We present an overview of searches for violation of lepton flavor universality with a focus on low energy precision probes using π, K, τ, and nuclear beta decays. We review the current experimental results, summarize the theoretical status within the context of the Standard Model, and discuss future prospects (both experimental and theoretical). We review the implications of these measurements for physics beyond the Standard Model by performing a global model-independent fit to modified W couplings to leptons and four-fermion operators. We also discuss new physics in the context of simplified models and review Standard Model extensions with a focus on those that can explain a possible deviation from unitarity of the Cabibbo–Kobayashi–Maskawa quark mixing matrix.

中文翻译：

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用 Pion、Kaon、Tau 和 β 衰变测试 Lepton 风味的普遍性


我们概述了违反轻子风味普遍性的搜索，重点介绍使用 π、K、τ 和核 β 衰变的低能量精确探针。我们回顾了当前的实验结果，总结了标准模型背景下的理论现状，并讨论了未来的前景（实验和理论）。我们通过对改进的 W 耦合到轻子和四费米子算子进行独立于模型的全局拟合，回顾了这些测量对标准模型之外的物理学的影响。我们还在简化模型的背景下讨论了新的物理场，并回顾了标准模型扩展，重点关注那些可以解释 Cabibbo-Kobayashi-Maskawa 夸克混合矩阵可能偏离幺正性的扩展。