Schwinger parametrization

Schwinger parametrization is a technique for evaluating loop integrals which arise from Feynman diagrams with one or more loops.

Using the well-known observation that

${\displaystyle {\frac {1}{A^{n}}}={\frac {1}{(n-1)!}}\int _{0}^{\infty }du\,u^{n-1}e^{-uA},}$

Julian Schwinger noticed that one may simplify the integral:

${\displaystyle \int {\frac {dp}{A(p)^{n}}}={\frac {1}{\Gamma (n)}}\int dp\int _{0}^{\infty }du\,u^{n-1}e^{-uA(p)}={\frac {1}{\Gamma (n)}}\int _{0}^{\infty }du\,u^{n-1}\int dp\,e^{-uA(p)},}$

for Re(n)>0.

Another version of Schwinger parametrization is:

${\displaystyle {\frac {i}{A+i\epsilon }}=\int _{0}^{\infty }du\,e^{iu(A+i\epsilon )},}$

which is convergent as long as ${\displaystyle \epsilon >0}$ and ${\displaystyle A\in \mathbb {R} }$.[1] It is easy to generalize this identity to n denominators.