Kinetic mixing between gauge fields of different U(1) factors is a well-studied phenomenon in 4d EFT. In string compactifications with U(1)s from sequestered D-brane sectors, kinetic mixing becomes a key target for the UV prediction of a phenomenologically important EFT operator. Surprisingly, in many cases kinetic mixing is absent due to a non-trivial cancellation. In particular, D3-D3 kinetic mixing in type-IIB vanishes while D3-anti-D3 mixing does not. This follows both from exact CFT calculations on tori as well as from a leading-order 10d supergravity analysis, where the key cancellation is between the C₂ and B₂ contribution. We take the latter approach, which is the only one available in realistic Calabi-Yau settings, to a higher level of precision by including sub-leading terms of the brane action and allowing for non-vanishing C₀. The exact cancellation persists, which we argue to be the result of \({\text{SL}}\left(2,{\mathbb{R}}\right)\) self-duality. We note that a B₂C₂ term on the D3-brane, which is often missing in the recent literature, is essential to obtain the correct zero result. Finally, allowing for \({\text{SL}}\left(2,{\mathbb{R}}\right)\)-breaking fluxes, kinetic mixing between D3-branes arises at a volume-suppressed level. We provide basic explicit formulae, both for kinetic as well as magnetic mixing, leaving the study of phenomenologically relevant, more complex situations for the future. We also note that describing our result in 4d supergravity appears to require higher-derivative terms — an issue which deserves further study.

不同 U(1) 因子的规范场之间的动力学混合是 4d EFT 中经过充分研究的现象。在使用来自隔离的 D 膜扇区的 U(1) 进行弦压缩时，动力学混合成为现象学上重要的 EFT 算子的 UV 预测的关键目标。令人惊讶的是，在许多情况下，由于不平凡的抵消，不存在动力混合。特别是，IIB 型中的 D3-D3 动力学混合消失，而 D3-抗 D3 混合则不会。这是根据环面的精确 CFT 计算以及主阶 10d 超重力分析得出的，其中关键抵消位于C ₂和B ₂贡献之间。我们采用后一种方法（这是现实 Calabi-Yau 设置中唯一可用的方法），通过包含膜作用的次主导项并允许非零C ₀达到更高的精度。确切的抵消仍然存在，我们认为这是\({\text{SL}}\left(2,{\mathbb{R}}\right)\)自对偶性的结果。我们注意到，D3 膜上的B ₂ C ₂项（在最近的文献中经常缺失）对于获得正确的零结果至关重要。最后，考虑到\({\text{SL}}\left(2,{\mathbb{R}}\right)\)破坏通量，D3 膜之间的动力学混合会在体积抑制的水平上出现。我们为动力学和磁力混合提供了基本的显式公式，将现象学相关的、更复杂的情况的研究留到了未来。我们还注意到，在 4d 超重力中描述我们的结果似乎需要更高的导数项——这是一个值得进一步研究的问题。