One-loop Feynman diagram

In physics, a one-loop Feynman diagram is a Feynman diagram with one independent "circle". Such a diagram can be obtained from a tree diagram by joining its two external legs.

Diagrams with loops correspond to the effects of quantum physics. Because one-loop diagrams only contain one loop, they express the next-to-classical contributions called the semiclassical contributions.

One-loop diagrams are usually computed as the integral over one independent momentum that can "run in the loop". The Casimir effect and Hawking radiation are examples of phenomena whose existence can be proved using one-loop Feynman diagrams.