Strong CP Problem
May 16, 2022
Quantum chromodynamics, the theory of strong interaction, admits a term which explicitly violates parity $P$ and time reflection invariance $T$ and due to the CPT theorem,
also $CP$. Naturalness would predict this term with strength one, whereas empirical evidence from the electric dipole moment
of the neutron yields an upper bound of almost vanishing strength of this term. This is
the so-called strong CP problem. In this report, I follow
the path from the initial $U(1)_A$ problem of strong interaction physics, via the $U(1)_A$ anomaly and
instantons, the theta vacuum, the electric dipole moment of the neutron and the
necessity of explicit symmetry breaking to the Peccei-Quinn model, extended to invisible axions and axion-like particles
and finally back to electric dipole moments of subatomic particles.
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