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Directed flow in Au+Au collisions at 62GeV

Directed flow in Au+Au collisions at 62GeV. Gang Wang for STAR Collaboration. Introduction  Directed Flow  Wiggle  Previous result  3 Methods  ZDC upgrade  ZDC-SMD method Physics results  10% - 70%  Centrality dependence

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Directed flow in Au+Au collisions at 62GeV

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  1. Directed flow in Au+Au collisions at 62GeV Gang Wang forSTAR Collaboration • Introduction Directed Flow  Wiggle  Previous result  3 Methods ZDC upgrade ZDC-SMD method • Physics results  10% - 70%  Centrality dependence  Pt dependence  Integrated v1  Limiting fragmentation • Summary

  2. Flow Among different kinds of flow, such as radial flow, directed flow, elliptic flow and higher harmonics, directed flow is the first discovered type of flow (20 years ago). Radial Flow (isotropic) Flow Directed Flow Anisotropic Flow Elliptic Flow Higher Harmonics Gang Wang, KSU

  3. <px> or v1 rapidity Directed Flow • Directed flow: the first harmonic (v1) in the Fourier expansion of the particle azimuthal anisotropy with respect to the reaction plane • It’s likely influenced by the the compression in the initial stages of the collision, and makes a probe for the early stage of the collision. v1 changes with the collision energy. J. -Y. Ollitrault, Nucl. Phys. A638, 195c (1998) Gang Wang, KSU

  4. Wiggle Models predicted that anti-flow/3rd flow component, with QGP  v1(y) flat or a wiggle structure. J. Brachmann et al. PRC 61, 024909 (2000) L.P. Csernai, D. Rohrich PRL 458 (1999) 454 Gang Wang, KSU

  5. Wiggle Models without QGP also predicted a wiggle structure in peripheral or mid-peripheral events. M. Bleicher and H. Stöcker, PLB 526, 309 (2002) RQMD Baryon stopping and positive space-momentum correlation uRQMD R. Snellings, H. Sorge, S. Voloshin, F. Wang, N. Xu, PRL 84, 2803 (2000) Gang Wang, KSU

  6. Previous result STAR Collaboration, PRL 92, 062301 (2004) Gang Wang, KSU

  7. Experimental complications At RHIC, most detectors cover the central rapidity region where the directed signal is small and the analysis procedures are easily confused by non-flow effects (azimuthal correlations not related to the reaction plane orientation). • We try to reduce sensitivity to non-flow effects: • v1{3} • the three-particle cumulant method • by Aihong Tang • v1{EP1, EP2} • the event plane method with mixed harmonics • by Markus Oldenburg, Ilya Selyuzhenkov • v1{ZDC-SMD} • the standard method with the 1st-order event plane reconstructed from neutral fragments of the incident beams • the event plane is determined from the sideward deflection of spectator neutrons (“bounce-off”) measured in ZDC-SMD • by Gang Wang Gang Wang, KSU

  8. STAR ZDC • Each of the RHIC experiments has a pair of Zero Degree Calorimeter • Each of the RHIC experiments has a pair of Zero Degree Calorimeter detectors for beam monitoring, triggering, and locating interaction vertices. • ZDCs detect neutrons emitted along beam directions and measure their total energy (multiplicity) . • Baseline ZDCs have no transverse segmentation, which motivates upgrade. Gang Wang, KSU

  9. STAR ZDC-SMD SMD is an 8 channel by 7 channel hodoscope that sits directly on the face of the 2nd ZDC module Gang Wang, KSU

  10. Raw event plane from ZDC-SMD The raw event plane distributions are not flat. We can use the inverse of the bin content as psi weight in the analysis. STAR Preliminary Gang Wang, KSU

  11. Flatten event plane distribution E877 Collaboration, Phys. Rev. C 56, 3254 (1997) STAR Preliminary Gang Wang, KSU

  12. Comparison between 2 analysis The analysis with psi weight gives the same result as the one with flat psi. 50% -- 60% STAR Preliminary Gang Wang, KSU

  13. Systematic study: 4 terms In analysis: To systematically study the method with ZDC-SMD, we can use the sub event plane from only east or west ZDC-SMD, instead of the full event plane, and even break down the correlation into X and Y direction. For example: Definitions of 4 terms: Gang Wang, KSU

  14. Systematic study : Corrections East and west correction: When east and west ZDC-SMD have different RP resolutions, we have to use a 3rd 1st-order event plane ( ) as an independent reference to correct this effect. E877 Collaboration, Phys. Rev. C 55, 1420 (1997) They are inverse of each other where denotes another 1st-order event plane, e.g. from FTPC Gang Wang, KSU

  15. Systematic study : Corrections X - Y correction: (from Sergei V. and Ilya S.) When detectors like TPC and FTPC don’t have the same sensitivity in X and Y direction, we have to replace the “2” in the numerator with for X direction or for Y direction. For example: Gang Wang, KSU

  16. Systematic study : Comparison After corrections applied, the 4 separate terms get much closer to each other. 50% -- 60% STAR Preliminary Gang Wang, KSU

  17. Systematic study : Comparison The average of the 4 separate terms almost coincides with the result of the full-event-plane analysis. 50% -- 60% STAR Preliminary Gang Wang, KSU

  18. Charged hadrons: Eta dependence STAR Preliminary Gang Wang, KSU

  19. NA49, QM99 Charged hadrons : Eta dependence Centrality by Centrality STAR Preliminary Gang Wang, KSU

  20. Charged hadrons: Pt dependence The magnitude of v1 is observed to increase with particle transverse momentum and then appears to saturate. STAR Preliminary Gang Wang, KSU

  21. STAR Preliminary Charged hadrons: Integrated v1 • v1 is integrated over • 0.15 GeV < pt < 2 GeV. • The magnitude of v1 decreases with centrality. • v1 in the more forward pseudorapidity region varies more strongly with centrality than in the region closer to midrapidity. Gang Wang, KSU

  22. Limiting Fragmentation It has been observed that particle emission(both spectra and flow) as a function of rapidity in the vicinity of beam rapidity appears unchanged over a wide range of beam energies, a pattern known as limiting fragmentation. STAR Preliminary The hypothesis holds here. Gang Wang, KSU

  23. Summary • We present the first measurements of charged particle directed flow in Au+Au collisions at 62GeV. • The analysis has been performed using three different methods and the results agree very well with each other. • STAR ZDC-SMD now allows to determine the reaction plane from the bounce-off of fragmentation neutrons, the first measurement of this type at RHIC. • Charged particles in the pseudorapidity region covered by the STAR TPC and FTPCs (up to eta = 4.0) flow in the opposite direction to the fragmentation nucleons. • Over the pseudorapidity range studied, the v1 for charged particles is found to vary monotonically with pseudorapidity at all centralities. • There’s no evidence so far for pion wiggle. We are working on v1 of identified particles. • Our data provide further support for the limiting fragmentation picture. Gang Wang, KSU

  24. Backup Slides Gang Wang, KSU

  25. Event Plane Resolution for ZDC-SMD STAR Preliminary Gang Wang, KSU

  26. E895 data EOS data: pion flow Gang Wang, KSU

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