  The latest LHC mono-jet searches place stringent bounds on the pp -> chibar
chi cross section of dark matter. Further properties such as the dark matter
mass or the precise structure of the interactions between dark matter and the
standard model can however not be determined in this manner. We point out that
measurements of the azimuthal angle correlations between the two jets in 2 j +
chibar chi events may be used to disentangle whether dark matter pair
production proceeds dominantly through tree or loop diagrams. Our general
observation is illustrated by considering theories in which dark matter
interacts predominantly with the top quark. We show explicitly that in this
case the jet-jet azimuthal angle difference is a gold-plated observable to
probe the Lorentz structure of the couplings of dark matter to top quarks, thus
testing the CP nature of the particle mediating these interactions.
