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U(2)^5 flavor symmetry and lepton universality violation in W -> tau nu_tau

The seeming violation of universality in the tau lepton coupling to the W boson suggested by LEP II data is studied using an Effective Field Theory (EFT) approach. Within this framework we explore how this feature fits into the current constraints from electroweak precision observables using different assumptions about the flavor structure of New Physics, namely [U(2) x U(1)]^5 and U(2)^5. We show the importance of leptonic and semileptonic tau decay measurements, giving 3-4 TeV bounds on the New Physics effective scale at 90% C.L. We conclude under very general assumptions that it is not possible to accommodate this deviation from universality in the EFT framework, and thus such a signal could only be explained by the introduction of light degrees of freedom or New Physics strongly coupled at the electroweak scale.