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The effect of ‘partonic wind’ in nuclear collisions on charm quark correlations

The effect of ‘partonic wind’ in nuclear collisions on charm quark correlations. Xianglei Zhu, DEP, Tsinghua University. Strangeness in Quark Matter, Beijing, Oct. 6-10, 2008. Outline. Introduction Charm correlations as a probe of thermalization

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The effect of ‘partonic wind’ in nuclear collisions on charm quark correlations

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  1. The effect of ‘partonic wind’ in nuclear collisions on charm quark correlations Xianglei Zhu, DEP, Tsinghua University Strangeness in Quark Matter, Beijing, Oct. 6-10, 2008

  2. Outline • Introduction • Charm correlations as a probe of thermalization • Effect of radial flow or ‘partonic wind’ on charm correlations • Summary X. Zhu, SQM2008

  3. PHENIX: PRL91, 182301(03) STAR: PRL92, 052302(04) Greco et al, PRC68, 034904(03) Y. Lu et al. J. Phys. G 32 1121 (2006) What RHIC has told us … STAR:PRL91, 072304 (03) • Jet quenching • Large v2 values • NCQ-scalings of v2 • Large radial flow STAR models Au+Au √sNN=200 GeV STAR: NPA757, 102 (05) But whether thermalization is reached is still unknown X. Zhu, SQM2008

  4. Why heavy quarks? • Symmetry is broken: EW Higgs mass QCD dynamical mass • In QGP, heavy quarks stay heavy • At RHIC, heavy quarks are mostly pair produced in the early stage hard processes. • If heavy quarks interact with the medium frequently:  frequent interactions among all quarks or partons light quarks (u,d,s) likely to be thermalized Total quark mass (MeV) B. Mueller, nucl-th/0404015 X. Zhu, SQM2008

  5. STAR hadrons pT> 6 GeV/c B. Zhang et al. Phys. Rev. C72 (2005) V. Greco et al. PLB 595(2004)202 RAA and v2 of non-photonic electron STAR: Phys. Rev. Lett. 98 (2007) 192301 PHENIX: Phys.Rev.Lett.98 (2007) 172301 • The non-photonic electron RAA shows strong interaction of heavy flavors with the medium • NPE v2 favors non-zero charm v2 at pT(NPE) < 2 GeV/c X. Zhu, SQM2008

  6. Charmed mesons correlation • Heavy quarks are pair created in the initial hard processes. • Similar to jets, they should have strong back-to-back angular correlations. • The interaction of heavy quarks with the medium will change the initial correlations. D p p Dbar X. Zhu, SQM2008

  7. Data on D - Meson Pair Correlations • DDbar pairs are back-to-back correlated in π- A collisions at lower energies • Pythia predictions agree with the data well. C. Lourenço and H. Wöhri, Phys. Rep. 433 (2006) X. Zhu, SQM2008

  8. Pythia predictions for RHIC p+p @ 200 GeV • p +p : Pythia predicts(g+g->c+cbar LO pQCD) DDbar – correlations • Back-to-back correlation • Stronger at larger pT How does the QGP modify the angular correlation? How does the HG modify the angular correlation? XZ, M. Bleicher, S.L. Huang, K. Schweda, H. Stöcker, N. Xu, P. Zhuang, Phys. Lett. B647. 366 (2007) X. Zhu, SQM2008

  9. Back-to-back c - cbar in thermalized medium • Put back-to-back c-cbar pairs in QGP. • Use non-relativistic Langevin approach to describe the random walk of charm quarks in QGP [B. Svetitsky and A. Uziel, PRD55]. Fluctuation-dissipation relation in equilibrium: • Use 1+1D Bjorken’s hydrodynamics to simulate the time evolution of QGP: • QGP in thermal equilibrium occupying a cylinder of fixed radius R(~7fm). • Since the time τ0, its temperature evolves like • For RHIC, T0=300MeV (τ0 =0.5fm/c); LHC, T0=700MeV (τ0 =0.2fm/c) X. Zhu, SQM2008

  10. Back-to-back c - cbar in QGP RHIC LHC • For small pT pairs, correlations smear out • For large pT pairs, correlations preserved • Change of initial correlations are sensitive to the drag coefficient, and the temperature XZ, M. Bleicher, S.L. Huang, K. Schweda, H. Stöcker, N. Xu, P. Zhuang, Phys. Lett. B647. 366 (2007) X. Zhu, SQM2008

  11. Drag coefficient G. Moore and D. Teaney, PRC71 D (2πT)=1.5 ~ a = 12 x 10-6 D (2πT)=3 ~ a = 6 x 10-6 D (2πT)=6 ~ a = 3 x 10-6 X. Zhu, SQM2008

  12. Drag coefficient PHENIX, Phys. Rev. Lett.98 (2007) 172301 John Lajoie, QM06 Radiative energy loss only fails to reproduce v2HF. Heavy quark transport model has reasonable agreement with both RAA and v2HF. Small relaxation time t or diffusion coefficient DHQinferred for charm. D (2πT)=1.5 ~ a = 12 x 10-6 D (2πT)=3 ~ a = 6 x 10-6 D (2πT)=6 ~ a = 3 x 10-6 X. Zhu, SQM2008

  13. Back-to-back c - cbar in QGP RHIC LHC • For small pT pairs, correlations smear out • For large pT pairs, correlations preserved • Change of initial correlations are sensitive to the drag coefficient, and the temperature XZ, M. Bleicher, S.L. Huang, K. Schweda, H. Stöcker, N. Xu, P. Zhuang, Phys. Lett. B647. 366 (2007) X. Zhu, SQM2008

  14. Hadronic Re-scattering (UrQMD simulation) • D mesons (D, D*,Ds,…) are from Pythia included in UrQMD. • D mesons collide with the hadrons elastically. • Different cross sections are tried (2mb, 20mb). • Different final state angular distributions are tried [M. Bleicher, et.al., Phys. Lett. B 447] • Forward: • Isotropic: is a constant • UrQMD model handles other collisions between hadrons dynamically. See also similar HSD simulation, E. Bratkovskaya et al., PRC71 X. Zhu, SQM2008

  15. Hadronic Re-scattering (UrQMD simulation) Hadronic re-scatterings have much less effects on DD-correlations XZ, M. Bleicher, S.L. Huang, K. Schweda, H. Stöcker, N. Xu, P. Zhuang, Phys. Lett. B647. 366 (2007) X. Zhu, SQM2008

  16. DDbar correlation function will be modified by the hot/dense medium created in the high-energy collisons, most probably because of the frequent partonic re-scattering. X. Zhu, SQM2008

  17. Discussion (Heavy quark productions in pp collisions – higher order contribution) Leading Order and Next to Leading Order Q g Q g g g Flavor excitation radiative corrections Flavor creation Gluon splitting X. Zhu, SQM2008

  18. Pythia predictions for p+p @ 200 GeV, 5.5TeV PYTHIA modes:*MSEL=1 including NLO contribution *MSEL=4 LO only RHIC: Back-to-back correlations are suppressed a bit, but there are still obvious back-to-back correlations left (especially for high pT pairs). LHC: The NLO contribution becomes important. There are no obvious back to-back correlations. How does the expanding QGP fireball modify the angular correlation? XZ, N. Xu, P. Zhuang, Phys. Rev. Lett. 100, 152301(2008) X. Zhu, SQM2008

  19. c - cbar pairs in expanding thermalized medium • Put c-cbar pairs from PYTHIA in QGP. • Use non-relativistic Langevin approach to describe the random walk of charm quarks in QGP [B. Svetitsky and A. Uziel, PRD55]. Fluctuation-dissipation relation in equilibrium: • Use 2+1D Bjorken’s hydrodynamics to simulate the time evolution of QGP which is producded in central collisions of heavy ion collision at RHIC and LHC. [X. Zhu, P. Zhuang and N. Xu, PLB607]. The transverse expansion of the fireball is considered. X. Zhu, SQM2008

  20. c-cbar correlations in fast expanding thermalized medium 1) At LHC, partonic density will be larger, temperature will be higher, collective expansion will be stronger. 2) Partonic wind pushes produced ccbars to near side! X 10-6 XZ, N. Xu, P. Zhuang, Phys. Rev. Lett. 100, 152301(2008) X. Zhu, SQM2008

  21. Trigger on high pT charm quark • In the model, high pT charm quarks mostly come from the surface of the fireball, with pT pointing outside. • Its partner charm quark will be stopped by the medium, even follow the radial flow, which has the same direction as the trigger charm quark. For LHC (Pb+Pb √sNN=5.5 TeV, most central), a=6 x 10-6 fm-1 MeV-2 X. Zhu, SQM2008

  22. p+p at s = 200 GeV L = 9 pb-1 statistical errors only NPE – charm correlation • STAR has measured NPE-hadron (NPE-D) correlations e-D correlations in p+p 200 GeV e-h correlations in Au+Au 200 GeV STAR Preliminary A. Mischke, QM08 G. Wang, QM08 X. Zhu, SQM2008

  23. NPE – charm correlation For LHC (Pb+Pb √sNN=5.5 TeV, most central), a=6 x 10-6 fm-1 MeV-2, pTc > 3 GeV/c. NPE is the decay product of high pT charm quark. Original high pT triggered c-cbar correlations are mostly preserved in electron – charm (or D) correlation! X. Zhu, SQM2008

  24. Radial flow effect on hadron pair correlations • “As all particles from the same NN collision are produced at the same position in the transverse plane, they get a similar radial push during the expansion stage. It creates rapidity, azimuthal angle, and transverse momentum correlations. …” [S. Voloshin, Phys. Lett. B632 (2006)] • Non-flow correlations in elliptic flow measurement with two particle correlations. • One possible mechanism of “Ridge” phenomenon. [E. Shuryak, PRC76; A. Dumitru, et.al., arXiv:0804.3858; C.A. Pruneau, et.al., Nucl. Phys. A802; S. Gavin, et.al., arXiv:0806.4718]. • This kind of correlation will be sensitive to thermalization and particle diffusion in the thermalized matter during the expansion. The identified particle azimuthal correlation is especially interesting. X. Zhu, SQM2008

  25. Summary • Hot and dense medium created in heavy ion collisions at RHIC reduces the initial back-to-back D-Dbar angular correlations. • In heavy ion collisions at LHC, strong partonic wind pushes the initial uncorrelated D-Dbar to the near side. X. Zhu, SQM2008

  26. Backup slides… X. Zhu, SQM2008

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