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β Decays as Sensitive Probes of Lepton Flavor Universality


Crivellin, Andreas; Hoferichter, Martin (2020). β Decays as Sensitive Probes of Lepton Flavor Universality. Physical Review Letters, 125(11):111801.

Abstract

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 gA 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 Vus from kaon decays, Vud 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 Vus from kaon decays over Vus from β decays (assuming CKM unitarity) is extremely sensitive to LFU violation (LFUV) in W−μ−ν couplings thanks to a CKM enhancement by (Vud/Vus)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.

Abstract

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 gA 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 Vus from kaon decays, Vud 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 Vus from kaon decays over Vus from β decays (assuming CKM unitarity) is extremely sensitive to LFU violation (LFUV) in W−μ−ν couplings thanks to a CKM enhancement by (Vud/Vus)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.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Physics Institute
Dewey Decimal Classification:530 Physics
Scopus Subject Areas:Physical Sciences > General Physics and Astronomy
Uncontrolled Keywords:General Physics and Astronomy
Language:English
Date:10 September 2020
Deposited On:05 Nov 2020 16:24
Last Modified:06 Nov 2020 21:00
Publisher:American Physical Society
ISSN:0031-9007
OA Status:Hybrid
Free access at:Publisher DOI. An embargo period may apply.
Publisher DOI:https://doi.org/10.1103/physrevlett.125.111801
Project Information:
  • : FunderSNSF
  • : Grant IDPCEFP2_181117
  • : Project TitleStrong-Interaction Effects in the Search for Physics beyond the Standard Model

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