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Homework Statement
I'm trying to show that the general form of the propagator is
[itex]D(x) = - \int \frac{d^3k}{(2\pi)^32\omega_k}[e^{-i(\omega_k t - \vec{k}\cdot\vec{x})}\theta(x^0) + e^{i(\omega_k - \vec{k}\cdot\vec{x})}\theta(-x^0)][/itex]
but my answers always seem to differ by a sign.
Homework Equations
[itex]D(x-y) = \int \frac{d^4k}{(2\pi)^4}\frac{e^{ik(x-y)}}{k^2-m^2+i\epsilon}[/itex]The Attempt at a Solution
If we take [itex]x^0 > 0[/itex] then Zee states that we take the integration over [itex]k^0[/itex] to be in the upper half of the complex plane.
This gives
[itex]D(x) = \int \frac{d\vec{k}dk^0}{(2\pi)^4}\frac{e^{i k\cdot x}}{k^2 - m^2 + i\epsilon}[/itex]
[itex]D(x) = \int \frac{d\vec{k}}{(2\pi)^4}2\pi i\,Res\left(\frac{e^{i k\cdot x}}{(k^0)^2 - \vec{k}^2-m^2 + i \epsilon} , -\sqrt{\omega_k^2-i\epsilon} \right)[/itex]
[itex]D(x) = -i\int \frac{d\vec{k}dk^0}{(2\pi)^3}\frac{e^{-i(\omega_k + \vec{k}\cdot\vec{x})}}{2\omega_k}[/itex]
Zee gets
[itex]D(x) = -i\int \frac{d\vec{k}dk^0}{(2\pi)^3}\frac{e^{i(\omega_k - \vec{k}\cdot\vec{x})}}{2\omega_k}[/itex]