Although there are many stages to High-Energy Physics (HEP) predictions, the fundamental physics are described in terms of probabilities (cross sections) calculated from perturbative Quantum Field Theories (QFTs). However, since HEP processes are stochastic, it is insufficient to evaluate the total cross section --- rather, to compare statistically compare experiment with theory we need to
sample the underlying distribution, known as the
differential cross section dheta (also known as the scattering amplitude or matrix element of the process).