Almost all the Universe's visible matter is comprised from nucleons and the nuclei they constitute; however an intimate QCD based understanding of these principal building blocks is only just beginning. Nevertheless, hadron physics has recently entered a new era, with the emergence of a comprehensive approach to the description of nucleon and nuclear structure, through the Wigner distributions of the fundamental constituents. Wigner distributions are a quantum mechanical concept analogous to the classical notion of a phase space distribution and encode information on both the spatial and momentum tomography of a bound state. A simpler, precursor concept is that of hadron light-front wave functions. They have a probability interpretation and therefore provide a connection between dynamical properties of a quantum field theory and notions familiar from quantum mechanics. An important example is the pion's parton distribution amplitude (PDA), which is a critical piece of the explanation for the behaviour of this peculiar hadron. Enigmatically, the nearly-massless pion is the cleanest expression of the mechanism that is responsible for almost all the visible mass in the Universe. This presentation will draw these threads together, tying the pion's PDA to real-world observables and statements about just when QCD might be perturbative; illustrating the connection between Wigner distributions and transverse momentum dependent parton distribution functions for the nucleon; and describing novel insights into the quark substructure of nuclei.

Argonne Physics Division Colloquium Schedule