TL;DR
This paper explores the strong CP problem in QCD, emphasizing that the most viable solution involves a spontaneously broken chiral symmetry, specifically the Peccei-Quinn symmetry.
Contribution
It clarifies the connection between the $U(1)_A$ problem, the $ heta$-vacuum, and the strong CP problem, advocating for the Peccei-Quinn mechanism as the only viable solution.
Findings
The $ heta$-vacuum leads to the strong CP problem.
Various solutions are examined and found lacking.
Spontaneous breaking of $U(1)_{PQ}$ remains the most viable solution.
Abstract
I discuss how the solution of the problem of QCD through the existence of the -vacuum gave rise to the strong CP problem. After examining various suggested solutions to this problem, I conclude that the only viable solution still is the one that involves the existence of a spontaneously broken chiral symmetry:
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