It is well known that in b hadron decays with a single unreconstructible
final state particle, the decay kinematics can be solved up to a quadratic
ambiguity, without any knowledge of the b hadron momentum. We present a
method to infer the momenta of b hadrons produced in hadron collider
experiments using information from their reconstructed flight vectors. Our
method is strictly agnostic to the decay itself, which implies that it can be
validated with control samples of topologically similar decays to fully
reconstructible final states. A multivariate regression algorithm based on the
flight information provides a b hadron momentum estimate with a resolution of
around 60% which is sufficient to select the correct solution to the quadratic
equation in around 70% of cases. This will improve the ability of hadron
collider experiments to make differential decay rate measurements with
semileptonic b hadron decays.Comment: 18 pages, 17 figures. Updated version to be published in JHE