Feasible Deviations from Unitarity with Vector-Like Quark Singlets

TL;DR

This paper investigates how vector-like quark singlets can induce deviations from CKM unitarity, analyzing their compatibility with flavor observables and identifying parameter regions with significant unitarity deviations.

Contribution

It provides a detailed analysis of models with minimal vector-like quark singlets, exploring their potential to cause observable deviations from CKM unitarity and confronting these with experimental constraints.

Findings

Second-row unitarity deviations must accompany first-row deviations in simple models.

More complex models can produce large deviations, especially in the second row.

Certain VLQ-singlet combinations can significantly alter flavor observables.

Abstract

We deduce pertinent relations between the elements of the CKM matrix, and find that not all of these are totally compatible with experiment and/or the assumption of the $3 \times 3$ unitarity. We identify complex phases in the CKM-elements which may signal deviations from unitary (DU). We focus on DUs induced by VLQ-singlets, and the possibility of having significant DUs of the first and second rows of the CKM matrix, together with DUs in its columns. We make a thorough analysis of models with the lowest amount of singlets and find a useful set of parametrizations crucial in coherently exploring the parameter-space of the proposed cases. We test the feasibility of each model, confronting them with the restrictions imposed by several important flavor observables. Special attention is given to the neutral kaon and $D^0$-meson sectors, particularly to the parameters $\epsilon_K$ and $x_D$. We find that for the most elementary VLQ-singlet cases, the DUs in the second row must roughly accompany the DUs of the first row. However, in cases with more elaborate combinations of VLQ-singlets, the DUs in the second row may be very large, and even substantially exceed those of the first row. This is what happens in models with sufficient mingling of the two sectors, e.g. in a 2-up-1-down VLQ-singlet scenario. Until now, the analysis of this joining of the up and down sectors with VLQs has not been described in the literature in great detail.