We analyze the sensitivity of hadronic $\tau$ decays to nonstandard interactions within the model-independent framework of the standard model effective field theory. Both exclusive and inclusive decays are studied, using the latest lattice data and QCD dispersion relations. We show that there are enough theoretically clean channels to disentangle all the effective couplings contributing to these decays, with the $\tau \to \pi \pi {\nu }_{\tau }$ channel representing an unexpected powerful new physics probe. We find that the ratios of nonstandard couplings to the Fermi constant are bound at the subpercent level. These bounds are complementary to the ones from electroweak precision observables and $pp\to \tau {\nu }_{\tau }$ measurements at the LHC. The combination of $\tau$ decay and LHC data puts tighter constraints on lepton universality violation in the gauge boson-lepton vertex corrections.

• Received 15 September 2018

DOI:https://doi.org/10.1103/PhysRevLett.122.221801

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society