Revealing the signal of QCD phase transition in heavy-ion collisions

We propose a novel criterion to identify the first-order QCD phase transition and the critical end-point (CEP) via directly the net-proton number fluctuations measured in relativistic heavy-ion collision (RHIC) experiments. Using this method, we show that there has not yet been a direct signal of the CEP of QCD according to the currently available data accumulated in the beam energy scan experiments with sNN⩾7.7 GeV. However, there is still a possibility for the CEP to appear in the matter generated by the collisions with energy below 7.7 GeV, and its identification requires future measurements with high statistics for the high-order cumulants at low collision energies.