Probing Light Dark Fermions in $B \to D^{(*)}\ell X_{\rm inv}$ via Rate Distributions

Experimental analyses of the semileptonic decays $ B \to D^{(*)} \ell \barν$ typically rely on the assumption that the missing energy originates from a massless neutrino, as predicted by the Standard Model. However, this assumption may not hold in scenarios where the invisible final-state particle i