The pion occupies a special place in nuclear physics: to many it is the "simplest hadron'', however this is a misnomer. The pion encapsulates the myriad complexities of Quantum Chromodynamics (QCD), the theory of the strong interaction, which, among other things, describes how quarks and gluons bind together to form nucleons and nuclei. The pion is both a bound-state of a dressed-quark and a dressed-antiquark in quantum field theory and the Goldstone mode associated with dynamical chiral symmetry breaking (DCSB) in QCD. Enigmatically, in coming to understand the pion's peculiarly low (lepton-like) mass, DCSB has been exposed as the origin of more than 98% of the mass in the visible Universe. This demonstrates theoretically that the Higgs mechanism plays just a minor role for the bulk of normal matter in the Universe.

Fig. 1: Experimental and theoretical knowledge of the pion's electromagnetic form factor. If our modern understanding of the origin of mass for visible matter is correct, then future measurements of the charge distribution of the pion will track the solid red curve.

Fig. 2: An example of the Sullivan process: used here to measure F_π(Q²).

An immediate consequence of this modern understanding is the prediction for the pion's electromagnetic form factor, F_π(Q²), displayed as the solid line in Fig. 1. F_π(Q²) is a representation of the distribution of electric charge within the pion. The dashed green curve in Fig. 1 represents F_π(Q²) in a Universe in which the Higgs mechanism is the only source of visible mass. These theoretical predictions will be tested experimentally, for example, at the upgraded Thomas Jefferson National Accelerator acility (JLab). In probing deeper into the interior of the pion; therefore, by increasing the momentum transfer, Q², in the scattering process, which is illustrated in Fig. 2, experiment will be able to directly measure the strength of DCSB and verify our theoretical picture of the origin of mass for visible matter.