Weak Annihilation Contribution to Angular Observables in $B_{c}^+\to D^{\ast+}\ell^{+}\ell^{-}$ Decays

Abstract

We analyze the rare semileptonic decays $B_{c}^+ \to D^{\ast+}(\to P_1 P_2)\ell^{+}\ell^{-}$, with $P_1 P_2 = D^+ \pi^0$ or $D^0 \pi^+$, and $\ell=\mu, \tau$. We focus on the impact of weak annihilation contributions alongside penguin, box, and long-distance effects. Using the effective Hamiltonian for $b \to d \ell^+ \ell^-$ transitions and $B_c \to D^{*}$ form factors from covariant confined quark model inputs, we compute the differential branching ratios, forward-backward asymmetry, longitudinal helicity fraction of the $D^{\ast}$, and various normalized angular coefficients. The results of the observables show that weak annihilation effects are sizable, particularly at low $q^2$, significantly modifying several observables and shifting zero-crossings. Resonance effects dominate at high $q^2$, 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 $B_c^+ \to D^{*+} \ell^+ \ell^-$ decays.