Underlying event

In physics, the underlying event (UE) is all what is seen in a hadron collider event which is not coming from the primary hard scattering (high energy, high momentum impact) process.[1]

It was first defined in 2002.[2][3]

Further explanation

Underlying events can be thought of as the remnants of scattering interactions.[4][5] The UE may involve contributions from both "hard" and "soft" processes (here “soft” refers to interactions with low p-T, i.e. transverse momentum, transfer[6]).

These are important both in the simulation of particle experiments (often using event generators); and interpretation and analysis of data so as to filter out the desired signals.[7]

Features

Contents of UE include initial and final state radiation, beam-beam remnants, multiple parton interactions, pile-up, noise.[5]

References
  1. "Underlying Event – a definitive study by CDF - Collider Blog". Collider Blog.
  2. Barnaföldi, Gergely Gábor; Agócs, András G.; Lévai, Péter (2011). Underlying Event Studies for LHC Energies. 1348. AIP. p. 124. arXiv:1101.4155v1. doi:10.1063/1.3579440.
  3. Affolder, T.; Akimoto, H.; Akopian, A.; Albrow, M. G.; Amaral, P.; et al. (CDF Collaboration) (2002-04-22). "Charged jet evolution and the underlying event in proton-antiproton collisions at 1.8 TeV". Physical Review D. American Physical Society (APS). 65 (9): 092002. doi:10.1103/physrevd.65.092002. hdl:1969.1/181493. ISSN 0556-2821.
  4. Lecture 10: QCD at Colliders, page 20. Particle Physics, Dr Victoria Martin, Spring Semester 2013.
  5. http://www.tifr.res.in/~mazumdar/talks/UEDY_IIT.pdf
  6. Aad, G.; Abbott, B.; Abdallah, J.; Abdelalim, A. A.; Abdesselam, A.; et al. (The ATLAS Collaboration) (2011-05-31). "Measurement of underlying event characteristics using charged particles inppcollisions at GeV and 7 TeV with the ATLAS detector". Physical Review D. 83 (11): 112001. arXiv:1012.0791v2. doi:10.1103/physrevd.83.112001. ISSN 1550-7998.
  7. Butterworth, Jon (2006). "Underlying Events" (PDF). ucl.ac.uk. London. Wilhelm and Else Heraeus-Seminar on “New Event Generators for Particle Physics Experiments”
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