Exploring the Structure of Hadrons Through Spin Asymmetries in Hard Scattering Processes

Abstract

Almost all of the visible matter in the universe is built from strongly interacting particles known as hadrons. For example, protons and neutrons, which comprise the nucleus of an atom, fit into this class of particles. However, hadrons are not a fundamental form of matter in that they are composed of other particles collectively called partons. One of the main challenges we face is to understand this internal structure. In the past, focus was placed mainly on reactions that involved so-called collinear twist-2 two-parton correlators. Even though these functions provide valuable access to the interior of hadrons, they present us with only a 1-dimensional and, hence, incomplete view of this complex world. In recent years, two extensions of this formalism have become an active source of research: transverse momentum dependent (TMD) functions and three-parton correlations. The former allow us to obtain a 3-dimensional image of hadrons (in momentum space), while the latter give us insight into the rich interactions that occur inside of them, and the two are closely connected. In this dissertation we analyze spin asymmetries in hard scattering processes, which address both types of functions, in order to understand the parton dynamics that cause these phenomena and to determine what these observables reveal about the inner-workings of hadrons. Namely, we analyze (almost back-to-back) di-hadron production in electron-positron annihilation as well as discuss momentum sum rules for some of the associated TMD functions. We also investigate transverse single-spin asymmetries in lepton-nucleon and proton-proton collisions. The study of the former allows us to comment on the general role of parton re-scattering in such effects, while a discussion of the latter includes a new computation of an important piece to that observable. Furthermore, we present pioneering calculations on longitudinal-transverse double-spin asymmetries in nucleon-nucleon collisions.