Measurements of e+e- Pair Production at the Bevalac
It has come to be generally recognized that a relativistic nuclear collision (RNC) can only be described as a complex dynamic evolution of the not-so-many-body system, whether the components of the system are taken at the hadronic level or at the quark level. The evolution proceeds from an initial state of two separated nuclei, through stages of: first nucleon-nucleon collisions; increasing rates of collisions and particle production (heating and compression); gradually decreasing rates of collision and particle production rates (cooling and expansion); and, finally, decoupling of various particle types from the overall system (freeze out). (Parentheses enclose the adiabatic thermodynamic terms that may in some circumstances be relevant descriptions for parts of this process.) It is, in particular, the operation of the dynamics during the stages of high excitation energy and high particle density that are of interest in studying such collisions. Unfortunately we cannot build a detector that will produce information about a particular part of this process. Our detectors integrate over the entire space-time history of a collision — only the dynamics itself determines how the final-state phase space will be populated with particles.
KeywordsVirtual Photon Lepton Pair Integrate Cross Section Real Photon Pion Pair
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