For massless $\phi^4$ theory, if exterior momentums are going to zero, then this diagram will be
$$\int \frac{dk^4}{k^4}$$ will suffer from infrared divergence. Because the infrared divergence, unlike UV divergence, often has physical meaning, how do I handle this divergence? What's the meaning?
IR divergences, if unavoidable, usually means the point about which perturbation theory is constructed is an inappropriate one.
– vik Sep 03 '15 at 16:33