Course Outline for Physics 880.05

II. D. Nucleus-Nucleus Collisions (Chs. 12,13) Part 1

Continuation: Soft Processes

Last period we considered the idea of strings in QCD as color electric flux tubes between quarks. Summary of basic points:

In summary, we understand (qualitatively at least) why pions are produced the most and why the transverse momentum (p_T) distributions decrease rapidly with increasing p_T, with an average p_T around 3-400 MeV/c. Other aspects such as the plateau in the multiplicity distributions can also be explained by models. We'll briefly discuss Chapter 6 and the other pictures on the handout from last time.

Now we turn from pp scattering to protons on nuclei (pA) and nucleus-nucleus scattering.

Chapter 12: The Glauber Model of Nucleus-Nucleus Collisions

Let's think about nucleus-nucleus collisions now. From Chapter 3 and the figures we learned that a nucleon colliding with another at high energy loses a significant fraction of its energy, which goes into creating other particles. We can imagine that a nucleus-nucleus collision will involve many nucleon-nucleon collisions, so that we can deposit a tremendous amount of energy in the region of the collision. This is our proposed method for reaching high temperatures or densities, with the hope of reaching the QGP phase transition.

Let's start with proton-nucleus collisions to see the patterns. Consider figure 12.1. This shows the cross section for protons on a variety of targets ranging from a proton to nuclei from carbon (A=12) to lead (A=208). The cross sections are all at fixed transverse momentum p_T=0.3 GeV/c and are plotted as a function of x. Recall that "x" here is a shorthand for x+. (How is dx d^p_T related to the invariant cross section?) Some observations:

How can we understand what is happening in Fig. 12.1 and, more generally, how much energy is lost in a nucleus-nucleus collision? If we know how many collisions the incident nucleon makes, and we can model how x changes with each collision, then we should be able to reproduce (at least semi-quantitatively) Fig. 12.1 and generalize to nucleus-nucleus collisions. We'll use the Glauber model of multiple-collision processes, which is a high-energy scattering approximation.


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Copyright © 1997,1998 Richard Furnstahl and James Steele.
E-mail: furnstah@mps.ohio-state.edu