Cumulants of net-baryon distributions are predicted to be sensitive observables in the study of the QCD phase diagram. The cumulant ratios are related to the thermodynamic susceptibilities which can be obtained from QCD based models, HRG, and lattice QCD calculations. QCD thermodynamics with a cross-over predicts negative sign of fifth- and sixth-order baryon number fluctuations. Furthermore, higher order proton factorial cumulants are also suggested to carry signals of the first-order phase transition between hadronic phase and the QGP, where the proton multiplicity distributions could develop a two-component structure.

We report the measurements of fifth- and sixth-order cumulants of net-proton distributions in Au+Au collisions from √sNN = 7.7 - 200 GeV, recorded by the STAR detector in the phase I of Beam Energy Scan (BES-I) program at RHIC. In addition, factorial cumulants of proton distributions for Au+Au collisions at √sNN = 7.7 GeV are also presented. While C5/C1 of net- proton distributions in 0-40% centrality shows a weak collision energy dependece and fluctuates around zero, the C6/C2 values are increasingly negative with decreasing energy for the same centrality. Results of the two ratios for peripheral 70-80% collisions are positive at all energies. Within large uncertainties, the proton factorial cumulant κ5 shows agreement with expectation from first-order transition inspired two-component model calculations while κ6 remains 1.8σ away from the predictions for 0-5% centrality.