New QCD frontiers in understanding the nucleon structure

Protons and neutrons, the building blocks of matter, are themselves bound states of quarks and gluons whose dynamics is described by QCD, the theory of strong interactions. Some of the most challenging questions in physics and focus areas of the past, present, and future US nuclear physics program are: How do quarks and gluons form nucleons and nuclei? How can the properties of nucleons and nuclei be described by the QCD dynamics? Although the rich and nonlinear equations of motion make it very difficult, if not impossible, to solve QCD analytically, asymptotic freedom ensures that QCD perturbation theory is reliable in the high-energy regime. Many of the remarkable success stories of the theory of strong interactions are rooted in perturbative QCD. In this talk I will demonstrate how QCD perturbation theory can be used to map out the internal landscape of protons and neutrons and to describe the quantum many body interactions that take place in high-energy electron-nucleus and proton-nucleus collisions. The two specific research directions, where the efforts of the community have been focused, are spin physics and small-x physics. I will discuss these new frontiers in QCD theory along with the exciting experimental programs and demonstrate how they open new windows to study QCD dynamics and nucleon structure. As examples of recent significant progress in these fields, on the spin physics side, I will discuss the transverse spin asymmetry observables and how they can be used to map out the transverse motion of the quarks and gluons. On the small-x physics side, I will present our recently developed factorization formalism for forward hadron production in proton-nucleus collision and discuss how it can be used to better understand the saturated gluon density regime. Toward the end of the talk I will discuss the future scientific opportunities in these rapidly developing fields.