Craig D. Roberts (Argonne Natl. Lab., USA)

Aspects of QCD Theory and Application

I will explain that the chiral-limit vacuum quark condensate is qualitatively equivalent to the pseudoscalar meson leptonic decay constant in the sense that they are both obtained as the chiral-limit value of well-defined gauge-invariant hadron-to-vacuum transition amplitudes that possess a spectral representation in terms of the current-quark mass. Thus, whereas it might sometimes be convenient to imagine otherwise, neither is essentially a constant mass-scale that fills all spacetime. This means, in particular, that the quark condensate can be understood as a property of hadrons themselves, which is expressed, for example, in their Bethe-Salpeter or light-front wavefunctions. In addition, using the pion elastic electromagnetic form factor, I will illustrate how an interplay between modern experiment and theory can be used to chart the interaction between light-quarks at long range.