With updated experimental data and improved theoretical calculations, several significant deviations are being observed between the Standard Model predictions and the experimental measurements of the branching ratios of $$ {\overline{B}}_{(s)}^0\to {D}_{(s)}^{\left(\ast \right)+}{L}^{-} $$ B ¯ s 0 → D s ∗ + L − decays, where L is a light meson from the set {π, ρ, K(∗)}. Especially for the two channels $$ {\overline{B}}^0\to {D}^{+}{K}^{-} $$ B ¯ 0 → D + K − and $$ {\overline{B}}_s^0\to {D}_s^{+}{\pi}^{-} $$ B ¯ s 0 → D s + π − , both of which are free of the weak annihilation contribution, the deviations observed can even reach 4–5σ. Here we exploit possible new-physics effects in these class-I non-leptonic B-meson decays within the framework of QCD factorization. Firstly, we perform a model-independent analysis of the effects from twenty linearly independent four-quark operators that can contribute, either directly or through operator mixing, to the quark-level b →$$ c\overline{u}d(s) $$ c u ¯ d s transitions. It is found that, under the combined constraints from the current experimental data, the deviations observed could be well explained at the 1σ level by the new-physics four-quark operators with γμ(1 − γ5) ⨂ γμ(1 − γ5) structure, and also at the 2σ level by the operators with (1 + γ5) ⨂ (1 − γ5) and (1 + γ5) ⨂ (1 + γ5) structures. However, the new-physics four-quark operators with other Dirac structures fail to provide a consistent interpretation, even at the 2σ level. Then, as two specific examples of model-dependent considerations, we discuss the case where the new-physics four-quark operators are generated by either a colorless charged gauge boson or a colorless charged scalar, with their masses fixed both at the 1 TeV. Constraints on the effective coefficients describing the couplings of these mediators to the relevant quarks are obtained by fitting to the current experimental data.

We update the Standard Model predictions for the branching ratios of the rare top-quark decays t → cg(g), and evaluate the maximum rates that can be reached in the aligned as well as in the four conventional two-Higgs-doublet models (2HDMs) with Z 2 symmetries. Taking into account the relevant constraints on the model parameters from precision flavour observables, we find that the branching ratios of t → cg and t → cgg decays can reach up to 3.36 × 10 −9 and 2.95 × 10 −9 respectively, being therefore of the same order, in the aligned 2HDM (A2HDM). This is obviously different from the SM case, where the predicted branching ratio of the three-body decay t → cgg is about two orders of magnitude larger than that of the two-body decay t → cg. On the other hand, compared with the SM predictions, no significant enhancements are observed in the four conventional 2HDMs with Z 2 symmetries for the branching ratios of these two decays. Nevertheless, the predicted branching ratios of t → cg and t → cgg decays in the A2HDM are still out of the expected sensitivities of the future HL-LHC and FCC-hh.

We update the Standard Model (SM) predictions for the branching ratios of the rare top-quark decays $$t \rightarrow cg(g)$$ t → c g ( g ) , and evaluate the maximum rates that can be reached in the aligned as well as in the four conventional two-Higgs-doublet models (2HDMs) with $$\mathcal {Z}_2$$ Z 2 symmetries. Taking into account the relevant constraints on the model parameters resulting from a global fit obtained at the $$95.5\%$$ 95.5 % confidence level, we find that the branching ratios of $$t \rightarrow cg$$ t → c g and $$t \rightarrow cgg$$ t → c g g decays can reach up to $$3.36\times 10^{-9}$$ 3.36 × 10 - 9 and $$2.95\times 10^{-9}$$ 2.95 × 10 - 9 respectively, being therefore of the same order, in the aligned 2HDM (A2HDM). This is obviously different from the SM case, where the predicted branching ratio of the three-body decay $$t \rightarrow cgg$$ t → c g g is about two orders of magnitude larger than that of the two-body decay $$t \rightarrow cg$$ t → c g . On the other hand, compared with the SM predictions, no significant enhancements are observed in the four conventional 2HDMs with $$\mathcal {Z}_2$$ Z 2 symmetries for the branching ratios of these two decays. Nevertheless, the predicted branching ratios of $$t \rightarrow cg$$ t → c g and $$t \rightarrow cgg$$ t → c g g decays in the A2HDM are still out of the expected sensitivities of the future high-luminosity Large Hadron Collider and the Future Circular Collider in hadron-hadron mode.