Quark-hadron duality addresses some of the most fundamental issues in strong interaction physics, in particular the nature of the transition from the perturbative to non-perturbative regions of QCD. I summarize recent developments in quark-hadron duality in lepton–hadron scattering, and outline how duality can be studied at future high-luminosity facilities such as Jefferson Lab at 12 GeV or an...

It is shown that the euclidean origin of the instanton leads to the appearence of a large imaginary part in some diagrams for hadron production in quark-quark scattering. This phenomenon and instanton–induced quark spin-flip give the fundamental QCD mechanism for the observed large single-spin asymmetries in hadron-hadron and lepton-hadron interactions. 1 Talk presented at the Workshop ”Polariz...

Exclusive two-photon reactions such as Compton scattering at large angles, deeply virtual Compton scattering, and hadron production in photon-photon collisions provide important tests of QCD at the amplitude level, particularly as measures of hadron distribution amplitudes and skewed parton distributions. Invited talk presented at Photon 2000 Ambleside, England August 26–31, 2000 ∗Work supporte...

After a brief review (in the first part) of some relevant properties of the high–energy parton– parton scattering amplitudes, in the second part we shall discuss the infrared finiteness and some analyticity properties of the loop–loop scattering amplitudes in gauge theories, when going from Minkowskian to Euclidean theory, and we shall see how they can be related to the still unsolved problem o...