J/ψ Production At High Transverse Momentum In P+P And A+A Collisions

A Quark-Gluon Plasma (QGP) is believed to have existed before the universe cooled and free quarks and gluons combined into protons and neutrons, which then bound together to form light nuclei. Lattice QCD calculations predict a phase transition from hadronic matter to the deconfined and locally thermalized QGP state at high temperature and small baryon density. The Relativistic Heavy Ion Collider (RHIC) is built to search for the QGP in laboratory through the high energy heavy ion collisions. Suppression of the c(?) bound state J/ψmeson production in relativistic heavy-ion collisions arising from J/ψdissociation due to color screening of the cc binding potential in the de-confined medium has been proposed as a signature of QGP formation.Beside the color screening effect, other mechanisms may contribute to the J/ψsuppression in heavy-ion collisions. For example, the decreasing J/ψsuppression with increasing p^T observed in Pb + Pb collisions at 17.3 GeV at CERN-SPS can be explained by initial state effects of finite J/ψformation time and the finite space time extent of the hot dense medium. At higher beam energy, RHIC observes similar J/ψsuppression at lower p^T at 200 GeV Au + Au and Cu + Cu collisions with that at CERN-SPS, even though the energy density is significant higher at RHIC. This may be due to the counter-balancing of dissociation by the recombination of thermal c and ¹c in the medium, which are abundant at higher energy. The strong suppression of non-photonic electrons from heavy-flavor decay observed at RHIC suggests that the J/ψmay be also suppressed due to loss energy in the medium if its formation proceeds through a channel carrying color.The medium generated in RHIC heavy-ion collisions is thought to be strongly coupled, making accurate QCD calculations of quarkonium propagation difficult. The AdS/CFT duality for QCD-like theories may provide insight into heavy fermion pair propagating in a strongly coupled liquid. One such calculation predicts that the dissociation temperature decreases with increasing J/ψp^T {or velocity). The temperature achieved at RHIC (-1.5 Tc) is below this dissociation temperature at low J/ψp^T, and above it at p^T＞5 GeV/c. Consequently, J/ψproduction is predicted to be more suppressed at high p^T, in contrast to the standard suppression mechanism. This prediction can be tested with measurements of J/ψover a broad kinematic range, both in p+p and nuclear collisions. The interpretation of J/ψsuppression observed at the SPS and by the PHENIX collaboration requires understanding of the quarkonium production mechanisms in hadronic collisions, which include direct production via gluon fusion and color-octet (CO) and color-singlet (CS) transitions, as described by Non-Relativistic Quantum ChromoDynamics (NRQCD); parton fragmentation; and feed-down from higher charmonium states (χ^c,ψ(2S)) and B meson decays. No model at present day fully explains the J/ψsystematics observed in elementary collisions. J/ψmeasurements at high- p^T both in p+p and nuclear collisions may provide additional insights into the basic processes underlying quarkonium production.In this thesis, we reports the measurement of J/ψin high transverse momentum in p+p and Cu + Cu collisions at√S_NN= 200 GeV at STAR. The inclusive cross section and J/ψ-hadron correlations are presented.The high-p^T J/ψis reconstructed through the J/ψ→e⁺e^- channel (Branching ratio = 5.9%). One electron daughter is triggered online by the Barrel Electromagnetic Calorimeter by requiring its deposit energy above certain threshold. This trigger enriches the high p^T electron sample. The electron identification for this triggered electron is provide by the combination of ionization energy loss dE/dx measured by the Time Projection Chamber (TPC), shower energy measured by the BEMC and shower shape measured by the Barrel Shower Maximum Detector (BSMD). Its purity is＞70 % with high efficiency. The other electron daughter has no BEMC trigger requirement. It is identified by using the dE/dx alone. This J/ψ→e⁺e^- reconstruction method allows us to measurement the J/ψ→e⁺e^- at high p^T(p^T＞5 GeV/c) with good signal to background (S/B) ratio. The J/ψ→e⁺e^- cross section at high-p^T is extracted using RHIC 2005 p+p and Cu + Cu data and RHIC 2006 p+p data, and compared to model calculations. Its xt scaling behavior is also tested. We found the J/ψ→e⁺e^-production in p+p collisions obey x_T scaling at p^T＞5 GeV/c with power value n = 5.6±0.2. The low p^T J/ψsignificantly deviates from the x_T scaling, suggesting the soft process could affect the J/ψproduction at low p^T although it must originate from a hard process due to the mass scale. The J/ψnuclear modification factor R_AA in Cu + Cu is found to increase from low to high p_T. The average of R_AA at p^T＞5 GeV/c is 1.4±0.4{stat.)±0.2{syst.) and is 1.1±0.3±0.2 from combined RHIC data. This is consistent with no J/ψsuppression, in contrast to the prediction from a theoretical model of quarkonium dissociation in a strongly coupled liquid using an AdS/CFT approach. The two-component model including color screening, hadronic phase dissociation, statical c(?) coalescence at the hadronization transition, J/ψformation time and B-meson feed-down can describe the overall trend of the data.Thanks to the high S/B ratio, we measured the azimuthal correlation between high p^T J/ψand charged hadrons. We observed an absence of charged hadrons accompanying high p^T J/ψon the near side, in contrast to the di-hadron correlation. From comparison with the model simulations, we estimate the fraction of J/ψfrom B-meson decay to be 13±5% at p^T＞5 GeV/c.