Large mixing angles in a SU(2)_L gauge theory of weak interactions as a resonant effect of 1-loop transitions between quasi-degenerate fermions

TL;DR

This paper demonstrates that 1-loop quantum effects between nearly degenerate fermions can cause large mixing angles, including maximal mixing, within the Standard Model without requiring new physics, through a resonant enhancement mechanism.

Contribution

It introduces a resonant effect of 1-loop transitions that can produce large mixing angles from quasi-degenerate fermions, providing a novel explanation within the Standard Model.

Findings

1-loop transitions can induce large mixing angles.

Resonant enhancement leads to maximal mixing.

The mechanism is perturbatively stable within the Standard Model.

Abstract

We show that 1-loop transitions between two quasi-degenerate fermions can induce a potentially large renormalization of their mixing angle, and a large renormalized Cabibbo (or PMNS) angle when the second fermion pair in the same two generations is far from degeneracy. At the resonance, the "Cabibbo angle" gets maximal and simply connected to masses without invoking any new physics beyond the standard model. This solution appears as the only one "perturbatively stable" (mixing angles are then renormalized with respect to their classical values by small amounts).