TRANSVERSE SPIN AND MOMENTUM STRUCTURE OF HADRONS IN QCD

Transverse Spin and Momentum Structure of Hadrons in QCD Leonard Gamberg, Pennsylvania State University (Principal Investigator) This research focuses on theoretical and phenomenological studies to quantify the internal structure of nucleons and other hadrons in terms of their elementary constituents. Theoretical and experimental studies of hadron structure reveal that hadrons have a complex internal structure involving quarks, antiquarks and gluons (generically partons) and their strong interactions. In addition to the partons' collinear momentum, which is highly correlated with the direction of a fast-moving parent hadron, partons possess intrinsic transverse motion and structure. Several types of high-energy scattering experiments are known to be sensitive to this intrinsic transverse momentum: semi-inclusive deep-inelastic leptoproduction of hadrons, inclusive electron-positron annihilation to almost back-to-back hadrons, and Drell-Yan lepton-pair or weak gauge boson production in hadron-hadron scattering, where the hadrons represent a proton or neutron or nucleas in the initial state. These studies reveal that the transverse motion of partons in hadrons is essential to unfolding a comprehensive picture of hadron sub-structure. Connecting these measurements to an underlying theoretical framework relies on the QCD factorization theorems. Factorization and evolution equations enable us to exploit the universality of these correlation functions to unfold the so called three dimensional (3D) (longitudinal and transverse momentum and spatial degrees of freedom) partonic structure of hadrons across available energy scales of experiments in terms of correlation functions such as transverse momentum dependent parton distributions (TMDs), transverse momentum weighted TMDs, and collinear multi-parton correlations functions. These parton correlation functions characterize the intrinsic spin and momentum correlations and dynamics of quarks and gluons in hadrons, while the perturbative (hard) scattering amplitudes probe the short distance dynamics of these partons. With new observables of hadron structure proposed to be measured at the future Electron Ion Collider (EIC), we propose to develop, quantify, and test these factorization theorems through first principle calculations from the gauge theory of elementary partonic interactions, Quantum Chromdynamics (QCD), as well as perform phenomenological simulations to predict the outcome of proposed experiments. In particular, proposed measurements of hadron structure at subleading power in the hard scale, proposed at an EIC--which are based on some of the earliest attempts to study the internal structure of hadrons--have not been established. From our preliminary work, in this funding period we will perform calculations to rigorously establish these factorization from first principle QCD calculations. The ultimate goal of these studies is to map out the partonic structure of hadrons in terms of its QCD degrees of freedom, quarks and gluons and discover novel correlations. This research is a high priority in the near-term, and the long range plan for nuclear physics as indicated in the National Academy's, 'An Assessment of U.S.-Based Electron-Ion Collider Science', the recently published Science Requirements and Detector Concepts for the Electron-Ion Collider: EIC Yellow Report, as it relates to both present and future studies on the transverse momentum and spin structure of the nucleon: That is, to carry out research on the internal landscape of the nucleon by understanding the structure of hadrons in terms of QCD's quarks and gluons through multidimensional analysis. This research has impact for present and future hadron structure and spin physics experiments at Jefferson- JLAB 12 GeV, present and future spin physics programs at the Relativistic Heavy Ion Collider at Brookhaven National Lab, Drell-Yan experiments both in the US and internationally, at Fermi Lab and the COMPASS experiment at CERN. With the site selection by the DOE of Brookhaven National Laboratory for building an Electron-Ion Collider, a one-of-a-kind nuclear physics research facility, our work provides theory support for this highest priority initiative of the next-generation U.S. based particle physics collider.