Phenomenological study of diquark effects in $\bar{B}_{(s)}^0 \to D_{(s)}^{(*)+} L^-$ decays
Motivated by the deviations between experimental data and SM predictions for the branching fractions of $\bar{B}_{(s)}^{0} \to D_{(s)}^{(*)+}L^-$ decays with $L=π,K^{(*)},ρ$, we investigate whether the type-IV scalar diquark can offer a plausible explanation for them. By matching the renormalized amplitudes in the full theory onto those in the low-energy effective theory formulated in the diquark operator basis, we extract the corresponding Wilson coefficients up to NLO in $α_s$ at the NP scale $μ_ϕ=M_ϕ$. We then rewrite the obtained operators in the SM basis and apply the renormalization-group running to evolve the Wilson coefficients down to the hadronic scale $μ_b=m_b$. For the hadronic matrix elements of the relevant NP operators, we employ the QCD factorization approach to evaluate the non-factorizable one-loop vertex corrections, and confront our theoretical predictions to the latest data to constrain the diquark parameter space. In the single-coupling case, we find that the purely left-handed coupling $Δ_{d(s)}^{LL}$ can reconcile the discrepancies at $1σ$, while the mixed right-left coupling $Δ_{d(s)}^{RL}$ remains compatible with data only at about $2σ$. Extending to the two-coupling case, we find viable parameter regions as long as $Δ_{d(s)}^{LL}\neq 0$. Incorporating the LHCb measurement of the ratio $R_{π/μν_μ}$ further reduces the allowed range of $Δ_{d}^{LL}$, but does not yield meaningful extra restriction on $Δ_{d}^{RL}$. Hence, while either $Δ_{d(s)}^{LL}$ or $Δ_{d(s)}^{RL}$ can separately provide an acceptable NP interpretation, our phenomenological results favor a scenario dominated by purely left-handed diquark-quark couplings. Finally, we present updated predictions for the branching fractions and the ratios $R_{(s)L}^{(*)}$, which can be probed by forthcoming LHCb and Belle II measurements.