Interplay between soft and hard processes in quantum chromodynamics

Sammanfattning: In QCD, the theory of strong interactions, hard processes with large momentum transfers can be calculated using perturbation theory, while soft processes with small momentum transfers are poorly understood. In this thesis, phenomenological models of soft and hard QCD processes are developed and tested.A new model is presented for the distribution of quarks and gluons in hadrons. They are derived from a spherically symmetric, Gaussian, distribution of the quark and gluon momenta in the hadron rest frame. The proton structure function F2 is calculated from the model and evolved using next-to-leading order QCD, resulting in a successful comparison-with deep inelastic scattering data.A unified description is given of the hadronic final states observed in deep inelastic electron-proton scattering at HERA, regarding both the large forward energy flow and the rapidity gap events with no particles in a large forward angular region. The model is based on perturbative QCD complemented with a new "soft colour interaction" mechanism which affects the hadronic final state, and provides a continuous description of the transition from diffractive to non-diffractive hard scattering.The model for soft colour interactions also describes the surprisingly high production rate of heavy quarkonia observed in proton-antiproton collisions at the Tevatron.