Signatures of Invisible Fermions in ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\mathrm{\ell }}^{-}{\bar{X}}_{\mathrm{i}\mathrm{n}\mathrm{v}}$ Decays

We investigate the effects of a massive invisible fermion in ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\mathrm{\ell }}^{-}{\bar{X}}_{\mathrm{i}\mathrm{n}\mathrm{v}}$ decays using a model-independent weak effective theory. Through a comprehensive angular analysis, we show that a nonzero invisible-particle mass leaves characteristic imprints on angular observables that can distinguish massless and massive invisible states. We further demonstrate that these observables provide strong discrimination among vector/axial-vector, scalar/pseudoscalar, and tensor interactions, as well as between left- and right-handed quark and lepton current operators.