Quantum Chromodynamics has yielded a huge amount of understanding about how the nucleon and other hadrons are built from quarks and gluons, the fundamental degrees of freedom in QCD.
To investigate hadron structure we study deep inelastic scattering processes, where individual quarks and gluons are resolved. The parton densities extracted from such processes encode the distribution of longitudinal momentum and polarization carried by quarks, antiquarks and gluons within a fast moving hadron.
They have helped shaping our physical picture of hadron structure. One of the main motivations to improve our understanding of the internal structure of the proton is provided by the fact that PDFs and their uncertainties play a decisive role in several collider physics applications, in searches for Physics beyond the Standard Model as well as in astroparticle physics.
PDFs will keep playing an important role for any future higher–energy collider involving hadrons in the initial state and we need to keep improving on our understanding in order to fully capitalize the potential of the future colliders. We plan to obtain ab-initio information about these quantities through our lattice simulations which are now feasible due to the existence of petascale computers.