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.
Highlights
We study renormalizable extensions of the standard model that contain vectorlike fermions in a complex representation of the standard model gauge group
This motivates a study of extensions of the standard model with additional fermions that reside in a single vector-like representation of the SM gauge group with a mass M . (For some earlier studies of such models, see, e.g., refs. [1,2,3,4,5,6,7,8,9,10].) We focus on vector-like fermions that can have Yukawa couplings involving the Higgs field and the SM fermions
These models have been studied in the literature previously, but here we consider all such models and compare the patterns of deviations from the SM they would give rise to. These models predict a very specific pattern for the contributions of beyond the standard model (BSM) physics to ∆F = 1 flavor-changing neutral-current (FCNC) processes compared to ∆F = 2 neutral meson mixings
Summary
There are 11 renormalizable models with vector-like fermions in complex representations of the standard model gauge group, where the vector-like fermions have renormalizable Yukawa couplings to the SM fermions through the Higgs doublet. Model III has vector-like fermions, , with same gauge quantum numbers as the SM left-handed doublets, (1, 2, −1/2) They Yukawa couple to the SM right-handed charged lepton fields eiR. Models V and VI contain vector-like fermions D and U with the same quantum numbers as the SM right-handed down and up-type quarks, (3, 1, −1/3) and (3, 1, 2/3). They Yukawa couple to the SM left-handed quark doublets QiL. Model X has a vector-like quark doublet qu with quantum numbers (3, 2, 7/6) that Yukawa couple through the Higgs doublet to the right-handed up-type quarks. Corrections to meson mixing of order λiλ∗j /(4πM ) that are suppressed by weak mixing angles and/or quark masses were considered for Model V in ref. [16]
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