Vector-like Quark Interpretation for the CKM Unitarity Violation, Excess in Higgs Signal Strength, and Bottom Quark Forward-Backward Asymmetry

Due to a recent more precise evaluation of $V_{ud}$ and $V_{us}$, the unitarity condition of the first row in the Cabibbo-Kobayashi-Maskawa (CKM) matrix: $|V_{ud}|^2 + |V_{us}|^2 + |V_{ub}|^2 = 0.99798 \pm 0.00038$ 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 ${\cal A}_{\rm FB}^b$ in $Z\to b\bar b$ 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 ${\cal A}_{\rm FB}^b$, $R_b$ and $Γ_{\rm had}$, Electro-Weak precision observables $ΔS$ and $ΔT$, $B$-meson observables $B_d^0$-$\overline{B}_d^0$ mixing, $B^+ \to π^+ \ell^+ \ell^-$ and $B^0 \to μ^+ μ^-$, 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.

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