2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/113715In general a decay with a missing (not detected) particle can not be fully reconstructed apart from a few exceptions. For example, if the momentum of the decaying particle is known or if the missing energy in an event is measured precisely, then the missing particle 4-momentum can be determined. Here a new method is proposed that utilizes additional information about the topology of a decay: The direction from the primary to the secondary vertex combined with momentum conservation allows the determination of the missing particle momentum up to a twofold ambiguity. The semileptonic decay of the Bs mesons is considered as an example to illustrate this method and to compare its performance against conventional approaches.8 pages, 5 figures corrected typos, updated figuresHigh Energy Physics - Phenomenologytext