New Directions in QCD and the Electron-Ion Collider

Electron-proton collisions have historically provided the most detailed constraints on the fundamental constituent structure of hadrons and nuclei, as well as testing fundamental aspects of quantum chromodynamics. Many QCD phenomena have been discovered or confirmed in electroproduction, including DGLAP evolution, duality, spin anomalies, leading-twist diffraction, color transparency, nuclear shadowing and anti-shadowing, the scaling behavior of hard exclusive hadron and nuclear reactions, jet hadronization, and hard pomeron phenomena. The electroproduction field has now been extended to many new areas, particularly diffractive phenomena, single spin asymmetries, semi-exclusive reactions, and deeply virtual Compton scattering. Although there has been great progress in understanding the quark and gluon structure of proton and nuclei, many fundamental questions concerning QCD remain, such as hadronization at the amplitude level, the nature of the running coupling and masses at low scales, the division of the proton's angular momentum among its constituents, the role of hidden-color degrees of freedom in nuclei, distinguishing renormalon-induced versus dynamical higher twist effects, the intrinsic heavy-quark structure of hadron wavefunctions, quark-antiquark asymmetries, single-spin asymmetries and spin-spin correlations, anomalously large heavy quark production cross sections, heavy-quark threshold effects, the observed breaking of gauge-coherent color transparency [1], the origin of nuclear shadowing and anti-shadowing, and the physicsmore » of leading-twist diffraction, including hard and soft pomeron and odderon phenomena. An electron-ion collider [2] with proton and electron polarization capabilities will greatly illuminate these questions [3].« less