Theory of density fluctuations in the O(4)-model of the QCD phase transition

Density fluctuations in small phase space bins for centrally produced particles are conveniently parametrized by a Gaussian free energy functional of the density fluctuation field. This observation leads to the investigation of the O(N) linear σ-model; the O(4) version serves as a prototype theory of the QCD chiral order parameter. We derive an exact, non-perturbative and finite equation for its finite temperature effective action. In the...