Study Of Higher Moments Of Net-proton Multiplicity Distributions And Mid-rapidity K~-/Ï€ Ratio In Heavy-ion Collisions

Scientists believe the Quantum Chromodynamics (QCD) is an appropriate theory which can successfully explain the strong interaction. QCD matter has multiple phase structure, such as hadron matter phase, Quark Gluon Plasma (QGP) phase and so on.QCD matter generally exists in the form of hadron gas with the range of observation, we could use the QCD phase diagram with two dimension to demonstrate the nuclear matter. There is a end point of the first order phase transition boundary, it is also the critical point of the second order phase transition, so we call it the QCD Critical Point (CP). However, thus far we still can’t find the position of the CP, even whether it exists or not.Recent research finds the higher moments such as Variance(σ2), Skewness(s) and Kurtosis(κ) of the high temperature and high density nuclear matter’s higher order fluc-tuate conserved quantities distributions (as baryon number) have directly relation to the Lattice QCD theory. And also find that the experimentally observable fluctuation of net proton number could approximately express the fluctuation of net baryon and net charged number better. The STAR research group has systematically measured the higher moments of net proton multiplicity distribution of the high-energy heavy ion collision experiment by Relativistic Heavy Ion Collider (RHIC). scientists use the measurement to find the CP of QCD, and finally find that the CP exist, it is located in the region that chemical potential of baryon greater than200MeV.One of my work is simulating the high temperature and high density nuclear matter state that after ultra-relativistic particle collision, by means of Monte Carlo models-AMPT, RQMD and UrQMD, and analyzing the higher moments of net proton multiplicity distri-bution. The analysis of Variance(σ2), Skewness(S) and Kurtosis(κ) of net-proton from Monte Carlo models could a valuable reference to the basic theory and can help us develop ideas. It also could be a baseline prediction for the currently on-going low energy heavy ion runs at RHIC.The another work is studying mid-rapidity K-/π ratio in heavy ion collisions by Monte Carlo models-AMPT, RQMD and UrQMD. Lattice QCD theory predicts matter with high energy density should be in the state of quark gluon deconfinement. It was suggested that strangeness production is a sensitive probe to deonfined state long time ago. K-/π ratio is one of the useful observables to search for strangeness enhancement. Scientist put forward a observational experimental variable ω, K-/π ratio is believed as a function of experimental variable ω. Experiment observes K-/π ratio increases linearly with ω at low energies and saturates at RHIC energies. We study the Dependency between mid-rapidity K-/π ratio and experimental variable ω, centrality, energy, collision system etc. The result could be a support to the new theory with experimental variable ω, and it also can help us develop ideas to the experimental result.