Measurements Of Collectivity In Au+Au Collisions By The STAR Experiment At RHIC

A new form of matter with partonic degrees of freedom is generally believed to have been produced in relativistic heavy-ion collisions at the Large Hadron Collider(LHC)and the Relativistic Heavy Ion Collider(RHIC).This new form of matter is referred as a deconfined,nearly locally thermalized,and strongly coupled Quark-Gluon Plasma(QGP)according to the Quantum Chromodynamics(QCD)theory.Since the year 2005,lots of exciting results from the RHIC have claimed that such QGP matter with strong collective motion,cannot be described with hadronic degree of freedom,have been discovered in the experiment.Lattice QCD calculations predict that,the phase transition from hadronic matter into the QGP phase is a smooth crossover at vanishing baryon chemical potential(μB)region.A first-order phase transition is expected at finite baryon chemical potential region,and the critical end point may exist at the end of the first-order phase transition.Searching for the onset of QGP and studying the properties of QCD medium have been the focus of the RHIC Beam Energy Scan(BES)program.The anisotropic flow,especially the first two Fourier expansion coefficients directed flow(v1)and elliptic flow(v2),are excellent probes for studying properties of the nuclear matter created in high-energy nuclear collisions owing to their sensitivity to the expansion dynamics.In addition,the v1 and v2 are particularly sensitive to the Equation-of-State(EoS)and degrees of freedom of nuclear matter of the heavy-ion collisions.Large positive v2,especially for multi-strange hadrons,along with the observation of its number-of-constituent-quarks(NCQ)scaling are strong evidence for the formation of a hydro-dynamically expanding QGP phase with partonic degrees of freedom.This has been demonstrated and observed at the top RHIC energy and LHC energy.On the other hand,the hydrodynamic model calculations show v1(y)in the mid-rapidity region provide sensitivity to the expansion dynamics of participant matter.They also predict that the minimum in the v1 slopes(dv1/dy)near mid-rapidity as a function of the collision energy,indicates the first-order phase transition of the QCD phase diagram.In the BES-I program,the STAR experiment reported that the v1 slopes(dv1/dy)for identified hadrons as a function of collision energy.A non-monotonic behavior of dv1/dy as a function of collision energy for proton and Λ is observed and the dv1/dy reaches a minimum around(?)=10-20 GeV.This is consistent with the hydrodynamic model predictions.In addition,at lower collision energies,the nuclear mean-field effects will become important and the nuclear incompressibility will significantly affect the observed v1 and v2 according to the transport model calculations.The v1 and v2 measurements over a large energy span will provide effective information that the created nuclear matter is dominated by hadronic or partonic degrees of freedom,thus one can explore the QCD phase structure.In this thesis,we will present new measurements of v1 and v2 for identified hadrons(π±,K±,p)in Au+Au collisions at(?)=3 GeV,and v2 for identified hadrons(π±,K±,KS0,p,φ,Λ,Ξ±,and Ω±)in Au+Au collisions(?)=27 and 54.4 GeV from the STAR experiment.The data sets at 3,27,and 54.4 GeV are 260,560,600 millions events with a minimum-bias trigger,respectively.The 3 GeV is the lowest collision energy with the beam energy of 3.85 GeV per nucleon that RHIC has reached,corresponding to a baryon chemical potential μB～750 MeV in the QCD phase diagram.The main detector of STAR is a cylindrical Time Projection Chamber(TPC)with 4 meters in diameter and 4 meters in length.The data at 3 GeV were taken in the year 2018 under the fixed-target(FXT)configuration covering the full acceptance from mid-rapidity to target rapidity region.The target,with a thickness of 0.25 mm corresponding to a 1%interaction probability,is positioned inside the beam pipe near the edge of the TPC,at 200.7 cm from the TPC center along the beam axis.The 27 and 54.4 GeV data were taken in the collider mode in 2018 and 2017,respectively,where the beam bunch crossing was restricted to the TPC central region,yielding an acceptance of |η|<1 in pseudo-rapidity in the lab frame.The standard event plane method was employed for the identified hadron v1 and v2 measurements in this thesis.At(?)=27 and 54.4 GeV,the second order event plane is reconstructed with tracks measured with TPC and the resolution is determined from two independent sub-events,from η ranges-1<η<-0.05 and 0.05<η<1,respectively.At(?)=3 GeV,the first order event plane is determined with the Event Plane Detector(EPD)covering the pseudo-rapidity region of-5.3<η<-2.6 in the lab frame.The large η-gap between particle interest and particle used for event plane can effectively suppress the non-flow effect.For the particle identification,the PID information of pion,kaon and proton can be directly measured by the TPC and TOF detector.The φ mesons are reconstructed through the decay channel,φ→ K++K-,where the combinatorial background is estimated using the mixedevent technique.The weak decay particles’ lifetime is longer than the detector resolution,so that we can control these decay topological cuts to suppress the background.At the two higher collision energies,it is found that the observed v2 values at mid-rapidity is positive and the v2(pT)results are consistent with the previous measurements.Meanwhile,the fact that the v2 NCQ scaling holds well,indicates the collectivity has been built-up in the partonic stage.Contrary to the results from high collision energies,the measured v2 values at midrapidity is negative,the NCQ scaling is absent for positively charged particles in 3 GeV Au+Au collisions.Furthermore,the v1 slopes at midrapidity for almost all observed hadrons are found to be positive,implying dominant repulsive baryonic interactions.Furthermore,calculations of hadronic transport models(UrQMD)qualitatively reproduced the data.These observations imply the vanishing of partonic collectivity and a new EoS,likely dominated by baryonic interactions in the high baryon density region.The thesis is organized as follows:Chapter 1 is an introduction to the physics of heavyion collisions and the experimental observables.Chapter 2 will shortly introduce the RHIC experiment and the STAR detector.The analysis technique and detail will be presented in Chapter 3.In Chapter 4,we will discuss the experimental results and the model calculations.Finally,the summary and outlook will be discussed in Chapter5.