Theoretical issues at high energy heavy ion collisions

Abstract

In the session I chaired one can find a stream of ideas and resultsfollowed by open questions. This is an indication of an active, healthyfield. I will very briefly summarize the main points, and I will followthat with a question and a suggestion.J Rafelski spoke about how phase space seems to be populated statistically inhigh energy nuclear collisions.C Gao basically raised the question: What is the available phase space? MBleicher emphasized that high mass resonances are produced in abundance by theevent simulator UrQMD, and probably in other approaches too. K Redlichanalyzed data using the canonical ensemble (fixed particle numbers) and showedthat it may be useful for understanding trends in particle production withincreasing energy or decreasing impact parameter. T Biro presented a novelapproach to rate equations simulating confinement to see the approach toequilibrium. M Bleicher and J Rafelski coupled reaction rates and freezeoutconditions to the dynamical evolution of the system. The difficult problem ofhow particle production rates are modified in a dense medium was addressedby S Soff.It has long been appreciated in high energy collisions that observables may be expressed as observables= d LIPS | |2.Oftentimes the Lorentz invariant phase space d LIPS dominates the slowvariation of the dynamical matrix element so that the observable isbasically determined by phase space up to an overall constant. This is afeature we have been seeing in high energy heavy ion collisions. The majorquestion is: What is the dynamics leading to the statistical population ofphase space in heavy ion collisions?I propose that a general program be developed to solve rate equations forpartons and for hadrons for any given space-time evolution of matter.Nucleosynthesis codes have been developed for community-wide use in the contextof the early universe and in supernovae. Why not in heavy ion collisions?