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M. Oldenburg Probing QCD with High Energy Nuclear Collisions, Hirschegg, Austria, January 2005 1 Directed Flow in Au+Au Collisions Markus D. Oldenburg Lawrence Berkeley National Laboratory Probing QCD with High Energy Nuclear Collisions Hirschegg, Austria, January 2005

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M. Oldenburg Probing QCD with High Energy Nuclear Collisions, Hirschegg, Austria, January 2005 2 Overview Introduction Model Predictions for Directed Flow Measurements & Results Model comparisons to data Summary and Outlook

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M. Oldenburg Probing QCD with High Energy Nuclear Collisions, Hirschegg, Austria, January 2005 3 Anisotropic Flow v 1 : directed flow v 2 : elliptic flow peripheral collisions produce an asymmetric particle source in coordinate space spatial anisotropy momentum anisotropy sensitive to the EoS Fourier decomposition of azimuthal particle distribution in momentum space yields coefficients of different order x y z z x

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M. Oldenburg Probing QCD with High Energy Nuclear Collisions, Hirschegg, Austria, January 2005 4 Antiflow of nucleons and 3 rd flow component Au+Au, E kin Lab = 8 A GeV L. P. Csernai, D. Röhrich, PLB 45 (1999), 454. J. Brachmann, S. Soff, A. Dumitru, H. Stöcker, J. A. Maruhn, W. Greiner, L. V. Bravina, D. H. Rischke, PRC 61 (2000), 024909. QGP v 1 (y) flat at mid-rapidity. Bounce off: nucleons at forward rapidity show positive flow. If matter is close to softest point of EoS, at mid-rapidity the ellipsoid expands orthogonal to the longitudinal flow direction. (GeV/c) y/y cm Softening of the EoS can occur due to a phase transition to the QGP or due to resonances and string like excitations. At mid-rapidity, antiflow cancels bounce off. Models with purely hadronic EoS dont show this effect.

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M. Oldenburg Probing QCD with High Energy Nuclear Collisions, Hirschegg, Austria, January 2005 5 Stopping and space-momentum correlation collective expansion of the system implies positive space-momentum correlation wiggle structure of v 1 (y) develops R. Snellings, H. Sorge, S. Voloshin, F. Wang, N. Xu, PRL 84 (2000), 2803. RQMD v2.4 (cascade mode) shape of wiggle depends on centrality, system size, and collision energy even pion v 1 (y) shows a wiggle structure or flatness at mid-rapidity No QGP necessary v 1 (y) wiggle.

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M. Oldenburg Probing QCD with High Energy Nuclear Collisions, Hirschegg, Austria, January 2005 6 Directed flow (v 1 ) at RHIC at 200 GeV J. Adams et al. (STAR collaboration), PRL 92 (2004), 062301. charged particles shows no sign of a wiggle or opposite slope at mid-rapidity Predicted magnitude of a wiggle couldnt be excluded. v 1 signal at mid- rapidity is rather flat

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M. Oldenburg Probing QCD with High Energy Nuclear Collisions, Hirschegg, Austria, January 2005 7 Charged particle v 1 (η) at 62.4 GeV Three different methods: –v 1 {3} –v 1 {EP 1,EP 2 } –v 1 {ZDCSMD} Sign of v 1 is determined with spectator neutrons. v 1 at mid-rapidity is not flat, nor does it show a wiggle structure STAR preliminary charged particles

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M. Oldenburg Probing QCD with High Energy Nuclear Collisions, Hirschegg, Austria, January 2005 8 Centrality dependence of v 1 (η) at 62.4 GeV Different centrality bins show similar behavior. Methods agree very well. Most peripheral bin shows largest flow. STAR preliminary charged particles

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M. Oldenburg Probing QCD with High Energy Nuclear Collisions, Hirschegg, Austria, January 2005 9 Centrality dependence of integrated v 1 integrated magnitude of v 1 increases with impact parameter b The strong increase at forward rapidities (factor 3-4 going from central to peripheral collisions) is not seen at mid-rapidities. !Note the different scale for mid-rapidity and forward rapidity results! midrapidity forward rapidity STAR preliminary charged particles

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M. Oldenburg Probing QCD with High Energy Nuclear Collisions, Hirschegg, Austria, January 2005 10 Comparison of different beam energies Data shifted with respect to beam rapidity. good agreement at forward rapidities, which supports limiting fragmentation in this region STAR preliminary charged particles NA49 data taken from: C. Alt et al. (NA49 Collaboration), Phys. Rev. C 68 (2003), 034903. y diff = y 200GeV – y 17.2,62.4GeV y 200GeV = 5.37 y 62.4GeV = 4.20 y 17.2GeV = 2.92

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M. Oldenburg Probing QCD with High Energy Nuclear Collisions, Hirschegg, Austria, January 2005 11 v 1 data and simulations at 62.4 GeV All models reproduce the general features of v 1 very well! At high η: Geometry the only driving force? [see Liu, Panitkin, Xu: PRC 59 (1999), 348] At mid-rapidity we see more signal than expected by the models. STAR preliminary charged particles

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M. Oldenburg Probing QCD with High Energy Nuclear Collisions, Hirschegg, Austria, January 2005 12 RQMD simulations for 62.4 GeV I Hadron v 1 is very flat at mid-rapidity. Pion v 1 is very flat at mid- rapidity, too. (There is a very small positive slope around η=0.) Proton v 1 shows a clear wiggle structure at mid- rapidity. The overall (= hadron) behavior of v 1 gets more and more dominated by protons when going forward in pseudorapidity.

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M. Oldenburg Probing QCD with High Energy Nuclear Collisions, Hirschegg, Austria, January 2005 13 Summary I Directed flow v 1 of charged particles at 62.4 GeV was measured. The mid-rapidity region does not show a flat signal of v 1. A finite and non-zero slope is detected. The centrality dependence of v 1 (η) shows a smooth decrease in the signal going from peripheral to central collisions. At mid-rapidity theres no significant centrality dependence of v 1 observed, while at forward rapidities directed flow increases 3-fold going from central to peripheral collisions. At forward rapidities our signal at 62.4 GeV agrees with (shifted) measurements at 17.2 and 200 GeV.

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M. Oldenburg Probing QCD with High Energy Nuclear Collisions, Hirschegg, Austria, January 2005 14 Summary II Model predictions for the pseudorapidity dependence of v 1 agree very well with our data, especially at forward rapidities. The very good agreement between different models indicates a purely geometric origin of the v 1 signal. RQMD simulations show a sizeable wiggle in protons v 1 (η), only. Measurements of identified particle v 1 at mid-rapidity will further constrain model predictions. High statistics measurement of v 1 at 200 GeV to come.

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M. Oldenburg Probing QCD with High Energy Nuclear Collisions, Hirschegg, Austria, January 2005 15 midrapidity forward rapidity both plots for centrality 10-70% Directed flow v 1 vs. transverse momentum p t magnitude of v 1 increases with p t and then saturates !Note the different scale for mid-rapidity and forward rapidity results! STAR preliminary p t -dependence of v 1 still awaits explanation by models!

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