Probing Invisible Fermions in $B \to D^{*}\ell X_{\text{inv}}$ via Angular Observables

Abstract

Semileptonic decays $B \to D^{*} \ell X_{\text{inv}}$ provide a sensitive probe of light invisible particles, such as sterile neutrinos or dark-sector fermions. Within a general weak effective theory framework, we show that a massive invisible fermion induces distinctive modifications in the angular distributions. We identify observables with enhanced sensitivity to the invisible particle mass, allowing a clear discrimination of such scenarios, and highlight angular structures that differentiate left- and right-handed lepton-dark-sector currents.