Semileptonic decays of light quarks beyond the Standard Model

We describe non-standard contributions to semileptonic processes in a model independent way in terms of an SU(2)LxU(1)Y invariant effective lagrangian at the weak scale, from which we derive the low-energy effective lagrangian governing muon and beta decays. We find that the deviation from Cabibbo universality, ΔCKM≡|Vud|2+|Vus|2+|Vub|2-1, receives contributions from four effective operators. The phenomenological bound ΔCKM=(-1±6)x10-4 provides strong constraints on all four operators, corresponding to an effective scale Λ>11 TeV (90% CL). Depending on the operator, this constraint is at the same level or better then the Z pole observables. Conversely, precision electroweak constraints alone would allow universality violations as large as ΔCKM=-0.01 (90% CL). An observed ΔCKM≠0 at this level could be explained in terms of a single four-fermion operator which is relatively poorly constrained by electroweak precision measurements.