Constraining freeze-out in heavy ion collisions with yields and fluctuations

Giorgio Torrieri

The following questions, and the dependance of the answer on energy and system size, define the study of chemical freeze-out in heavy ion collisions: a) Does the statistical model underlie real physics or is it's good fit to data a fortuitus occurrence? b) What is the freeze-out temperature? c) Does freeze-out occur in chemical equilibrium? d) Is strangeness chemically equilibrated? What is it's correlation volume at chemical freeze-out? e) How long is the phase between hadronization and freeze-out? How important is this interacting hadron gas phase in determining final-state observables? Presently, these questions are subject of lively controversy. We argue that an unambiguus answer to each of them can be found by experimentally studying both yields and event-by-event fluctuations within the same statistical model. We outline the caveats in the study of fluctuations and present a quantitative comparison between the statistical model and SPS and RHIC data.