friese-Oct-2005

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					    Elliptic Flow of Λ Hyperons
 in Pb+Pb collisions at 158 AGeV


Volker Friese
                                     for the                  Collaboration

 v.friese@gsi.de




                   Analysis by G. Stefanek, Kielce




   VI-SIM Workshop "Correlations and Fluctuations", Monbachtal, October 2005
                             Introduction
                                             Initial spatial anisotropy in non-
                                             central collisions is converted by
                                             collective effects to momentum
                                             space

                                             Macroscopic (equilibrium) picture:
                                             pressure gradient in the
                                             interaction region → sensitive to
                                             EOS

                                             Microscopic picture: scattering of
                                             hadrons / partons

                                             Degree of thermalisation?

                                             heavy / strange particles: insight
                                             into early stage of collision
Volker Friese   VI-SIM Workshop, Monbachtal, October 2005             2
                                      Introduction




                                                      smooth increase of v2 with collision
                                                      energy from AGS to RHIC

                                                      no indication of soft region in EOS
                                                      as seen in mean mt

h- / π v2, mid-rapidity, pt integrated

         Volker Friese   VI-SIM Workshop, Monbachtal, October 2005            3
                                 Introduction

  RHIC, Au+Au 200 AGeV, min.bias



                                                                At RHIC:

                                                                mass ordering below
                                                                pt ≈ 1.5 GeV

                                                                agreement with hydro
                                                                calculations up to pt ≈ 2
                                                                GeV

J. Adams et al., nucl-ex/0409033


    Volker Friese   VI-SIM Workshop, Monbachtal, October 2005                  4
                        The NA49 Experiment




 Two Vertex TPC (VTPC-1,VTPC-2)                     Δp/p2 = 7 (0.3) 10-4 (GeV/c)-1
       inside magnetic field                           (VTPC-1, VTPC+MTPC)
 Two Main TPC (MTPC-L, MTPC-R)
                                                     dE/dx resolution 3-6 %
       outside magnetic field
 Veto Calorimeter (VCAL)                            Identification of π+, π- , K+, K-, p,
       detects projectile                            K0s, Λ, Ξ, Ω, φ
       spectators
        Volker Friese   VI-SIM Workshop, Monbachtal, October 2005               5
                Centrality Determination

    Pb+Pb 158 AGeV, 3 M events

                                                             centrality selection
                                                             based on zero-degree
                                                             energy (projectile
                                                             spectators)

                                                             This dataset:

                                                             semi-central online
                                                             trigger

                                                             σ/σtot < 23.5 %



Volker Friese    VI-SIM Workshop, Monbachtal, October 2005             6
                           Elliptic Flow Analysis

• estimate of the reaction plane by the second harmonic
  event plane (Φ2 EP)

• determination of the reaction plane resolution
  (<cos(2(2EP - 2RP)) >) by correlation of sub-events

• evaluation of the Fourier coefficient v2' from
  Λ azimuthal distribution with respect to the event plane
     dN/d(φlab-Φ2 EP) ~ 1 + 2v2' cos[2(φlab-Φ2 EP)]
                          + 2v4' cos[4(φlab-Φ2 EP)]

• correction for the event plane resolution
     v2 = v2' / <cos(2(EP - RP))>

     Details in A.M.Poskanzer and S.A.Voloshin, Phys. Rev. C58 (1998) 1671.

       Volker Friese     VI-SIM Workshop, Monbachtal, October 2005            7
                        Event Plane Determination

                                                                     Primary pions are used
                                                                     2.4 < y < 5; pt < 1 GeV
                                                                     identification by dE/dx
                                                                     quality criteria for
                                                                     suppression of
                                                                     secondary tracks
                                                                     remove tracks with
                                                                     common points with Λ
                                                                     daughter candidates
                                                                     (avoid autocorrelation)
extraction of the azimuthal angle of the event plane:
       X2 = Q2 cos(22 EP) = i pTi [cos(2ilab) - <cos(2lab)> ]
       Y2 = Q2 sin(22 EP) = i pTi [sin(2ilab) - <sin(2lab)> ]
       2 EP = (tan-1 Y2 / X2 ) / 2
        Volker Friese    VI-SIM Workshop, Monbachtal, October 2005                  8
        Acceptance Correction for the Event Plane

Acceptance correction by recentering the distribution
   – <cos(2lab)>, <sin(2lab) > averaged over all events,
     stored in a matrix
       • pt = 0.0-1.0 GeV/c        - 20 bins
       • y = 1- 6                  - 50 bins
       • centrality                 - 8 bins
       • elapse time                - 10 bins




      Volker Friese   VI-SIM Workshop, Monbachtal, October 2005   9
   Acceptance Correction for the Event Plane




 additional acceptance correction by artificial mixed-events
    dN/d(φlab-Φ2 EP) = dNreal/d(φlab-Φ2 EP) / dNmix/d(φlab-Φ2 EP)


Volker Friese   VI-SIM Workshop, Monbachtal, October 2005           10
                       Event Plane Resolution

determined from two random subevents (equal multiplicity):


ΔΦ2 = Φ2A – Φ2B


Fit:
1 + 2 <cos 2ΔΦ2> cos(2ΔΦ2)




<cos 2(ΦEP-ΦRP)> = [2 <cos 2ΔΦ2]1/2
       Volker Friese   VI-SIM Workshop, Monbachtal, October 2005   11
                Selection of Λ candidates




      selection by decay topology Λ→pπ-

      preselection of daughter candidates by dE/dx

      suppression of random background by geometrical
      cuts
Volker Friese    VI-SIM Workshop, Monbachtal, October 2005   12
                Selection of Λ candidates

                                              invariant-mass cut
                                              (1.108 – 1.124 GeV)
                                              bin-by-bin background subtraction
                                              acceptance range:
                                              -1.5 < y < 1.0; 0.4 GeV < pt < 4 GeV
                                              Large anisotropy in azimuthal
                                              distribution




Volker Friese    VI-SIM Workshop, Monbachtal, October 2005              13
                            Extraction of v2
      /TOT = 12.5 - 23.5 %
                                                Fit of


                                                       dN/d(φlab-Φ2 EP) ~
                                                        1 + 2v2' cos[2(φlab-Φ2 EP)]
                                                          + 2v4' cos[4(φlab-Φ2 EP)]

      /TOT = 5 - 12.5 %                              Correction for event plane
                                                       resolution:

                                                       v2 = v2' / <cos(2(EP -
                                                       RP))>


Volker Friese      VI-SIM Workshop, Monbachtal, October 2005             14
                  Rapidity Dependence


                                                            no significant
                                                            dependence of Λ
                                                            v2 on rapidity
                                                            observed

                                                            similar to protons,
                                                            but in contrast to
                                                            pions




Volker Friese   VI-SIM Workshop, Monbachtal, October 2005              15
                       Rapidity Dependence (p, π)

         proton v2                                                    pion v2




C.Alt et al., Phys. Rev. C 68 (2003) 034903


       Volker Friese      VI-SIM Workshop, Monbachtal, October 2005             16
                 pt Dependence of Λ v2



                                                       increase with pt from ≈
                                                       0.5 GeV on

                                                       stronger increase for
                                                       more peripheral
                                                       collisions




Volker Friese   VI-SIM Workshop, Monbachtal, October 2005             17
                         Comparison with CERES




Good agreement with CERES for comparable event selections

      Volker Friese   VI-SIM Workshop, Monbachtal, October 2005   18
                          Comparison with RHIC
                 STAR data: J.Adams et al., Phys. Rev. Lett. 92 (2004) 052302




central events: similar v2 at SPS and RHIC
semicentral events: stronger increase with pt at RHIC
(mostly not due to different centrality)
 Volker Friese           VI-SIM Workshop, Monbachtal, October 2005              19
      Ellipitic Flow for Different Species

                                                  linear increase with pt for all
                                                  particle species

                                                  mass hierarchy π > p > Λ
                                                  for pt < 2 GeV

                                                  data well reproduced by blast-
                                                  wave fit (T = 92 MeV, <ρ0> =
                                                  0.8); parameters similar to
                                                  those obtained from pt spectra
                                                  fit

                                         Model:
                                         P.Huovinen et al., Phys. Lett. B 503 (2001) 58
                                         C.Adler et al., Phys. Rev. Lett. 87 (2001) 182301
                                         F.Retiere, A.M.Lisa, Phys. Rev. C 70 (2004) 044907

Volker Friese   VI-SIM Workshop, Monbachtal, October 2005                    20
           Comparison with Hydrodynamics



                                         • hydrodynamic calculations with
                                           Tf =120 MeV reproduce pT spectra
                                           but overpredict v2(pT) SPS data

                                         • predicitions with higher temperature
                                           Tf =160 MeV closer to Λ v2 (pT) data
                                           but cannot reproduce pT spectra



                                       Hydrodynamic calculations by P.Huovinen:
                                        1-st order phase transition, Tc=165 MeV


Volker Friese   VI-SIM Workshop, Monbachtal, October 2005                 21
                Naive Coalescence Model


                                             • v2 of protons and Λ hyperons agree
                                               with naive quark coalescence model

                                             • pions show a larger elliptic flow
                                               below pT = 2 GeV/c
                                                 → possible explanation by resonance
                                                   decays and quark momentum
                                                   distribution in hadrons
                                              V.Greco, C.M.Ko Phys. Rev. C 70 (2004) 024901




Volker Friese   VI-SIM Workshop, Monbachtal, October 2005                    22
                                          Summary


• v2 is flat versus rapidity
• it grows linear with pt up to ≈ 2.5 GeV
• weaker pt dependence than at RHIC
• it increases with decreasing centrality
• NA49 results are in agreement with CERES
• simultaneous description of v2 and pt spectra by Blast-
  Wave fit
• no consistent decsription by hydro models


       Volker Friese   VI-SIM Workshop, Monbachtal, October 2005   23

				
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