Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: CP-violating theta parameter in the domain model of the QCD vacuum   A non-zero CP-violating $\theta$ parameter is treated in the domain model
which assumes a cluster-like vacuum structure whose units are characterised in
particular by a topological charge which is not necessarily an integer number.
In the present paper we restrict consideration to rational values of the
charge. The model has previously been shown to manifest confinement,
spontaneous chiral symmetry breaking and the absence of an axial U(1) Goldstone
boson. We find that the specific structure of the minima of the free energy
density of the domain ensemble forces a $2\pi$-periodicity of observables in
$\theta$ for any number of light quarks, that vacuum doubling occurs at
$\theta=\pi$ for any $N_f>1$ and any value of topological charge $q$. These
features are in agreement with expectations based on anomalous Ward identities
and large $N_c$ effective theories. We find also additional values of $\theta$
depending on $q$ for which vacuum doubling occurs.

28
The categories of this paper are: alg-geom
4
The categories of this paper are: cond-mat
30
The categories of this paper are: q-alg
14
The categories of this paper are: math-ph
35
The categories of this paper are: mtrl-th
24
The categories of this paper are: dg-ga
3
The categories of this paper are: physics
20
The categories of this paper are: acc-phys
34
The categories of this paper are: chem-ph
29
The categories of this paper are: funct-an
10
The categories of this paper are: q-bio
32
The categories of this paper are: atom-ph
8
The categories of this paper are: hep-ex
31
The categories of this paper are: ao-sci
1
The categories of this paper are: cs
26
The categories of this paper are: bayes-an