- #1

Gary Weiss

- 2

- 0

I don't understand this bit about the derivation of propagator expressions.

Bjorken and Drell describe the step function as:

[tex]\theta(\tau)=lim_{\epsilon \to 0}\frac{-1}{2\pi i}\oint_{-\infty}^{\infty}\frac{d\omega e^{-i\omega r}}{\omega + i \epsilon } [/tex]

the singularity is at [tex] -i \omega \epslion [/tex]

I understand that if I evaluate this integral with a path above the real axis

I get zero, and that if I evaluate it below the real axis I get 1.

however I don't understand why it's ok to choose the paths in this way.

What happened at tau<0 that allowed me to integrate only the upper half of

the real/imaginary axis?

thanks!