Tree level

In the context of perturbation theory, a tree-level Feynman diagram is a topology contributing to the probability amplitude for a scattering process which contains no closed loops. If the S-matrix elements for a process with given initial and final states receive contributions from tree-level diagrams (i.e. the tree-level diagrams are also the lowest order in perturbation theory in which this process is first described), such graphs also constitute the leading-order diagrams.