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arXiv · 2609.00620

Status of the strong $CP$ problem

Wen-Yuan Ai
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Abstract

The strong $CP$ problem remains one of the major open questions in the Standard Model of particle physics. Its best-known solution is the Peccei--Quinn mechanism, which predicts a new pseudoscalar boson, the QCD axion. In recent years, alternative solutions and new perspectives on the foundations of the strong $CP$ problem have also received renewed attention. In this review, we provide a pedagogical account of the underlying physics and summarise the current theoretical and phenomenological status of the problem.

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Wen-Yuan Ai. 2026-09-01. Status of the strong $CP$ problem. https://arxiv.org/abs/2609.00620

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