Before answering the question more or less directly, I'd like to point out that this is a good question that provides an object lesson and opens a foray into the topics of *singular integral equations*, *analytic continuation* and *dispersion relations*. Here are some references of these more advanced topics: Muskhelishvili, *Singular Integral Equations*; Courant & Hilbert, *Methods of Mathematical Physics, Vol I*, Ch 3; Dispersion Theory in High Energy Physics, Queen & Violini; Eden et.al., *The Analytic S-matrix*. There is also a condensed discussion of `invariant functions' in Schweber, *An Intro to Relativistic QFT Ch13d*.

The quick answer is that, for $m^2 \in\mathbb{R}$, there's no "shortcut." One must *choose* a path around the singularities in the denominator. The appropriate choice is governed by the boundary conditions of the problem at hand. The $+i\epsilon$ "trick" (it's not a "trick") simply encodes the boundary conditions relevant for causal propagation of particles and antiparticles in field theory.

We briefly study the analytic form of $G(x-y;m)$ to demonstrate some of these features.

Note, first, that for real values of $p^2$, the singularity in the denominator of the integrand signals the presence of (a) branch point(s). In fact, [Huang, *Quantum Field Theory: From Operators to Path Integrals*, p29] the Feynman propagator for the scalar field (your equation) may be explicitly evaluated:
\begin{align}
G(x-y;m) &= \lim_{\epsilon \to 0} \frac{1}{(2 \pi)^4} \int d^4p \, \frac{e^{-ip\cdot(x-y)}}{p^2 - m^2 + i\epsilon} \nonumber \\
&= \left \{ \begin{matrix}
-\frac{1}{4 \pi} \delta(s) + \frac{m}{8 \pi \sqrt{s}} H_1^{(1)}(m \sqrt{s}) & \textrm{ if }\, s \geq 0 \\
-\frac{i m}{ 4 \pi^2 \sqrt{-s}} K_1(m \sqrt{-s}) & \textrm{if }\, s < 0.
\end{matrix} \right.
\end{align}
where $s=(x-y)^2$.

The first-order Hankel function of the first kind $H^{(1)}_1$ has a logarithmic branch point at $x=0$; so does the modified Bessel function of the second kind, $K_1$. (Look at the small $x$ behavior of these functions to see this.)

A branch point indicates that the Cauchy-Riemann conditions have broken down at $x=0$ (or $z=x+iy=0$). And the fact that these singularities are logarithmic is an indication that we have an endpoint singularity [eg. Eden et. al., Ch 2.1]. (To see this, consider $m=0$, then the integrand, $p^{-2}$, has a zero at the lower limit of integration in $dp^2$.)

Coming back to the question of boundary conditions, there is a good discussion in Sakurai, *Advanced Quantum Mechanics*, Ch4.4 [NB: "East Coast" metric]. You can see that for large values of $s>0$ from the above expression that we have an outgoing wave from the asymptotic form of the Hankel function.

Connecting it back to the original references I cited above, the $+i\epsilon$ form is a version of the Plemelj formula [Muskhelishvili]. And the expression for the propagator is a type of Cauchy integral [Musk.; Eden et.al.]. And this notions lead quickly to the topics I mentioned above -- certainly a rich landscape for research.

This post imported from StackExchange Physics at 2014-07-13 04:38 (UCT), posted by SE-user MarkWayne