Higher Moments Of Net Kaon Multiplicity Distributions At RHIC

The main goals of the high energy nuclear collisions are to explore the phase structure of strongly interacting hot and dense nuclear matter and map the quantum chromodynamics(QCD)phase diagram which can be displayed by the temperature(T)and baryon chemical potential(?B)).The calculations from Lattice QCD suggest that the phase transition between the hadronic phase and Quark-Gluon Plasma(QGP)phase at large ?B and low T is of first order.While at low ?B and high T region,the phase transition is a smooth crossover.The point at the end of first order phase boundary towards crossover region is called QCD Critical Point(CP).Experimental discovery of the critical point is one of the central targets of the Beam Energy Scan(BES)program at the Relativistic Heavy-Ion Collider(RHIC)facility.Fluctuations of conserved quantities,such as net-baryon(B),net-charge(Q)and net-strangeness(S),have been predicted to be sensitive to the QCD phase transition and QCD critical point.Experimentally,one can measure various order moments(Variance(?2),Skew-ness(S),Kurtosis(?))of the event-by-event conserved quantities distributions in heavy-ion collisions.These moments are sensitive to the correlation length(?)of the hot dense matter created in the heavy-ion collisions and also connected to the thermodynamic susceptibilities computed in Lattice QCD and in the Hadron Resonance Gas(HRG)model.For instance,the variance,skewness and kurtosis have been shown to be related to the different power of the correlation length as ?2,?4.5 and ?7,respectively.Theoretically,the nth order susceptibilities ?(n)are related to cumulant as ?(n)=Cn/VT3,where V,T are the volume and temperature of the system,Cn is the nth order cumulants of multiplic-ity distributions.In order to compare with the theoretical calculations,cumulant ratios(S?=C3/C2,??2=C4/C2)are constructed to cancel the volume effects.Thus,those moment products are also directly related to the ratios of various order susceptibilities as??2=?B(4)/?B(2)and Due to the high sensitivity to the correlation length and the connection with the susceptibilities,one can use the moments of the conserved quantity dis-tributions to search for the QCD critical point and the QCD phase transition.Experimentally,it is very hard to measure the net-baryon(?NB)and the net-strangeness(?NS)distributions,so we use net-proton(?NP)and net-kaon(?NK)as proxies respectively.These have been widely studied experimentally and theoretically.The first phase of the beam energy scan program has started in the year 2010 at RHIC.It tunes the Au+Au colliding energies from 200 GeV down to 7.7 GeV,which correspond to a baryon chemical potentials range from 20 to 420 MeV.In this thesis,we performed the first measurements by the STAR experiment for the high moments of net-kaon multiplicity distributions in Au+Au collisions at(?)=7.7,11.5,14.5,19.6,27,39,62.4,and 200 GeV.From the energy dependence of cumulant ratios,one can see that the values of ?2/M increase as the collision energy increases,and are larger for peripheral collisions compared with the central collisions.Within uncertainties,the values of S? and ??2 are consistent with both the Poisson and NBD baselines in central collisions.For peripheral collisions,both the Poisson and NBD baselines underestimate the measured S? values.The UrQMD calcula-tions for ?2/M,Sa,and ??2 are consistent with the measured values within uncertainties except for ?2/M at(?)=200 GeV.The manuscript is organized as follows.In the chapter ?,? give the brief introduction about the analysis background and the analysis motivation.In the chapter ?,? show the basic structure of the STAR detector and the sub-detector which will be used in my analysis.Next,in the chapter ?,? will explain the analysis method and analysis details used in the moments calculation of net kaon multiplicity distributions,including the data preparation,particle identification,error estimation,and the correction methods.I will also give the analysis results in this chapter.In the chapter ??,we calculate the various order of cumulants and their ratios of net proton,net kaon,and net charge multiplicity distributions with UrQMD model.In chapter ?,? give the overview of the Event Plane Detector,which will be installed for Beam Energy Scan phase ? program.It will provide the reconstruction of event plane,centrality determination,and the trigger.Finally,I will give the summary of this thesis at chapter ?.