Measurements Of Electrons From Open Heavy Flavor Hadron Decays In P+p And Au+Au Collisions At?^SNN?^1/2?200 GeV By The STAR Experiment

Quantum ChromoDynamics(QCD)is a basic gauge field theory of strong interac-tion,a fundamental force describing the interactions between quarks and gluons.The strong interaction has two distinctive features:asymptotic freedom and color confine-ment.Due to color confinement,free quarks have not been observed experimentally under normal conditions.Instead,they are confined within the color-neutral hadrons.The Lattice QCD calculation predicts a phase transition from hadronic gas to a new mat-ter Quark Gluon Plasma(QGP),in which quarks are deconfined,at high temperature and energy density.Experimentally,QGP could be created and studied via high-energy heavy-ion collisions.The Relativistic Heavy Ion Collider(RHIC)at the Brookhaven National Laboratory(BNL)in the United States is a dedicated machine to study the properties of QGP and QCD phase diagram through nucleus-nucleus collisions.Plenty of measurements from RHIC experiments,such as "jet-quenching" and Number of Con-stituent Quark(NCQ)scaling of elliptic flow,show that a strongly coupled QGP(sQGP)has been formed at RHIC.The next step is to study the QGP properties and the QCD phase structure.Due to the masses of the heavy quarks(charm and bottom quark)are larger than the AQCD(one of QCD scale parameters)and TQGP,they are believed to be dominantly produced in hard scatterings via gluon fusions before the creation of QGP at the early stage of heavy-ion collisions.This means these heavy quarks experience the entire QGP evolution and their kinematics carry the information about their interactions with QGP.Thus,the heavy quarks are suggested as an ideal probe for studying the properties of the QGP.Meanwhile,due to the high transverse momentum transfers when heavy quark produced,their cross-section can be calculated by perturbative QCD(pQCD)theory.Theory predicts heavy quarks lose less energy than light quarks through gluon radiation due to the suppressed radiation angel of the gluon,the so-called "dead cone"effect.Therefore,measuring the nuclear modification factors of the semi-leptonic de-cayed electrons from open heavy flavor quark hadrons is very important to understand the parton energy loss mechanism and QGP properties in nucleus-nucleus collisions at RHIC energies.In particular,the separate measurements for the nuclear modification factors of heavy quarks are crucial to test the mass hierarchy of the parton energy loss in the QGP.In this thesis,the data taken by the Solenoidal Tracker at RHIC(STAR)experiment are used to study the production of electrons from open heavy flavor hadron decays at low transverse momentum(PT)in p+p collisions and at high pT in Au+Au collisions at(?)= 200 GeV.The experimental data used for these analyses are from the minimum biasp+p collisions and the high-tower triggered Au + Au collisions collected by STAR in years 2012 and 2014,respectively.Electrons are identified using the Time Projection Chamber combined with the Time Of Flight detector at low pT and with the Barrel Elec-tromagnetic Calorimeter(and the Shower Maximum Detector in Au + Au collisions)at high pT.Meanwhile the photonic background electrons(gamma conversion,light neutral meson ?0 and ? Dalitz decays)are statistically subtracted by reconstructing the invariant mass of tagged e± and every other partner e±.The efficiencies used in these two analyses are estimated from pure electron data sample and STAR Geant + Monte Carlo embedding data.Then,the pT spectra of electrons from open heavy flavor hadron decays at low pT in p+ p collisions and at high pT for different centralities in An + Au collisions are obtained.In p + p collisions,the measured spectrum of electrons from open heavy flavor hadron decays is consistent with the calculation of pQCD at pT>0.5 GeV/c.And the excess above the pQCD calculation at pT<0.5 GeV/c can provide constraints for the future theoretical model development.Moreover,this measurement also provides a high-precision reference for the measurements of nuclear modification factors in Au +Au collisions.This is the first time that STAR extends the measurement of electrons from open heavy flavor hadron decays to the low pT region.In An + Au collisions,the more precise measurements of the invariant yields and the nuclear modification factors for electrons from open heavy flavor hadron decays are obtained compared to the published STAR measurements.For the invariant yield in central Au+Au collisions,there is significant difference between Au+Au measurement and the pQCD calculation scaled by the number of nucleon-nucleon collisions(Ncoll),indicating existence of hot medium effects.From central to peripheral collisions,the difference is getting smaller,which is consistent with the expectation of less medium effects in peripheral collisions.And a strong suppression of the nuclear modification factor of electrons from open heavy flavor hadron decays at high pT in central Au + Au collisions is observed,which is consistent with the energy loss of the heavy quarks in the hot and dense matter.This measurement provides an important experimental reference for theoretical study about the energy loss mechanism of heavy flavor quarks through interactions with the QGP.Furthermore,the measurements of the nuclear modification factors are used in separate measurements of open charm and bottom hadron production in Au + Au collisions.This paves the way for testing the mass hierarchy of the parton energy loss in the QGP.