We present measurements of event-by-event fluctuations on hadron multiplicity ratios (K/π, p/π, K/p) in Au+Au collisions at $\sqrt {s_{NN} } $ and 200 GeV using the STAR detector at RHIC. The magnitude of dynamical fluctuations σ dyn for p/π and K/p ratios change smoothly from a large negative value at 7.7 GeV to a smaller negative value at 200 GeV while that for the K/π ratios exhibits no significant beam energy dependence. The dynamical fluctuations related to pair production σ dynpair dyn for the p/K, K/p, K −/K + and p/p ratios at 200 GeV all exhibit a maximum at the mid-central collisions and decrease at the most peripheral and most central collisions.
We argue that by measuring higher moments of the net proton number fluctuations in heavy ion collisions (HIC) one can probe the QCD chiral cross-over transition experimentally. We discuss the properties of fluctuations of the net baryon number in the vicinity of the chiral cross-over transition within the Polyakov loop extended quark-meson model at finite temperature and baryon density. The calculation includes non-perturbative dynamics implemented within the functional renormalization group approach. We find a clear signal for the chiral cross-over transition in the fluctuations of the net baryon number. We address our theoretical findings to experimental data of the STAR Collaboration on energy and centrality dependence of the net proton number fluctuations and their probability distributions in HIC.
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