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arXiv · 2603.25729

$\theta$ Angle and Axial Anomaly in Holographic QCD

Csaba Csáki·Eric Kuflik·Wei Xue·Taewook Youn
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Abstract

We present a bottom-up holographic description of the QCD $\theta$-vacuum and the $U(1)_A$ anomaly in five dimensions. The multi-branched $\theta$-vacuum structure emerges geometrically from a higher-dimensional gauge field, while the axial anomaly is realized through a St\"uckelberg coupling that is dual to a Chern-Simons term. In this framework, the $\eta'$ meson appears as a zero mode of bulk fluctuations, and its mass arises from the anomaly-induced St\"uckelberg term. The construction provides a transparent holographic derivation of the anomaly contribution to the $\eta'$ mass and naturally reproduces the Witten-Veneziano relation between the $\eta'$ mass and the Yang-Mills topological susceptibility.

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Csaba Csáki, Eric Kuflik, Wei Xue, Taewook Youn. 2026-03-26. $\theta$ Angle and Axial Anomaly in Holographic QCD. https://arxiv.org/abs/2603.25729

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