Faculty of Social Sciences

Disputation: The interplay between quark and hadronic degrees of freedom and the structure of the proton

We study the low-energy sector of the strong interaction which is the least understood part of the Standard Model, the theory that describes the interactions of all known particles. The ideal particles for this study are the proton and the neutron, collectively called the nucleon.

They make up the nucleus of all the atoms of our world and understanding them has been of high priority ever since their discovery. We show that one cannot neglect the effects of other hadrons, such as neutrons and pions when studying the proton. A large part of the proton's hadronic wavefunction is shown to consist of the wavefunctions of other hadrons. In other words, when probing the proton there is a sizeable probability that one is probing some other hadron surrounding the proton as a quantum fluctuation.

The nucleon itself consists of elementary particles known as quarks and gluons, collectively called partons. Exactly how the properties of these partons make up the properties of the nucleon has been the subject of active research ever since their discovery. Two main issues are the flavor asymmetry of the proton sea and the spin structure of the nucleon. To address these questions we study the interplay between the partonic and hadronic degrees of freedom. We introduce a model based on a convolution between hadronic quantum fluctuations as described by chiral perturbation theory, and partonic degrees of freedom motivated by a physical model of the nucleon having only few physically constrained parameters.

We present the hadronic distribution functions and the parton distribution functions. The results are in agreement with a large set of experimental data. These include the structure functions of the proton and the neutron. Agreement with the sum rules of the spin structure functions offers new insight into the spin structure of the nucleon.