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# A challenge to lepton universality in B-meson decays

## Abstract

One of the key assumptions of the standard model of particle physics is that the interactions of the charged leptons, namely electrons, muons and taus, differ only because of their different masses. Whereas precision tests comparing processes involving electrons and muons have not revealed any definite violation of this assumption, recent studies of B-meson decays involving the higher-mass tau lepton have resulted in observations that challenge lepton universality at the level of four standard deviations. A confirmation of these results would point to new particles or interactions, and could have profound implications for our understanding of particle physics.

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## Acknowledgements

We recognize the contributions and dedication of our colleagues in the large international collaborations supporting the operation of the BaBar (M.F.S., R.K., V.L.), Belle (T.K., Y.S.) and LHCb (G.C., B.H.) detectors, the data processing and the data analyses on which the results presented in this Review are based. None of this would have been achieved without the efforts of the teams at SLAC, KEK and CERN who achieved excellent beam conditions and delivered high luminosities of the e+e and pp storage rings over many years. We acknowledge support from the Organisation for Scientific Research (NWO) of the Netherlands, the US National Science Foundation and Department of Energy, the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Excellence Cluster of the DFG of Germany: Origin and Structure of the Universe, and the Japan Society for the Promotion of Science (JSPS).

## Author information

Authors

### Contributions

All authors contributed to writing and editing the manuscript.

### Corresponding author

Correspondence to Vera Lüth.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

Reviewer Information Nature thanks G. Isidori and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Ciezarek, G., Franco Sevilla, M., Hamilton, B. et al. A challenge to lepton universality in B-meson decays. Nature 546, 227–233 (2017). https://doi.org/10.1038/nature22346

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• DOI: https://doi.org/10.1038/nature22346

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