Semileptonic decays of light quarks beyond the Standard Model

TL;DR

This paper investigates non-standard contributions to semileptonic light quark decays beyond the Standard Model using an effective field theory approach, deriving constraints on new physics operators from precision measurements of CKM unitarity deviations.

Contribution

It provides a model-independent analysis of new physics effects on semileptonic decays and derives bounds on effective operators from CKM unitarity tests and electroweak precision data.

Findings

Deviations from CKM unitarity constrain new physics operators to scales above 11 TeV.

Electroweak precision tests are less restrictive than CKM unitarity bounds.

A nonzero CKM deviation could be explained by a single poorly constrained four-fermion operator.

Abstract

We describe non-standard contributions to semileptonic processes in a model independent way in terms of an SU(2)_L X U(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, \Delta_CKM = |V_ud|^2 + |V_us|^2 + |V_ub|^2 - 1, receives contributions from four effective operators. The phenomenological bound of \Delta_CKM = -1E-4 +- 6E-4 provides strong constraints on all four operators, corresponding to an effective scale greater than 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 \Delta_CKM = -0.01 (90% CL). An observed nonzero \Delta_CKM at this level could be explained in terms of a single four-fermion operator which is relatively poorly constrained by electroweak precision measurements.