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Introduction

Up to now, the focus of nuclear physics research has mostly been on the description of nuclei at the scale of 1 fermi in terms of colourless particles, nucleons and mesons. At this scale, quarks and gluons are confined within the nucleons, and nuclei may be described as interacting systems of many non-relativistic nucleons. High energy experiments have, however, revealed interesting dynamics at much shorter distance scales, where hadrons are composed of interacting quarks and gluons. In the past two decades, Quantum Chromo Dynamics (QCD) has emerged as the theory for the strong force with quarks and gluons as the building blocks of nuclear matter at large densities and high temperatures. One of the most exciting challenges for nuclear physics is the study of the non-perturbative regime of QCD. It is this regime which is relevant for understanding how the elementary fields of QCD - quarks and gluons - build up particles such as protons and neutrons. A basic theoretical difficulty is the non-existence of asymptotic, isolated, coloured objects. This is a feature of the richness of the vacuum structure of QCD. Understanding the different QCD phases and the transitions among them is the challenge of the modern study of strong interactions. At low energy, chiral symmetry can be used to build an effective theory of hadron interactions. At higher energies, the parton model uses non-perturbative quark and gluon distributions to describe hadronic scattering processes. 
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