Cabibbo angle anomalies and a global fit to vector-like quarks

TL;DR

This paper investigates the Cabibbo angle anomaly, a 3-sigma discrepancy in CKM matrix element determinations, exploring vector-like quarks as a promising new physics explanation through a comprehensive global fit and discussing future experimental prospects.

Contribution

It provides the first comprehensive global fit of vector-like quark models to address the Cabibbo angle anomaly, integrating various experimental constraints.

Findings

Vector-like quarks can potentially explain the anomaly.

Current constraints favor certain vector-like quark models.

Future experiments could clarify the viability of these models.

Abstract

The most recent determinations of $V_{ud}$ from superallowed beta decays lead to a discrepancy when compared to the value implied by mesonic CKM measurements combined with CKM unitarity. On top of this, improved precision in lattice QCD calculations have revealed another discrepancy between the $V_{us}$ determinations from kaon and pion semi-leptonic decays. The combination of these can be referred to as the Cabibbo angle anomaly, which we find has a significance of around $3\,\sigma$. After summarising the current state of these issues, I will talk about new physics models that modify semi-leptonic decays as potential explanations, and why vector-like quarks in particular appear the most promising candidates. I will then discuss the results of a global fit to various vector-like quark models, and how other constraints are important in determining the most likely explanation. Finally I will touch on future experiments that could shed further light on the situation.