  In 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.
