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I read in an article that the Standard Model leaves unanswered questions about the non-perturbative effects of the QCD. I have basic knowledge about the perturbative and non-perturbative QCD. Could you please tell me what are these effects and unanswered questions? Sorry in advance if my question is too broad.

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    $\begingroup$ Giving the reference of the article is the least you could do, don't you think? $\endgroup$ – user154997 Sep 1 '17 at 10:38
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It seems that one of the question is the strong CP-problem in the QCD. It reflects the non-trivial vacuum structure of the QCD, being composed of the different states labelling by discrete number $n$ with the weight $e^{i\theta n}$. This is reflected in appearance of the term $S= \theta\int d^{4}x G_{\mu\nu}^{a}\tilde{G}^{\mu\nu}_{a}$, with $G_{\mu\nu}^{a}$ being the gluon strength tensor and $\tilde{G}$ is the dual one. This term, being invisible within the perturbation theory (since can be expressed as the integral of the total derivative), manifests itself non-perturbatively. In particular, the dipole electric moment of the neutron is proportional to $\theta$.

From experiment we know that $|\theta| < 10^{-10}$. But this smallness isn't expected within the QCD, in which all dimensionless quantities are predicted to be of order of one...

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