Higher Order Statistics for Three-dimensional Shear and Flexion
  We introduce a collection of statistics appropriate for the study of
spinorial quantities defined in three dimensions, focussing on applications to
cosmological weak gravitational lensing studies in 3D. In particular, we
concentrate on power spectra associated with three- and four-point statistics,
which have the advantage of compressing a large number of typically very noisy
modes into a convenient data set. It has been shown previously by \cite{MuHe09}
that, for non--Gaussianity studies in the microwave background, such
compression can be lossless for certain purposes, so we expect the statistics
we define here to capture the bulk of the cosmological information available in
these higher-order statistics. We consider the effects of a sky mask and noise,
and use Limber's approximation to show how, for high-frequency angular modes,
confrontation of the statistics with theory can be achieved efficiently and
accurately. We focus on scalar and spinorial fields including convergence,
shear and flexion of 3D weak lensing, but many of the results apply for general
spin fields.
