Semileptonic decays of light quarks beyond the Standard Model Original Research Article

We describe non-standard contributions to semileptonic processes in a model independent way in terms of an image 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, image, receives contributions from four effective operators. The phenomenological bound image provides strong constraints on all four operators, corresponding to an effective scale image (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 image (90% CL). An observed image at this level could be explained in terms of a single four-fermion operator which is relatively poorly constrained by electroweak precision measurements.