  According to first principle Lattice QCD calculations, the transition from
quark-gluon plasma to hadronic matter is a smooth crossover in the region
$\mu_{\rm B}\leq T_{c}$. In this range the ratio, $C_{6}/C_{2}$, of net-baryon
distributions are predicted to be negative. In this paper, we report the first
measurement of the midrapidity net-proton $C_{6}/C_{2}$ from 27, 54.4 and 200
GeV Au+Au collisions at RHIC. The dependence on collision centrality and
kinematic acceptance in ($p_{T}$, $y$) are analyzed. While for 27 and 54.4 GeV
collisions the $C_{6}/C_{2}$ values are close to zero within uncertainties, it
is observed that for 200 GeV collisions, the $C_{6}/C_{2}$ ratio becomes
progressively negative from peripheral to central collisions. Transport model
calculations without critical dynamics predict mostly positive values except
for the most central collisions within uncertainties. These observations seem
to favor a smooth crossover in the high energy nuclear collisions at top RHIC
energy.

Nuclear Experiment;High Energy Physics - Experiment;High Energy Physics - Phenomenology;Nuclear Theory