transverse region is typically most UE-rich, while the toward and away regions contain much of the hard-scatter activity. Observables include charged-particle multiplicity and the scalar sum of transverse momentum, studied as functions of the leading object's pT or event energy.

Modeling and tuning: Modern event generators such as Pythia, Herwig, and Sherpa implement MPI models and color connections to describe UE. Parameters are tuned to UE-sensitive observables across energies, yielding more accurate predictions for backgrounds, jet energy scales, and isolation criteria in analyses.

Importance: Correct treatment of the underlying event is essential for precision measurements and new-physics searches, affecting jet reconstruction, pile-up mitigation, and background modeling. The concept remains active as collider energies rise and new data challenge existing tunes.