Measurement Of Charm Production Cross-section And Leptons From Its Semileptonic Decay At RHIC

The strong interaction, one of the four fundamental forces of nature, is confining: there is no single quark as a color-triplet state observed experimentally. Quantum Chromody-namics (QCD) is believed to be a correct basic gauge field theory of strong interactions. Lattice QCD calculations predict a phase transition from hadronic gas to a new matter, Quark- Gluon Plasma (QGP), at high energy nuclear-nuclear collisions. In this new form of matter, quarks are deconfined and approach local thermalization. One of the ultimate goals of the heavy ion collision experiments is to search for the QGP matter and study its properties. The Relativistic Heavy Ion Collider (RHIC) located at Brookhaven National Laboratory (BNL) provides a high energy density environment to create and search for the QGP matter by colliding ions like Au at energies up to (S_NN)^1/2=200 GeV. Recent experimental studies at RHIC have given strong evidences that the nuclear matter created in Au+Au collisions at(S_NN)^1/2=200 GeV has surprisingly large collectivity and opacity as reflected by its hydrodynamic behavior at low p_T and its particle suppression behavior at high p_T.Charm quarks provide a unique tool to probe the partonic matter created in relativistic heavy-ion collisions at RHIC energies. Due to their large quark mass ((?)1.3 GeV/c²), charm quarks are predicted to lose less energy than light quarks via only gluon radiation. A measurement of the nuclear modification factor for the charmed hadrons semileptonic decayed single electrons compared to light hadrons is valuably important to complete the picture of the observed jet-quenching phenomenon and help us better understand the energy-loss mechanisms at parton stage in Au+Au collisions at RHIC.Furthermore, the interactions between charm quarks and the medium could boost the radial and elliptic flow resulting in a different charm p_T spectrum shape. Due to its large mass, a charm quark could acquire flow from the sufficient interactions with the surrounding partons in the dense medium. The measurement of charm flow and freeze-out properties is vital to test light flavor thermalization and the partonic density in the early stage of heavy ion collisions.Charm quarks are believed to be produced only at early stages via initial gluon fusions and its cross section can be evaluated by perturbative QCD calculations. Thus study of the binary collision (N_bin) scaling properties for the total charm cross-section from different collision systems can test the theoretical assumptions and determine if charm quarks are indeed good probes to the partonic matter created in high energy heavy ion collisions. Charm total cross-section measurement is also essential for the separation of bottom contribution in non-photonic electron measurement, and for the model calculations, which tries to explain the observed similar suppression pattern of J/ψat RHIC and SPS.In this thesis, we present the measurements of D⁰→KÏ€at low p_T (p_T≤2 GeV/c) and non-photonic electron spectra (0.9≤P_T≤5 GeV/c) from D⁰ semi-leptonic decay. In addition, we use a newly proposed technique to identify muons from charm decays at low p_T The combination of all these three measurements stringently constrains the total charm production cross-section at mid-rapidity at RHIC. They also allow the extraction of the charmed hadron spectral shape and a study of possible charm radial flow in Au+Au collisions.D⁰ mesons were reconstructed from hadronic decay D⁰→K^-Ï€⁺ ((D|-)⁰→K⁺Ï€^-) with a branching ratio of 3.83% in minbias Au+Au collisions. The D⁰ yields were obtained from fitting a Gaussian plus a linear (or a second-order polynomial function) for residual background to the invariant mass distributions of kaon-pion pairs after mixed-event combinatorial background subtraction. The inclusive muons at 0.17≤p_T≤0.25 GeV/c in minbias Au+Au and central Au+Au collisions were analyzed by combining the ionization energy loss (dE/dx) measured in the Time Projection Chamber (TPC) and the mass calculated from the Time Of Flight (TOF) detector at STAR after the residual pion contamination subtracted. The dominant background muons from pion/kaon weak decays were statistically subtracted using the distribution of the distance of closest approach to the collision vertex (DCA).Inclusive electrons up to p_T= 5 GeV/c are identified by using a combination of velocity (β) measurements from the TOF detector and dE/dx measured in the TPC. Photonic background electrons are subtracted statistically by reconstructing the invariant mass of the tagged e^±and every other partner candidate e^? using a 2-dimensional invariant mass method. Partner track finding efficiency is estimated from STAR Geant + Monte Carlo embedding data. More than～95% of photonic background (photon conversion andÏ€⁰ Dalitz decay) can be subtracted through this method. The p_T spectra for non-photonic electrons in central Au+Au minbias Au+Au collisions and its subdivided centralities will be presented. In addition, the method to extract the elliptic flow v₂ of non-photonic electron is also developed in the thesis.By combining these three independent measurements: D→KÏ€, muons and electrons from charm semileptonic decays in minbias and central Au+Au collisions at RHIC, we observed that: The transverse momentum spectra from non-photonic electrons are strongly suppressed at 0.9≤P_T≤5 GeV/c in central Au+Au collisions relative to d+Au collisions. For electrons with p_T (?) 2 GeV/c, corresponding to charmed hadrons with P_T (?) 4 GeV/c, the suppression is similar to that of light baryons and mesons. The blast wave fit to the electron spectra with p_T (?) 2 GeV/c indicates that charmed hadrons may interact and decouple from the system differently from multi-strange hadrons and light hadrons. Future upgrades with a direct reconstruction of charmed hadrons are crucial for more quantitative answers. Charm differential cross-sections at mid-rapidity (dσ_cc|-^NN)/dy) are extracted from a combination of the three measurements covering～90% of the kinematics. The total charm cross-section per nucleon-nucleon collision (σ_cc|-^NN)) is reported as 1.40±0.11(stat.)±0.39(sys.) mb in 0-12% central Au+Au and 1.29±0.12±0.36 mb in minbias Au+Au collisions at (s_NN)^1/2=200 GeV. The charm production cross sections are found to follow the number of binary collisions scaling. This supports the assumption that hard processes scale with binary interactions among initial nucleons and charm quarks can be used as a probe sensitive to the early dynamical stage of the system.In the above measurements, the bottom contribution to the non-photonic electron spectrum is neglected. The separation of bottom and charm contributions in current non-photonic electron measurements is very difficult. There are large uncertainties in the model predictions for charm and bottom production in high-energy nuclear collisions. Thus identification of bottom from the non-photonic electron measurements is crucial to better understand charm physics. In the discussion section of this thesis, we will try a fit to non-photonic electron spectrum and estimate the bottom contributions. We will also compare the v₂ distribution from simulation to the experimental data and estimate the possible charm v₂.