Near threshold expansion of Feynman diagrams

E. Mendels
2005 Journal of Mathematical Physics  
The near threshold expansion of Feynman diagrams is derived from their configuration space representation, by performing all x integrations. The general scalar Feynman diagram is considered, with an arbitrary number of external momenta, an arbitrary number of internal lines and an arbitrary number of loops, in n dimensions and all masses may be different. The expansions are considered both below and above threshold. Rules, giving real and imaginary part, are derived. Unitarity of a sunset
more » ... m with I internal lines is checked in a direct way by showing that its imaginary part is equal to the phase space integral of I particles.
doi:10.1063/1.1947119 fatcat:lechlx7a2ve53bdmhw7dfrhghi