Crivellin Andreas, Hoferichter Martin
Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland.
Physik-Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland.
Phys Rev Lett. 2020 Sep 11;125(11):111801. doi: 10.1103/PhysRevLett.125.111801.
Nuclear β decays as well as the decay of the neutron are well-established low-energy probes of physics beyond the standard model (SM). In particular, with the axial-vector coupling of the nucleon g_{A} determined from lattice QCD, the comparison between experiment and SM prediction is commonly used to derive constraints on right-handed currents. Further, in addition to the CKM element V_{us} from kaon decays, V_{ud} from β decays is a critical input for the test of CKM unitarity. Here, we point out that the available information on β decays can be reinterpreted as a stringent test of lepton flavor universality (LFU). In fact, we find that the ratio of V_{us} from kaon decays over V_{us} from β decays (assuming CKM unitarity) is extremely sensitive to LFU violation (LFUV) in W-μ-ν couplings thanks to a CKM enhancement by (V_{ud}/V_{us})^{2}∼20. From this perspective, recent hints for the violation of CKM unitarity can be viewed as further evidence for LFUV, fitting into the existing picture exhibited by semileptonic B decays and the anomalous magnetic moments of muon and electron. Finally, we comment on the future sensitivity that can be reached with this LFU violating observable and discuss complementary probes of LFU that may reach a similar level of precision, such as Γ(π→μν)/Γ(π→eν) at the PEN and PiENu experiments or even direct measurements of W→μν at an FCC-ee.
核β衰变以及中子衰变是已被充分确立的超出标准模型（SM）的低能物理探针。特别是，利用从格点量子色动力学确定的核子轴矢耦合(g_{A})，实验与标准模型预测之间的比较通常用于推导对右手流的限制。此外，除了来自K介子衰变的CKM矩阵元(V_{us})，来自β衰变的(V_{ud})是检验CKM幺正性的关键输入量。在此，我们指出，关于β衰变的现有信息可以重新解释为对轻子味普适性（LFU）的严格检验。事实上，我们发现（假设CKM幺正性）K介子衰变中的(V_{us})与β衰变中的(V_{us})之比，由于((V_{ud}/V_{us})^{2}\sim20)的CKM增强效应，对W - μ - ν耦合中的轻子味普适性破坏（LFUV）极其敏感。从这个角度来看，最近关于违反CKM幺正性的暗示可以被视为LFUV的进一步证据，符合由半轻子B衰变以及μ子和电子的反常磁矩所展现的现有图景。最后，我们评论了利用这个违反轻子味普适性的可观测量所能达到的未来灵敏度，并讨论了可能达到类似精度水平的轻子味普适性的互补探针，例如在PEN和PiENu实验中的(\Gamma(\pi\rightarrow\mu\nu)/\Gamma(\pi\rightarrow e\nu))，甚至在FCC - ee上对W→μν的直接测量。
