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".