Weak annihilation contribution to angular observables in Bc⁺→ D*+ ⁺ ⁻ decays

We analyze the rare semileptonic decays Bc⁺ → D*+(→ P₁ P₂) ⁺ ⁻ B c + → D ∗ + ( → P 1 P 2 ) ℓ + ℓ - , with P₁ P₂ = D⁺ π ⁰ P 1 P 2 = D + π 0 or D⁰ π ⁺ D 0 π + , and =μ , τ ℓ = μ , τ . We focus on the impact of weak annihilation contributions alongside penguin, box, and long-distance effects. Using the effective Hamiltonian for b → d ⁺ ⁻ b → d ℓ + ℓ - transitions and Bc → D* B c → D ∗ form factors from covariant confined quark model inputs, we compute the differential branching ratios, forward–backward asymmetry, longitudinal helicity fraction of the D* D ∗ , and various normalized angular coefficients. The results of the observables show that weak annihilation effects are sizable, particularly at low q² q 2 , significantly modifying several observables and shifting zero-crossings. Resonance effects dominate at high q² q 2 , restricting reliable analysis windows. We conclude that the inclusion of weak annihilation is essential for precise Standard Model predictions and for isolating possible New Physics effects in Bc⁺ → D*+ ⁺ ⁻ <m