Search for the QCD Critical Point by Higher Moments of Net-proton Multiplicity Distributions at RHIC

Higher moments of net-proton multiplicity distributions are applied to search for the QCD critical point. In this paper, we will present measurements for kurtosis ($\kappa$), skewness ($S$) and variance ${\sigma }^2$) of net-proton multiplicity distributions at the mid-rapidity ($\mathrm{\mid }y\mathrm{\mid }<0.5$) and transverse momentum range $0.4<p_T<0.8$ {\gevc} for Au+Au collisions at {\sNN} = 7.7, 11.5, 39, 62.4 and 200 GeV, Cu+Cu collisions at {\sNN} = 22.4, 62.4 and 200 GeV, $d$+Au collisions at {\sNN} = 200 GeV and p+p collisions at {\sNN} = 62.4 and 200 GeV. The moment products $\kappa {\sigma }^2$ and $S\sigma$ of net-proton distributions, which are related to volume independent baryon number susceptibility ratios, are consistent with Lattice QCD and Hadron Resonance Gas (HRG) model calculations at high energies ({\sNN} = 62.4 and 200 GeV). Deviations of $\kappa {\sigma }^2$ and $S\sigma$ for the Au+Au collisions at low energies ({\sNN} = 7.7, 11.5 and 39 GeV) from HRG model calculations are also observed.