Conference talk Piet Mulders

Piet Mulders
VU Amsterdam, NIKHEK
Quantum Chromodynamics at Work

 The matter in our world and the visible matter in the universe is made up of atoms consisting of electrons and nuclei built from protons and neutrons. At and above the atomic scale the interactions are primarily gravity and the electroweak forces. The latter include the interactions between charges and photons, which are beautifully described in quantum electrodynamics (QED). The atomic world also beautifully illustrates how charged particles including their spins provide a basis for enormous complexity.

In the subatomic world, the theoretical framework is Quantum Chromodynamics (QCD) that describes the strong interactions between color charges, carried by quarks and the force mediators themselves, confining them inside the nucleons, protons and neutrons. The framework of QCD certainly rivals in beauty with QED and is in fact much richer being a non-abelian gauge theory based on the symmetry group of the three color charges.

In the description of high energy scattering processes, the most important input used to encode the structure of hadrons is a set of Parton Distribution Functions (PDFs) representing probabilities for finding specific quarks and gluons (partons) carrying fractions x of the parent hadron’s momentum (soft collinear part). The different functions in this set describe in essence spin-spin transfer probabilities. The interactions of the partons (hard part) are calculated using perturbation theory within the quantum field theoretical framework known as the Standard Model of particle physics.

One can go beyond this collinear approach. In that case one has to include the transverse momentum of the partons. It leads to a novel set of Transverse Momentum Dependent (TMD) distribution functions. Such functions take into account various spin-orbit correlations that may exist in a hadron, including socalled time-reversal odd functions describing spin – momentum transfer between hadrons and partonsIs this transverse momentum dependence a useful addition? Can it be measured? Can the formalism be set up and used in the same successful way as collinear PDFs, which are related to expectation values of field operators using the Operator Product Expansion in QCD? The answer is yes, but … . But after accounting for the complications, quark and gluon TMDs offer new insight into spin and orbital substructure of hadrons while they also may provide new tools to explore physics beyond the Standard Model, which is one of the ways to study the basic structure of our universe.

Wednesday, February 20, 2013 - 19:55