New Vector-Like Fermions and Flavor Physics

TL;DR

This paper explores extensions of the standard model with vector-like fermions, analyzing their flavor-changing effects, constraints from experiments, and identifying key one-loop contributions affecting meson mixing.

Contribution

It introduces 11 models with vector-like fermions coupling to the standard model without extra fine-tuning, analyzing their flavor physics implications and constraints.

Findings

Constraints from meson mixing are not sensitive to high scales for strongly interacting fermions.

One-loop contributions to four-quark operators are significant and scale as 1/(4 pi M)^2.

Models do not require additional fine-tuning and can lead to observable flavor-changing effects.

Abstract

We study renormalizable extensions of the standard model that contain vector-like fermions in a (single) complex representation of the standard model gauge group. There are 11 models where the vector-like fermions Yukawa couple to the standard model fermions via the Higgs field. These models do not introduce additional fine- tunings. They can lead to, and are constrained by, a number of different flavor- changing processes involving leptons and quarks, as well as direct searches. An interesting feature of the models with strongly interacting vector-like fermions is that constraints from neutral meson mixings (apart from CP violation in neutral kaon mixing) are not sensitive to higher scales than other flavor-changing neutral-current processes. We identify order 1/(4 pi M)^2 (where M is the vector-like fermion mass) one-loop contributions to the coefficients of the four-quark operators for meson mixing, that are not suppressed by standard model quark masses and/or mixing angles.