Constructing new heavy-quark sum rule for $b \to c$ meson and baryon decays

Abstract: We study heavy-hadron semileptonic decays proceeding via $b \to c$ transition, such as $B \to D^{(*)}\tau\bar{\nu}_\tau$ and $\Lambda_b \to \Lambda_c\tau\bar{\nu}\tau$. In the heavy-quark limit, where the heavy-quark symmetry holds, we construct a general heavy-quark sum rule for these decays based on the spin decomposition picture. The relation holds directly for the squared amplitudes without requiring phase-space integration. We then apply this relation to reproduce the sum rule among $B \to D^{(*)}\tau\bar{\nu}\tau$ and $\Lambda_b \to \Lambda_c\tau\bar{\nu}_\tau$. Furthermore, we extend the analysis to $\Omega_b \to \Omega_c^{(*)}$ transitions and those involving excited states, such as $B \to {D_0^,D_1^}$ and $B \to {D_1,D_2^*}$.