Editing S-matrix (section)

== Motivation ==

In high-energy [[particle physics]] one is interested in computing the [[probability]] for different outcomes in [[scattering]] experiments. These experiments can be broken down into three stages:
# Making a collection of incoming [[subatomic particle|particle]]s collide (usually two kinds of particles with high energies).
# Allowing the incoming particles to interact. These interactions may change the types of particles present (e.g. if an [[electron]] and a [[positron]] [[Annihilation|annihilate]] they may produce two [[photon]]s).
# Measuring the resulting outgoing particles.

The process by which the incoming particles are transformed (through their [[Fundamental interaction|interaction]]) into the outgoing particles is called [[scattering]]. For particle physics, a physical theory of these processes must be able to compute the probability for different outgoing particles when different incoming particles collide with different energies.

The ''S''-matrix in quantum field theory achieves exactly this. It is assumed that the small-energy-density approximation is valid in these cases.