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$b \to c$ semileptonic sum rule: Current status and prospects

The $b \to c$ semileptonic sum rules provide relations between the decay rates of $B \to D^{(*)} τ\barν$ and $Λ_b \to Λ_c τ\barν$. Starting from the heavy quark and zero-recoil limits, we revisit the derivation of the sum rule for total decay rates. We then examine deviations from the limits and investigate corrections arising from realistic hadron masses and higher-order contributions to form factors, taking account of uncertainties. We show that these corrections are negligible compared to current experimental uncertainties, indicating that the sum rule is useful for cross-checking experimental consistency and testing the validity of the Standard Model predictions. In future, precise determinations of the form factors particularly for the tensor operator will be necessary to compare the sum rule predictions with $Λ_b \to Λ_c τ\barν$ data from the LHCb experiment and the Tera-Z projects.