Abstract
Due to a recent more precise evaluation of Vud and Vus, the unitarity condition of the first row in the Cabibbo-Kobayashi-Maskawa (CKM) matrix: |Vud|2 +|Vus|2 +|Vub|2 = 0.99798 ± 0.00038 now stands at a deviation more than 4σ from unity. Furthermore, a mild excess in the overall Higgs signal strength appears at about 2σ above the standard model (SM) prediction, as well as the long-lasting discrepancy in the forward-backward asymmetry {mathcal{A}}_{mathrm{FB}}^b in Zto boverline{b} at LEP. Motivated from the above three anomalies we investigate an extension of the SM with vector-like quarks (VLQs) associated with the down-quark sector, with the goal of alleviating the tension among these datasets. We perform global fits of the model under the constraints coming from the unitarity condition of the first row of the CKM matrix, the Z -pole observables {mathcal{A}}_{mathrm{FB}}^b , Rb and Γhad, Electro-Weak precision observables ∆S and ∆T , B-meson observables {B}_d^0-{overline{B}}_d^0 mixing, B+→ π+ℓ+ℓ− and B0→ μ+μ− and direct searches for VLQs at the Large Hadron Collider (LHC). Our results suggest that adding VLQs to the SM provides better agreement than the SM.
Highlights
Precision observand B0 → μ+μ−, and direct searches for vector-like quarks (VLQs) at the Large Hadron Collider (LHC)
S, b couplings to the Z boson are modified, we find that the following observables are modified: 4The Zss coupling remains unchanged from the standard model (SM) value, because we keep the gb2 = 0, and no mixing between VLQ and K-Kmixing observables
We have advocated an extension of the SM with vector-like quarks, including a doublet and a singlet, in aim of alleviating a few experimental anomalies
Summary
The upper component of the doublet and the singlets have the same quantum numbers as the SM down-type quarks, and they are allowed to mix with the SM down-type quarks if nontrivial Yukawa interactions exist among them. It was pointed out that the Yukawa interaction between BL and bR will induce a mixing between the. Right-handed bR and bR, and so reduce the bottom Yukawa coupling. It will increase the coupling of the Z boson to the right-handed b quark [17]. The mixing between bL and bL is suppressed due to the absence of Yukawa interaction between BR and bL, and so the modification of CKM matrix is negligible. The Higgs-induced Yukawa interaction between bL,R and the SM down quarks will give a larger left-handed mixing than the right-handed one. The non-negligible left-handed mixing can further modify the original 3 × 3 CKM matrix and the extra VLQs can extend the CKM matrix to 5 × 5 to restore the unitarity
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