The proton and neutron, collectively called nucleons, are the fundamental building blocks of all atomic nuclei that make up essentially all the visible matter in the universe. By breaking the nucleon in high energy scattering experiments, we have learned that nucleons are composed of elementary particles called quarks and gluons, whose interactions and dynamics are governed by Quantum Chromodynamics (QCD). However, many profound questions remain, such as how QCD works to forbid us to see quarks and gluons in isolation in any modern detector and how quarks and gluons are bound together to form the nucleons and to make up their properties, including mass and spin. Developing a deeper understanding of these building blocks of matter is one of the grand challenges of modern science. In this talk, I will introduce a class of exclusive hard scattering processes to study the internal quark/gluon structure of nucleon and to explore its tomographic images of quarks and gluons without breaking the nucleon. Since we do not see any quarks and gluons in isolation, I will discuss the validity of QCD factorization to match the nucleon that we can measure in scattering experiments to the spatial distributions of quarks and gluons inside it. I will also introduce a new effort to pixelate such spatial distributions of quarks and gluons inside a colliding nucleon in slides of different momenta carried by these quarks and gluons.
