# Fourier Transform of propagators

1. Oct 1, 2013

### L0r3n20

I've been assigned the following homework:
I have to compute the spectral density of a QFT and in order to do so I have to compute Fourier tranform of the following quantity (in Minkowsky signature, mostly minus)

$\rho\left(p\right) = \int \frac{1}{\left(-x^2 + i \epsilon x_0\right)^{\Delta}} e^{i p \cdot x} d^4 x$

Using residual theorem I've been able to compute exactly the case for $\Delta = 1$ and it turns out to be $\rho(p) \propto \delta(p^2) \theta(p_0)$ .
For the case $\Delta \neq 1$ it's a bit tricky but I managed to perform the integration over $x_0$ using the residual once again and I found ($r^2 = x_i x^i$)

$\sum_{k=0}^{\Delta -1} {\Delta-1 \choose k} (i p_0)^k \frac{\Gamma(\Delta -1 - k)}{\Gamma(\Delta)^2} \frac{1}{(2 r)^{2 \Delta+1-k}} \left(e^{i p_0 r} + (-1)^{2 \Delta -1 - k} e^{-i p_0 r}\right)$

Probably something is wrong since when I perform the remaining integrations (in spherical coordinates) I do not recover the following result:

$\rho\left(p\right) = \frac{\Delta -1}{4^{\Delta} \Gamma(\Delta)^2} \theta(p_0) \delta(p^2) (p^2)^{\Delta -2}$
Any help would be great.