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arXiv · 2509.03613

Strong CP Violation and large-$N_c$ spin-flavor symmetry

Thomas R. Richardson
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Abstract

We revisit the contribution of the QCD $\bar \theta$ term to the CP violating pion-nucleon couplings and the nucleon electric dipole moment in a combined large-$N_c$ and chiral perturbation theory framework. In particular, we approach this issue through the emergent spin-flavor symmetry of the baryon sector at large but finite $N_c$. We obtain good agreement with previous analyses for the pion-nucleon couplings and show that the large-$N_c$ framework indicates that tree-level contributions to the electric dipole moment possibly play a dominant role. The spin-flavor symmetry also enables us to provide novel constraints on CP violating pion-$\Delta$ couplings as well as the $\Delta$ electric dipole moment and $\Delta N$ transition moment.

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Thomas R. Richardson. 2025-09-03. Strong CP Violation and large-$N_c$ spin-flavor symmetry. https://doi.org/10.1103/1jb4-tqpn

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