Explaining the Cabibbo Angle Anomaly

Abstract

The determinations of the Cabibbo angle from kaon, pion, D and tau decays disagree with the one from super-allowed beta decays. The resulting ≈ 3 σ deficit in first row (but also first column) CKM unitarity is known as the Cabibbo angle anomaly (CAA). Determining V ud from beta decays requires knowledge of the Fermi constant GF , usually extracted from muon decay. However, because new physics might also affect muon decay, and thus the determination of the Fermi constant, an interesting interplay with the global electroweak fit arises where GF is a crucial input. In these proceedings we review the CAA and its different interpretations in terms of physics beyond the SM, first discussing the corresponding effect in the effective field theory and then using (simplified) models.

Natural Suppression of FCNCs at the One-Loop Level in $Z^\prime$ Models with Implications for K, D and B Decays

“…There was recently a large activity in connection with the so-called Cabibbo anomaly (CAA) which is nicely reviewed in [30]. There exist tensions between different determinations of the elements |V us | and |V ud | in the CKM matrix from different decays.…”

“…As reviewed in [30] numerous authors made efforts in the literature to find the origin of this anomaly and to remove it with the help of NP models containing W , vector-like leptons, vector-like quarks, scalars, Z gauge bosond and leptoquarks.…”

Experimental overview of CKM metrology from kaon physics

“…The most precise experimental determination of 𝑉 𝑢𝑑 is derived from super-allowed 0 + → 0 + nuclear 𝛽 decays. Using a set of 15 of the most precisely measured of these processes [10] and applying the most up-to-date radiative corrections [7] |𝑉 𝑢𝑑 | 𝛽 = 0.97367 (11) exp (13) Rad.Cor. (27) Nucl.Struc.…”

“…However, the combined best fit region, shown in yellow, is inconsistent with the dashed line and numerically the best fit value from experimental measurements is 2.8 standard deviations (2.8𝜎) from the unitarity expectation. This significant tension is called the Cabibbo angle anomaly [13], with the implied link to the critical 𝜆 = 𝑉 𝑢𝑠 parameter.…”