# What is instanton effect in QCD?

In the context of the Peccei-Quinn Symmetry in solving the Strong CP problem, it is said that the Axion develops a mass due to QCD instanton effects below $$\Lambda_{QCD}$$, which is very much temperature-dependent.

I didn't understand what is the instanton effect and how does it give mass to the Axion.

• What is meant is that non-perturbatively $(\tilde G G) ^\#\neq 0$. Take a look at that: inspirehep.net/literature/142732 – nwolijin Mar 2 at 9:40
• Perhaps you can add the reference where that was stated, so that the community has a lead to follow. I have heard similar statements in relation to the eta prime particle for example, where such phenomena is explained in the literature. But I am also interested in the axion case... – ohneVal Mar 3 at 10:38