Chapter 3 - Geometry

Abstract

The chapter describes nuclear density distributions, and links between the centrality of a collision and observables. Experiments that measure charge density distributions study the Coulomb interactions of a charged particle with a nucleus. Electron scattering is one way to measure charge distribution. It is assumed that charge and matter distributions are identical so that fits of the nuclear charge density distributions can be applied equally well to the nuclear matter distributions. The Glauber model of multiple collisions is used to relate the impact parameter to the number of participants and collisions. It is a geometrical model based on the assumption of constant inelastic cross section for each subsequent collision. The number of participants in a collision can be obtained starting from the probability for a nucleon–nucleon collision in the interaction of a hadron with a nucleus. The amount of stopping in a collision can be related to a light-cone momentum fraction. This is not the same fraction as that entering the parton distribution functions. The transverse energy produced in the collision is also elaborated in the chapter.