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WS22/23 – Abstract
Björn Garbrecht
(Technische Universität München)
26.1.2023
Problems for strong CP (PRETALK for students at 15:30 h)
QCD with massive quarks contains a CP-odd rephasing invariant, commonly called $\bar \theta$. Potential effects of this parameter may be mediated to hadrons (e.g. to the neutron's electric dipole or $eta^\prime\to\pi\pi$) in the effective theory following the pattern by which axial symmetry is broken by the 't Hooft vertex. Calculating the latter reveals that CP-violating effects are absent in the Green's functions for the fermions---the phases from instanton effects and quark masses are aligned. Yet, one must still interfere between the different topological sectors. In the path integral quantization, integer topological sectors only follow when Euclidean time (or real time on a contour with ${\rm i} \epsilon$ prescription) is taken to infinity. This implies that the spacetime volume must be taken to infinity before interfering among the sectors. Taking this into account, CP-violating correlations are absent despite nonvanishing $\bar\theta$. Some remarks concerning canonical quantization will be made toward the end of the seminar.
