In this article we discuss the measurement of spin at the LHC, in events with
two unknown four-momenta. Central to this problem is the identification of
spin-dependent kinematic variables and the construction of a statistical test
that can distinguish between different spin hypotheses. We propose a method for
reconstructing kinematic variables that depend upon the unknown momenta. The
method is based upon a probabilistic reconstruction of each event, given the
masses of the final and intermediate states and the cross-section of the
assumed hypothesis. We demonstrate that this method can distinguish between two
spin hypotheses for a specific process, even after mass uncertainties and
Standard Model backgrounds are taken into account. We compare our method with
another that only utilises the observable momenta of each event. We will show
that our method permits an improved discrimination between hypotheses, with a
reduced probability of error.Comment: 20 pages, 13 figures. Minor changes to text, references adde