Has Tc been measured by heavy ion experiments?

Abstract

We discuss the role of cumulants of net baryon number fluctuations in the analysis of critical behavior in QCD and the study of freeze-out conditions in heavy ion experiments. Through the comparison of the current set of measurements of higher order cumulants of net baryon number fluctuations with lattice QCD calculations and results from hadron resonance gas model we can learn to what extent freeze-out as, determined by such cumulants, occurs close to the QCD transition temperature and thus can probe critical behavior at small values of the baryon chemical potential. Understanding how the relation between freeze-out conditions and the QCD crossover transition is reflected in properties of the experimentally determined cumulants is an important prerequisite to search for the QCD critical point. We point out that even if perfect continuum extrapolated lattice QCD results would be available, it would be inappropriate to use these observables to extract the value of the QCD transition temperature at vanishing baryon chemical potential from experimental data. We furthermore provide indications that a recently performed comparison of lattice QCD results on cumulants with data from heavy ion experiments suffer from systematic as well as statistical uncertainties in the lattice QCD calculations. This makes such comparison of lattice QCD calculations with experimental data at present not useful.