In a typical particle physics event, the incoming particles are
scattered or destroyed, and up to hundreds of particles can be produced, although few are likely to be new particles not discovered before. In the old
bubble chambers and
cloud chambers, "events" could be seen by observing
charged particle tracks emerging from the region of the event before they curl due to the
magnetic field through the chamber acting on the particles. At modern
particle accelerators, events are the result of the interactions which occur from a
beam crossing inside a
particle detector.
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. Individual particle physics events are modeled by
scattering theory based on an underlying
quantum field theory of the particles and their interactions. The
S-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 a
perturbative expansion in terms of
Feynman diagrams. Events occur naturally in
astrophysics and
geophysics, such as subatomic
particle showers produced from
cosmic ray scattering events. ==References==