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Elliptic and directed flow in heavy ion collisions

Elliptic and directed flow in heavy ion collisions. Hot Quarks 2006, 16.05.06, Villasimius, Sardinia Hannah Petersen, Universität Frankfurt . Thanks to the UrQMD group @ Frankfurt. Mohammed Abdel-Aziz (fluctuations) Marcus Bleicher Stephane Haussler (fluctuations)

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Elliptic and directed flow in heavy ion collisions

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  1. Elliptic and directed flow in heavy ion collisions Hot Quarks 2006, 16.05.06, Villasimius, Sardinia Hannah Petersen, Universität Frankfurt Hannah Petersen, Hot Quarks 2006

  2. Thanks to the UrQMD group@ Frankfurt • Mohammed Abdel-Aziz (fluctuations) • Marcus Bleicher • Stephane Haussler (fluctuations) • Qingfeng Li (EoS, HBT) • Diana Schumacher (dileptons) • Horst Stöcker • Sascha Vogel (resonances) • Xianglei Zhu (elliptic flow and charm)

  3. Outline • Motivation • Introduction • Equation of state (EoS) • Directed flow results • Elliptic flow results • Summary

  4. Motivation • No direct detection of the quark gluon plasma  indirect observables like flow are needed • Transverse collective flow is intimately connected to pressure • Flow is sensitive to changes in the equation of state and therefore to phase transitions (H.Stöcker,W.Greiner Phys.Rep. 137 (1986) 277) phase boundary Plot taken from H. Stöcker, E. Bratkovskaya et al.,J.Phys. G 31, 2005

  5. Introduction - directed flow Fourier expansion of the azimuthal distribution of the emitted particles : Directed flow with  measures the total amount of transverse flow Reaction plane (J.Y. Ollitrault, Phys. Rev. D, 46; A.M. Poskanzer, S.A. Voloshin, Phys. Rev. C, 58)

  6. Introduction - elliptic flow Second coefficient of the Fourier expansion of the azimuthal particle distribution: Coordinate space asymmetry  momentum space anisotropy

  7. Time evolution • Pressure develops sharp maximum in the early stage of the reaction • Pressure gradients lead to flow • v2 builds up directly after this maximum

  8. Equation of state Schematic picture of the EoS with a first order phase transition Connection between pressure and flow via P/e softest point QGP HG A : surface element QGP HG Mixed phase P: pressure e: energy density e

  9. The UrQMD model • Non-equilibrium transport model • All hadrons and resonances up to 2.2 GeV • String excitation and fragmentation • Cross sections are fitted to available data, parametrized via AQM or calculated by detailed balance • Generates full space-time dynamics of hadrons and strings • Known event-plane

  10. v1 of protons @ 40 AGeV Comparison of rapidity spectra between model and data: • Largest flow at high rapidity values • Centrality dependence visible Data from C.Alt et al., Phys. Rev. C 68, 2003

  11. v1 of protons Slope around midrapidity characterizes shape of the rapidity distribution Extracted from normalized rapidity distribution via polynomial fit At low energies: potentials are important At high energies: data developes negative slope  ´wiggle´ QGP-signal? (L.P. Csernai, Phys.Lett. B 458,1999)

  12. Elliptic flow Two competing effects lead to different signs of v2: Squeeze-out py2 > px2  v2 < 0 In-plane flow px2 > py2  v2 > 0

  13. v2 (y) of pions @ 40/160 AGeV Pb+Pb

  14. v2(pt) of pions @ 40/160 AGeV Pb+Pb

  15. Excitation function of elliptic flow At low energies: squeeze-out effect visible and inclusion of nuclear potential needed At high energies: underestimation of flow by calculation because of lack of pressure Data and calculation for mid-central events „HMw“= mean field from a hard equation of state with momentum dependence and medium-modified NN-cross section (Qingfeng Li, nucl-th/0602032)

  16. ‘Partonic’ dof already @ 40 AGeV • Underestimation of v2(pt) in model coincides with onset of ‘partonic’ matter signals onset of change of EoS in the early stage • ´partonic´fraction is always calculated at the time of highest energy density in the reaction (see also H. Weber et al., Phys. Lett. B 442, 1998)

  17. Summary • Flow is connected to pressure and therefore to the EoS • Slope of v1(y) becomes negative around 40 AGeV • Clear underestimation of elliptic flow at high energies in the transport model • Phase transition around Elab ~40 AGeV ?

  18. Backup slides Hannah Petersen, Hot Quarks 2006

  19. data for h- Elliptic flow scaling • Data shows saturation of scaled v2 • High mass resonances like in UrQMD can not explain v2 above 40 AGeV • Strong hint for initial QGP pressure from 30 AGeV on !

  20. Excitation function v2/<pt>

  21. Model UrQMD

  22. V2(y) of protons @40 AGeV • Experimental situation unclear • Transport model calculation is compatible with the data

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