New Physics bounds from CKM-unitarity

TL;DR

This paper uses an effective field theory approach to analyze how new physics could affect CKM unitarity tests, providing constraints on new physics operators from precision measurements of V_ud and V_us.

Contribution

It demonstrates that CKM unitarity tests are the most sensitive probes for flavor-blind new physics, constraining operator scales above 11 TeV.

Findings

CKM unitarity tests constrain new physics operators to scales >11 TeV.

Four operators can cause deviations, tightly constrained by current data.

Constraints are comparable or better than Z pole observables.

Abstract

Using an effective field theory approach, we study the new physics (NP) corrections to muon and beta decays and their effects on the extractions of V_ud and V_us. Assuming nearly flavor blind NP interactions we find that the CKM-unitarity test is the only way to expose NP. The four short-distance operators that can generate a deviation are strongly constrained by the phenomenological bound |Vud|^2 + |Vus|^2 + |Vub|^2 - 1 = (-0.0001 \pm 0.0006), corresponding to an effective scale > 11 TeV (90% CL). Depending on the operator, this constraint is at the same level or better than that generated by the Z pole observables.