Small violations of unitarity, the phase in Bs mixing and visible t->cZ decays at the LHC

TL;DR

This paper proposes a model with slight unitarity violations in quark mixing, introducing a new quark that explains B_s mixing phase anomalies, predicts observable rare top decays, and accounts for D^0 mixing without long-distance effects.

Contribution

It introduces a model with a new isosinglet quark that allows larger CP-violating phases and predicts observable rare top decays at the LHC, extending the Standard Model's explanations.

Findings

Allows for a larger phase in B_s mixing than the Standard Model.

Predicts observable t→cZ decays at the LHC.

Provides an explanation for D^0--D̄^0 mixing without long-distance contributions.

Abstract

We show that it is possible to accommodate the observed size of the phase in $B^0_s$--$\bar B^0_s$, mixing in the framework of a model with violation of $3\times 3$ unitarity. This violation is associated to the presence of a new $Q=2/3$ isosinglet quark $T$, which mixes both with $t$ and $c$ and has a mass not exceeding 500 GeV. The crucial point is the fact that this framework allows for $\chi\equiv\arg(-V_{ts}V_{cb}V_{tb}^*V_{cs}^*)$ of order $\lambda$, to be contrasted with the situation in the Standard Model, where $\chi$ is constrained to be of order $\lambda^2$. We point out that this scenario implies rare top decays $t\to cZ$ at a rate observable at the LHC and $|V_{tb}|$ significantly different from unity. In this framework, one may also account for the observed size of $D^0$--$\bar D^0$ mixing without having to invoke long distance contributions. It is also shown that in the present scenario, the observed size of $D^0$--$\bar D^0$ mixing constrains $\chi^\prime\equiv\arg(-V_{cd}V_{us}V_{cs}^*V_{ud}^*)$ to be of order $\lambda^4$, which is significantly smaller than what is allowed in generic models with violations of $3\times 3$ unitarity.