Particles, Astrophysics, and Nuclear Physics Seminar

The Internal Structure of the Nucleon in Hard Reactions

Quantum chromodynamics (QCD) describes — as a well established part of the standard model — the interaction of quarks, antiquarks, and gluons. But how these QCD degrees of freedom make up hadrons, i.e. the degrees of freedom of strong forces we observe, is still not fully understood.

Important insights into the dynamics of (and inside) hadrons are revealed by high-energy lepton-nucleon scattering experiments, in particular, when using polarized lepton beams and polarized targets —as done at Jefferson Lab and other recent experiments.

Data from these experiments give first insights into the spatial distribution of quarks in the transverse plane (via generalized parton distribution functions), or spin-orbit-interactions of quarks in the nucleon (accessible by means of transverse momentum dependent distribution and fragmentation functions), and confirm the existence of predicted effects due to final state interactions in hard QCD scattering processes (such as the Collins and Sivers effect).

• What do we learn about the nucleon structure from the first data?
• What are we going to learn from planned/proposed future experiments?
• What are, in particular, the prospects of the 12GeV upgrade of Jefferson Lab?

In the talk, I address these and other questions, and review recent advances in theory and phenomenology of hard exclusive and semi-inclusive scattering.