Title: Light new physics and the $\boldsymbolτ$ lepton dipole moments: prospects at Belle II Show Chinese title

Title: 轻新物理与$\boldsymbolτ$轻子磁矩：Belle II的前景

Abstract: While electron and muon dipole moments are well-established precision probes of physics beyond the Standard Model, it is notoriously challenging to test realistic New-Physics (NP) scenarios for the $\tau$ lepton. Constructing suitable asymmetries in $e^+e^-\to\tau^+\tau^-$ has emerged as a promising such avenue, providing access to the electric and magnetic dipole moment once a polarized electron beam is available, e.g., with the proposed polarization upgrade of the SuperKEKB $e^+e^-$ collider. However, this interpretation relies on an effective-field-theory (EFT) argument that only applies if the NP scale is large compared to the center-of-mass energy. In this Letter we address the consequences of the asymmetry measurements in the case of light NP, using light spin-0 and spin-1 bosons as test cases, to show how results can again be interpreted as constraints on dipole moments, albeit in a model-dependent manner, and how the decoupling to the EFT limit proceeds in these cases. In particular, we observe that the imaginary parts generated by light new particles can yield non-vanishing asymmetries even without electron polarization, presenting opportunities for NP searches that can be realized already with present data at Belle II. Show Chinese abstract

Abstract: 虽然电子和缪子的磁矩是标准模型之外物理现象的精确探测工具，但测试实际的新物理（NP）情景对于$\tau$轻子来说一直是一个臭名昭著的挑战。在$e^+e^-\to\tau^+\tau^-$中构建合适的不对称性已成为一种有希望的方法，一旦具备极化电子束，例如，通过 proposed 的 SuperKEKB $e^+e^-$对撞机极化升级，就可以获得电和磁偶极矩。然而，这种解释依赖于一个有效场论（EFT）的论点，该论点仅在 NP 能标远大于质心系能量时才适用。在本信中，我们针对轻 NP 的情况讨论不对称性测量的后果，使用轻自旋-0 和自旋-1 玻色子作为测试案例，以说明结果如何再次被解释为对偶极矩的约束，尽管是在模型依赖的方式下，并且说明这些情况下如何退耦到 EFT 极限。特别是，我们观察到由轻新粒子产生的虚部即使在没有电子极化的情况下也能产生非零不对称性，这为 NP 搜索提供了机会，这些搜索可以利用 Belle II 当前的数据实现。