Cabibbo-suppressed charged-current semileptonic decays of $Ξ_b$ baryons

We present the first perturbative QCD calculations of the $Ξ_b \to (Λ, Σ)$ transition form factors at leading order in $α_s$, which govern the Cabibbo-suppressed semileptonic decays $Ξ_b \to (Λ, Σ)\ell ν_\ell$ with $\ell = e, μ, τ$. Using these form factors, we evaluate differential and integrated branching fractions and angular observables within the helicity formalism. The branching ratios are predicted to be of order $10^{-4}$ for $Σ$ final states and $10^{-5}$ for $Λ$ final states, making them accessible to ongoing experiments such as LHCb. Ratios of decay rates between $τ$ and $e$ channels are also provided, offering new probes of lepton-flavor universality. Lepton-mass effects are found to significantly impact the integrated angular observables. Furthermore, a combined analysis of $b \to u$ and $b \to c$ transitions in $Ξ_b$ decays yields sub-percent precision for the ratios $\mathcal{R}_\ell(Σ/Ξ_c)$, enabling an independent determination of $|V_{ub}/V_{cb}|$ once the relevant decay-rate measurements become available.

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