# The CKM unitarity problem: A trace of new physics at the TeV scale?

## TL;DR

This paper investigates the 4-sigma deviation from CKM unitarity after recent precise measurements, exploring new physics scenarios involving a 4th quark or a horizontal family symmetry at the TeV scale.

## Contribution

It proposes and analyzes two potential beyond Standard Model solutions to the CKM unitarity problem involving either a new quark or a gauge symmetry at the TeV scale.

## Key findings

- A 4th quark with |V_{ub'}| ~ 0.04 could explain the deviation if its mass is below 6 TeV.
- A gauge horizontal family symmetry at ~6 TeV can reconcile CKM unitarity and muon decay measurements.
- The proposed models also relate to the neutron lifetime discrepancy.

## Abstract

After the recent high precision determinations of $V_{us}$ and $V_{ud}$, the first row of the CKM matrix shows more than $4\sigma$ deviation from unitarity. Two possible scenarios beyond the Standard Model can be investigated in order to fill the gap. If a 4th quark $b'$ participates in the mixing, with $\vert V_{ub'} \vert \sim0.04$, then its mass should be no more than 6 TeV or so. A different solution can come from the introduction of the gauge horizontal family symmetry acting between the lepton families and spontaneously broken at the scale of about 6 TeV. Since the gauge bosons of this symmetry contribute to muon decay in positive interference with Standard Model, the Fermi constant is slightly smaller than the muon decay constant so that unitarity is recovered. Also the neutron lifetime problem, that is about $4\sigma$ discrepancy between the neutron lifetimes measured in beam and trap experiments, is discussed in the light of the these determinations of the CKM matrix elements.