In particle physics, an event refers to the results just after a fundamental interaction takes place between subatomic particles, occurring in a very short time span, at a well-localized region of space.
Because of the uncertainty principle, an event in particle physics does not have quite the same meaning as it does in the theory of relativity, in which an "event" is a point in spacetime which can be known exactly, i.e., a spacetime coordinate.
Physical quantities used to analyze events include the differential cross section, the flux of the beams (which in turn depends on the number density of the particles in the beam and their average velocity), and the rate and luminosity of the experiment.
[dubious – discuss] Individual particle physics events are modeled by scattering theory based on an underlying quantum field theory of the particles and their interactions.
For suitable quantum field theories, the S-matrix may be calculated by a perturbative expansion in terms of Feynman diagrams.