Are the new particles heavy or light in $b \to s E_{\mathrm{miss}}$?

In this work, we study the $B^+\to K^+ E_{\mathrm{miss}}$, $B^0\to K^{*0} E_{\mathrm{miss}}$, and $\Lambda_b^0\to \Lambda^0 E_{\mathrm{miss}}$ decays under three different new physics hypotheses: the heavy new particles, the light neutral vector particles, and the axion-like particles. We find that all three hypotheses can resolve the Belle-II excess, and they can be clearly distinguished by the longitudinal polarization fraction of $K^*$. Furthermore, we discover that the longitudinal polarization fraction of $\Lambda$ can be used to distinguish the chirality of the effective operators.