New vector-like fermions and flavor physics

Abstract

Abstract:We study renormalizable extensions of the standard model that contain vectorlike 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 K 0 −K 0 mixing) are not sensitive to higher scales than other flavor-changing neutral-current processes. We identify order 1/(4πM ) 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.

Search for single production of vector-like top partner decaying to Wb at $$e\gamma $$ e γ collision

“…Generally, the top partner couplings to the SM particles are severely constrained by electroweak precision physics as well as by the direct measurement of V tb [25]. However, such constraints can be altered significantly in most realistic models including the vector-like quark with two or more partner multiplets [26][27][28][29][30][31][32][33]. Here we take a conservative limit for the coupling parameter g * ≤ 0.5, which is consistent with the current experiment bounds [34].…”

Flavourful Z ′ models for R K ∗ $$ {R}_{K^{\left(\ast \right)}} $$

“…Unlike the original vector-like fermion models [39,40], having non-universal U(1) ′ charges of the fourth vector-like family forbids mixing via the usual Higgs Yukawa couplings. Instead, new singlet scalars with appropriate U(1) ′ charges are added to generate mass mixing of quarks and leptons with the vector-like family.…”

“…This has motivated the phenomenological analysis of Z ′ models based on gauged L µ − L τ , possibly combined with vector-like quarks [38]. Without a Z ′ , vector-like quarks directly mixing with ordinary quarks via the Higgs Yukawa couplings can lead to FCNCs [39,40]. However, vector-like quarks with a gauged U(1) ′ typically forbids the Higgs coupling of vector-like quarks to ordinary quarks, but allows new possibilities [38].…”

Flavor Constraints on New Physics

“…Another illustration of the expected progress with well quantifiable increases in mass scale sensitivity, in both quark and lepton flavor experiments, is to consider extensions of the SM involving vector-like fermions, which can Yukawa couple to the SM [68]. These fermions can have masses M much greater than the weak scale, since they have a mass term even in the absence of Figure 11: One-loop vector-like fermion contributions to K and D mixing in Model V [68].…”