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3
votes
0answers
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Why does the axion solve the CP problem?

I don't understand why the CP violating term in QCD vanishes when we introduce the axion. The term is $$\left(\frac{a}{f_a}+\theta\right)G\tilde{G}$$ It is claimed that $\left\langle a\right\rangle=-\...
3
votes
1answer
167 views

How do we see that the axion is a pseudoscalar?

The axion is the pseudo-Goldstone boson associated to the breaking of the conjectured Peccei-Quinn Abelian symmetry. The axion couples to the SM gauge fields in a CP-invariant manner (e.g. $aF\tilde F ...
6
votes
1answer
192 views

CP-Symmetry of the axion field and decay constant

In the $U(1)$-Peccei-Quinn-Symmetry an axion field $a$ is introduced in order to solve the strong CP-Problem. It is said, that below a certain scale $f_{a}$ this symmetry is broken and you are able to ...
5
votes
1answer
338 views

Does the Peccei-Quinn (PQ) mechanism require fine-tuning?

The strong CP problem: the QCD theta parameter can not be arbitrarily small due to 't Hooft's doctrine of naturalness (a parameter can be arbitrarily small only when putting it to zero provides an ...