Addressing the CKM Unitarity Problem with a Vector-like Up Quark

TL;DR

This paper proposes that a single vector-like top quark can explain deviations from CKM unitarity, satisfying experimental constraints and predicting a testable mass below 7 TeV.

Contribution

It introduces a model with a vector-like top quark to address CKM unitarity deviations, consistent with experimental bounds and accessible to future collider tests.

Findings

Deviations from CKM unitarity can be explained by a vector-like top quark.

The model satisfies constraints from CP violation and meson mixing.

The new top quark mass is predicted to be below approximately 7 TeV.

Abstract

We point out that hints of deviations from unitarity in the first row of the CKM matrix may be explained by the presence of a single vector-like top. We study how the stringent experimental constraints arising from CP Violation in the kaon sector and from meson mixing such as $D^0$-$\overline{D}^0$, $K^0$-$\overline{K}^0$ and $B^0_{d,s}$-$\overline{B}^0_{d,s}$ can be satisfied in the proposed framework. In order for the deviations from unitarity to be of the required size while keeping the theory perturbative, the new top quark should have a mass $m_T \lesssim 7$ TeV which could be probed in upcoming experiments at the energy frontier.