# A Terminology of 1PI diagrams

1. Feb 15, 2017

### spaghetti3451

One-particle irreducible diagrams are diagrams that cannot be broken into two disconnected diagrams by cutting an internal propagator.

1. Why are the external lines on an one-particle irreducible diagram truncated/amputated?

2. What is the motivation for the word 'one-particle' in the terminology?

2. Feb 15, 2017

### A. Neumaier

because that's how they are used in scattering calculations.
One also defines $k$-particle irreducible diagrams (which cannot be made disconnected by removing up to $k$ internal lines). These describe the properties of $k$-point correlations (involving $k$ particles, hence the name), which are important for calculations in nonequilibrium quantum field theory.