Cabibbo-suppressed charged-current semileptonic decays of $\Xi_b$ baryons

Abstract

We present the first perturbative QCD calculations of the $\Xi_b \to (\Lambda, \Sigma)$ transition form factors at leading order in $\alpha_s$, which govern the Cabibbo-suppressed semileptonic decays $\Xi_b \to (\Lambda, \Sigma)\ell \nu_\ell$ with $\ell = e, \mu, \tau$. 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 $\Sigma$ final states and $10^{-5}$ for $\Lambda$ final states, making them accessible to ongoing experiments such as LHCb. Ratios of decay rates between $\tau$ 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 $\Xi_b$ decays yields sub-percent precision for the ratios $\mathcal{R}_\ell(\Sigma/\Xi_c)$, enabling an independent determination of $|V_{ub}/V_{cb}|$ once the relevant decay-rate measurements become available.