Protons and neutrons (“nucleons”) are the building blocks of atomic nuclei and account for most of the mass of the visible matter in the universe. Half a century of investigations revealed that the nucleons themselves are composed of more basic constituents (“partons”): quarks bound together by the exchange of gluons. Partons interact via the strong nuclear force and can possess three charge states, conventionally called “colors”. This has led to the development of the fundamental theory of strong interactions known as Quantum Chromo-Dynamics (QCD). Understanding how the inner structure and quanto-mechanical properties of the nucleons, such as the spin, originate from their constituent partons is one of the most challenging enterprises in nuclear physics. During the past decade, there have been tremendous efforts towards going beyond the one-dimensional structure of the proton to a three-dimensional tomographic imaging of the proton. The 3-D structure of the proton in coordinate and momentum space can be described respectively via the Generalized Parton Distributions (GPDs) and the Transverse Momentum Dependent (TMD) parton distribution functions. With advancements in theory and the development of phenomenological tools we are preparing for the next step in subnuclear tomographic imaging. The 2015 nuclear physics long-range plan endorsed the realization of a high-luminosity polarized Electron-Ion Collider (EIC) as the next large construction project in the United States. This will open a unique opportunity for very high precision measurements.
In this seminar I shall narrate my personal research experience in pursuing the multi-dimensional knowledge of the proton’s structure and its spin. I will start with my investigation of GPDs at the ZEUS experiment at HERA, the previous and only electron+proton collider, through measurements of the exclusive production of a real photon – a process known as Deeply Virtual Compton Scattering. I will also tell how we are now accessing the Sivers TMD function in polarized proton+proton collisions at RHIC through the measurement of transverse single spin asymmetries in Drell-Yan and weak boson production. This is an effective path to test the fundamental QCD prediction of the non-universality of the Sivers function. Finally, I shall give a brief description of the EIC project, highlight several key high precision measurements from its planned broad physics program and discuss the expected impact on our current understanding of the 3-D structure of nucleons and nuclei and the origin of the nucleon spin.
Argonne Physics Division Seminar Schedule