Beam Energy Dependence of Fifth and Sixth-Order Net-proton Number Fluctuations in Au+Au Collisions at RHIC

We report the beam energy and collision centrality dependence of fifth and sixth order cumulants (C₅, C₆) and factorial cumulants (κ₅, κ₆) of net-proton and proton distributions, from √sNN = 3 - 200 GeV Au+Au collisions at RHIC. The net-proton cumulant ratios generally follow the hierarchy expected from QCD thermodynamics, except for the case of collisions at √sNN = 3 GeV. C₆/C₂ for 0-40% centrality collisions is increasingly negative with decreasing √sNN, while it is positive for the lowest √sNN studied. These observed negative signs are consistent with QCD calculations (at baryon chemical potential, μB ≤ 110 MeV) that include a crossover quark-hadron transition. In addition, for √sNN ≥ 11.5 GeV, the measured proton κₙ, within uncertainties, does not support the two-component shape of proton distributions that would be expected from a first-order phase transition. Taken in combination, the hyper-order proton number fluctuations suggest that the structure of QCD matter at high baryon density, μB~ 750 MeV (√sNN = 3 GeV) is starkly different from those at vanishing μB~ 20MeV (√sNN = 200 GeV and higher).