description of matter, is inadequately understood. At RHIC, such high-energy densities will be created that the quarks and gluons are expected to become deconfined across a volume that is large compared to that of a hadron. By determining the conditions for deconfinement, experiments at RHIC will play a crucial role in understanding the basic nature of confinement and shed light on how QCD describes the matter of the real world. These experiments are complementary to the studies of the structure of the nucleon described in Chapter 2.

An exciting theoretical challenge in studying high-energy-density matter is to understand chiral symmetry. Massless quarks possess a handedness (i.e., right-handed or left-handed); this chirality is a fundamental symmetry of QCD. In the everyday world, particles have mass. How the massless quarks turn into particles with mass is not completely understood, but the process spontaneously violates the chiral symmetry of QCD. By probing the transition between states where chiral symmetry holds and where it is broken, insight can be gained about how particles acquire their masses. Although the connection between chiral symmetry and quark deconfinement is not well understood at present, chiral symmetry is expected to hold in the quark-gluon plasma.

Information gathered from high-energy heavy-ion collisions is potentially also important in astrophysics. It will help constrain the equation of state, the equation that relates the density of matter in neutron stars and supernovae, as well as in the first microseconds of the early universe, to pressure and temperature. This information will place stronger theoretical constraints on the maximum mass of a neutron star, improving the ability to distinguish neutron stars and black holes.

A transition from normal hadronic matter to a quark-gluon plasma at high densities or temperatures is expected because, generally, as matter is heated or compressed its degrees of freedom change from composite to more fundamental. For example, by heating or compressing a gas of atoms, one eventually forms an electromagnetic plasma in which the nuclei become stripped of electrons, thereby forming an electron gas. Similarly, when nuclei are squeezed (as happens in the formation of neutron stars in supernovae, where the matter is compressed by gravitational collapse) they merge into a continuous fluid of neutrons and protons—nuclear-matter liquid. Likewise, a gas of nucleons, when squeezed or heated, should turn into a gas of uniform quark matter, composed of quarks, antiquarks, and gluons.

The regions in temperature and baryon density where the transition to a quark-gluon plasma is expected are shown in Figure 4.1. Baryons are protons, neutrons, and other particles made up of three quarks. At low temperatures and baryon densities, the system can be described in terms of hadrons, nucleons, mesons, and internally excited states of nucleons. In the high-temperature (~ 150 MeV or 1012 K), high-baryon-density (~ 5-10 times the density of nuclear matter) region, the appropriate description is in terms of quarks and gluons. The transition between these regions may be abrupt, as in the boiling of water—with a



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