## 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.