Correlation function (quantum field theory)
For time-dependent correlation functions, the time-ordering operator is included.
Correlation functions are also called simply correlators. Sometimes, the phrase Green's function is used not only for two-point functions, but for any correlators.
The correlation function can be interpreted physically as the amplitude for propagation of a particle or excitation between y and x. In the free theory, it is simply the Feynman propagator (for n=2).
- Peskin, Michael; Schroeder, David. An Introduction to Quantum Field Theory. Addison-Wesley.