强-有重力和无重力时的$CP$
Strong-$CP$ with and without gravity
摘要：
传统上，强$CP$问题被假定为一个自然性难题，轴子解有时被视为一个特别的解决方案。引力要么被忽略，要么被认为是对全局佩切-奎因对称性的威胁。我们解释说，情况根本不同。在引力中，轴子是由$S$ -矩阵施加的一致性问题：每个规范扇区必须包含具有相应$\barθ$精确松弛的轴子。我们表明，这有利于一个替代的和非常简单的制定的轴子，完全固定的规范冗余的QCD，没有涉及的全球对称性。轴子机制是量子色动力学$3$ -形式的希格斯效应，确保了物理学在算符展开的所有阶次上都独立于$\barθ$。在不久的将来，对中子EDM的实验探测将是超越标准模型的$CP$违反物理学的明确信号。轴子耦合与引力的尺度有关。
英文摘要：
Conventionally, the strong-$CP$ problem is assumed to be a naturalness puzzle, with the axion solution sometimes viewed as an ad hoc fix. Gravity is either ignored or taken as a threat for the global Peccei-Quinn symmetry. We explain that the situation is fundamentally different. In gravity, axion is a matter of consistency imposed by the $S$-matrix: Each gauge sector must include axion with exact relaxation of the corresponding $\barθ$. We show that this favors an alternative and remarkably simple formulation of the axion, fully fixed by the gauge redundancy of QCD, without involvement of a global symmetry. The axion mechanism is a Higgs effect for the QCD $3$-form, ensuring that physics is independent of $\barθ$ to all orders in operator expansion. A near-future experimental detection of the neutron EDM will be an unambiguous signal of $CP$-violating physics beyond the Standard Model. The axion coupling is tied to the scale of gravity.
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