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Structure **Up:** Quark
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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.

**Next:** Nuclear
Structure **Up:** Quark
and Hadron Dynamics **Previous:** Quark
and Hadron Dynamics

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