Measurements Of Elliptic Flow Of Heavy Flavor Electrons And D^*+Production In Au+Au Collisions At RHIC

Lattice QCD calculations predict that,in relativistic heavy ion collisions where tem-perature is extremely high,quarks and gluons will deconfine from bound hadrons,and a new matter state,Quark-Gluon Plasma(QGP),will be formed.The matter in early universe is believed to be in QGP state a few microseconds after the Big Bang.This strong coupling QCD matter created in the heavy ion collisions is a natural laboratory to study strong interactions.One of the primary goals for relativistic heavy ion collisions,is to produce QGP and study its properties.The masses of heavy quarks(HQ)are much larger than QCD scale,so their produc-tion cross sections can be calculated by perturbative QCD.As their masses are much larger than QGP typical temperature,their thermal production in the medium is sup-pressed.Thus,they are dominantly produced in the initial hard scattering processes with large momentum transfer before the formation of QGP in the heavy ion collisions.The thermalization time of heavy quarks is comparable or longer than the lifetime of QGP created in heavy-ion collisions.Therefore,their final kinematics still carry dy-namic information about their interaction with the QGP medium.When heavy quarks transport through the QGP medium,they interact with the back-ground partons in the medium by elastic scattering and gluon radiation.At low mo-mentum,heavy quarks and medium interactions are dominated with elastic scattering.Since heavy quark masses are much larger than the medium temperature,their interac-tions with the medium can be approximated as multiple elastic scattering with relatively small momentum transfer(q2?T2),in analogy to "Brownian motion" in molecular physics.Heavy quark motion inside the QGP can therefore be described by a stochastic Langevin simulation.One of the transport parameters,often characterized by a spatial diffusion coefficient DS reveals the QGP coupling strength.The Ds can be constrained by analyzing the experimental data of nuclear modification factor RAA and elliptic flow v2 of heavy flavor hadrons or their decay daughters at low momentum,and compared to phenomenological models.The strength of measured D0 v2 at low pT at RHIC top energy and LHC is comparable to those of light hadrons.A local peak structure is ob-served in the D0 RAA around pT?2 GeV/c,known as "flow bump".It is inferred from current measurements in comparison with model calculations that D0 meson has obtained collectivity and may have been thermalized in the medium.Based on the avail-able experimental data,the current understanding of the 2?TDs has been constrained to be around 2-5 near the critical temperature Tc,while the temperature dependence of D5 has an appreciable uncertainty at higher temperature regions.The heavy quark(HQ)energy loss mechanism in the medium can be learned from the modification of pT spectrum of final state heavy flavor hadrons.The gluon radiation process is expected to be suppressed with the increase of parton mass at a given momen-tum,known as the "dead cone" effect.Therefore,the radiative energy loss of quarks and gluons should follow a mass hierarchy:?Eb<?Ec<?Eu,d,s<?Eg.Intuitively,HQ RAA is then expected to be less suppressed if only considering radiative energy loss.However,the measured RAA for charm mesons or their decay products from RHIC top energy and LHC is comparable to those of light hadrons up to pT?10 GeV/c.This striking observation reveals the importance of elastic energy loss for heavy quark energy loss mechanism in the QGP.In addition,the mass hierarchy of RAA is also affected by the initial parton spectra and hadronization process.Recently,a charm-bottom hierar-chy at intermediate pT is observed in measurements of RAA of bottom decay daughters from LHC.Based on the theory predictions,the collisional energy loss dominates at low momentum,while heavy quarks loss energy mainly through gluon radiation at high momentum.However,it is not very clear at which momentum will the radiative energy loss takes over the elastic ones.In this thesis,measurements of the elliptic flow of heavy flavor electron(eHF)in Au+Au collisions at(?)27,54.4 GeV are reported.By lowering the collision energy,one would expect the initial temperature the QGP medium can reach will be different.Therefore,the measurements from different collision energies are expected to offer sensitive information on the temperature dependence of various QGP proper-ties.The measurements are based on the data samples collected during RHIC year run 2017 and 2018 by the Soldnoidal Tracker at RHIC(STAR)detector with a statistics ten times of that in Au+Au(?)=62.4 GeV collisions collected in year 2010.In this analysis,electrons are identified by combining the energy loss information from the Time Projection Chamber(TPC)subsystem and particle velocity information from the Time Of Flight(TOF)subsystem.The main background sources in this analysis are so called photonic electrons,including electrons originated from photon conversions in the detector material and Dalitz decay electrons from light mesons ?0/?.The yield of photonic electrons can be evaluated by a reconstruction method and removed from the total electrons statistically.The reconstruction method involves a finite efficiency which is estimated by full detector simulation using the embedding technique.The v2 of inclusive electrons is calculated using the event plane ?-sub method.The elliptic flow of photonic electrons is computed using simulations with the input hadron v2 from real data.The results show that eHF has obtained a large v2 in Au+Au(?)54.4 GeV collisions that is comparable to the eHF v2 in 200 GeV collisions.The strength of eHF v2 is also comparable to those of light hadrons at pT>1 GeV/c in 54.4 GeV collisions.It indicates that charm quarks also have strong interactions with the QGP medium in Au+Au collisions at 54.4 GeV,although the collision energy is nearly a factor of 4 lower with respect to(?)=200 GeV.In Au+Au collisions at 27 GeV,the strength of eHF v2 is lower than 1.6? lower than that in(?)=54 GeV collisions,and consistent with 0 within uncertainty.This might be a hint for that charm quark is not fully thermalized in Au+Au(?)=27 GeV collisions.Measurements of D*+production in Au+Au(?)=200 GeV collisions are re-ported too.The analysis is based on data samples collected by STAR with the Heavy Flavor Tracker(HFT)detector installed during 2014-2016.This measurement is com-plementary to the D0 RAA measurements to confirm that suppression of Do RAA is not a result from the light quark component of D0.Measurements of various charm meson species are needed and nearly 23%charm quarks will be hadronized to D*+in ee/ep collisions.D*+has a sizable feed-down contribution to the final state D0 mesons with about 67%of D*+decay to D0.Model calculation predicts that D*+life time might be shortened in high temperature and density environment.The final observed D*+yield might get reduced if D*+ decays before kinematic freeze-out due to re-scatterings with the hadronic medium.D*+is reconstructed through the channel D*+? D0?+,D0?K-?+.The result shows that there is no obvious centrality dependence in the D*+/D0 yield ratio,which indicates that re-scattering effect does not have a significant impact on the final D*+ yields.Therefore the measurements of D*+production are able to be combined with D+and D0 measurements for better experimental precision to understand the charm quark energy loss as well as other transport properties.The pT dependence of D*+/D0 yield ratio is consistent with PYTHIA calculation in p+p colli-sions at 200 GeV.As there is no modification of D*+/D0 yield ratio observed in Au+Au collisions,it can be inferred that D*+RAA has similar suppression as D0 RAA and D0 RAA modification is not resulted from D*+feed down.It supports the conclusion that the charm quark has a strong interaction with the medium.Looking forward towards the future heavy flavor program in heavy ion collisions,the sPHENIX detector is a new generation detector for measurements utilizing hard probes.The innermost tracking system is the Monolithic Active Pixel Sensor Vertex Detector(MVTX).The sPHENIX detector also features as high trigger and DAQ rate.Precise heavy flavor measurements over a broad momentum range are expected in the future sPHENIX detector.In the last part of this thesis,the simulation of charm baryon measurements in the sPHENIX detecor is reported.We first extract the detector perfor-mance for single tracks through full detector simulation,which will be set as the input for the followed fast simulation.From the simulation,we predict the ?c+ signal significance and the statistic uncertainty of ?c+/D0 ratio in the future sPHENIX experiment in dif-ferent centrality and pT bins under different PID scenarios.A systematic measurement of charm baryon ?c+ and possibly production in p+p,p+A and A+A collisions with the future sPHENIX detector will offer new insights towards heavy quark hadronization and may potentially shed light on the understanding of color confinement in QCD.