Inparticle physics, anevent refers to the results just after afundamental interaction takes place betweensubatomic particles, occurring in a very short time span, at a well-localized region of space. Because of theuncertainty principle, an event in particle physics does not have quite the same meaning as it does in thetheory of relativity, in which an "event" is a point inspacetime which can be known exactly, i.e., a spacetime coordinate.
In a typical particle physics event, the incoming particles arescattered or destroyed, and up to hundreds of particles can be produced, although few are likely to be new particles not discovered before.[1]
In the oldbubble chambers andcloud chambers, "events" could be seen by observingcharged particle tracks emerging from the region of the event before they curl due to themagnetic field through the chamber acting on the particles. At modernparticle accelerators, events are the result of the interactions which occur from abeam crossing inside aparticle detector.
Physical quantities used to analyze events include thedifferential cross section, theflux of the beams (which in turn depends on thenumber density of the particles in the beam and their averagevelocity), and therate andluminosity of the experiment.[dubious –discuss]
Individual particle physics events are modeled byscattering theory based on an underlyingquantum field theory of the particles and their interactions. TheS-matrix is used to characterize the probability of various event outgoing particle states given the incoming particle states. For suitable quantum field theories, the S-matrix may be calculated by aperturbative expansion in terms ofFeynman diagrams.
Events occur naturally inastrophysics andgeophysics, such as subatomicparticle showers produced fromcosmic ray scattering events.