1 INTRODUCTION

Hadrons are strongly interacting particles made of quarks, antiquarks and gluons in a net color singlet state. The color forces between these color-carrying degrees of freedom, described by quantum chromodynamics, are responsible for the remarkable properties of hadrons. They lead to confinement of quarks and gluons in individual hadrons. The residual color forces between confined hadrons are the familiar "nuclear forces." The color forces also give rise to spontaneous breaking of chiral symmetry in nuclear and particle physics.

^[1]

The problem I would like to discuss in this chapter is the less well understood dynamical role of color degrees of freedom in the strong interactions of hadrons. In particular, when hadrons interact ultrarelativistically, their internal color configurations are frozen by Lorentz time dilation. When the configuration of a hadron is small in spatial extent, the hadron interacts weakly with other hadrons, giving rise to the phenomenon of color transparency. On the other hand, when its configuration is large the interaction of the hadron is stronger than average, a color opacity. Such varying interactions are described by fluctuations in the interaction cross sections of hadrons, which can be deduced from experiment, and which should give rise to important fluctuations in observed quantities, such as multiplicity and transverse energy, produced in ultrarelativistic heavy-ion collisions. The focus here is to introduce the basic ideas in order to enable the reader to...