Speaker
Description
Experimental studies of the semileptonic decays $B\to D^{(*)}\ell\bar{\nu}$ generally assume that the missing energy is carried by a massless neutrino, as predicted by the Standard Model (SM). This assumption may not hold if the invisible final-state particle is instead a massive fermion, such as a sterile neutrino or a dark-sector state. We investigate the effects of such a massive invisible particle on the kinematic and angular observables of $B\to D^{(*)}\ell \, X_{\rm inv}$ decays within the framework of the most general weak efgeneral weak effective theory framework and simplified models that can generate the relevant interactions. The resulting modifications to decay distributions, as well as their impact on the extraction of the CKM matrix element $|V_{cb}|$ and the determination of effective Wilson coefficients, are analysed as functions of the invisible-particle mass. Our study demonstrates that the conventional assumption of a massless invisible state can lead to non-trivial biases in the interpretation of semileptonic $B$-decay data and provides a general framework for probing such scenarios in current and future flavor experiments.