  Quantum tomography has become an indispensable tool in order to compute the
density matrix $\rho$ of quantum systems in Physics. Recently, it has further
gained importance as a basic step to test entanglement and violation of Bell
inequalities in High-Energy Particle Physics. In this work, we present the
theoretical framework for reconstructing the helicity quantum initial state of
a general scattering process. In particular, we perform an expansion of $\rho$
over the irreducible tensor operators $\{T^L_M\}$ and compute the corresponding
coefficients uniquely by averaging, under properly chosen Wigner D-matrices
weights, the angular distribution data of the final particles. Besides, we
provide the explicit angular dependence of both the normalised differential
cross section and the generalised production matrix $\Gamma$. Finally, we
re-derive all our previous results from a quantum-information perspective using
the Weyl-Wigner-Moyal formalism and we obtain in addition simple analytical
expressions for the Wigner $P$ and $Q$ symbols.
