Elliptic Flow of Hyperons in Pb Pb collisions at 158 AGeV

1
Elliptic Flow of ? Hyperonsin PbPb collisions
at 158 AGeV
for the Collaboration
Analysis by G. Stefanek, Kielce
VI-SIM Workshop "Correlations and Fluctuations",
Monbachtal, October 2005
2
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
3
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- / p v2, mid-rapidity, pt integrated
4
Introduction
RHIC, AuAu 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
5
The NA49 Experiment

• Two Vertex TPC (VTPC-1,VTPC-2) inside
  magnetic field
• Two Main TPC (MTPC-L, MTPC-R) outside magnetic
  field
• Veto Calorimeter (VCAL) detects projectile
  spectators

?p/p2 7 (0.3) 10-4 (GeV/c)-1 (VTPC-1,
VTPCMTPC) dE/dx resolution 3-6 Identification
of p, p- , K, K-, p, K0s, ?, ?, O, f
6
Centrality Determination
PbPb 158 AGeV, 3 M events
centrality selection based on zero-degree energy
(projectile spectators) This dataset semi-centr
al online trigger s/stot lt 23.5
7
Elliptic Flow Analysis

• estimate of the reaction plane by the second
  harmonic
• event plane (F2 EP)
• determination of the reaction plane resolution
• (ltcos(2(?2EP - ?2RP)) gt) by correlation of
  sub-events
• evaluation of the Fourier coefficient v2' from
• ? azimuthal distribution with respect to the
  event plane
• dN/d(flab-F2 EP) 1 2v2' cos2(flab-F2 EP)
  
• 2v4'
  cos4(flab-F2 EP)
• correction for the event plane resolution
• v2 v2' / ltcos(2(?EP - ?RP))gt

Details in A.M.Poskanzer and S.A.Voloshin, Phys.
Rev. C58 (1998) 1671.
8
Event Plane Determination
Primary pions are used2.4 lt y lt 5 pt lt 1
GeV identification by dE/dx quality criteria for
suppression of secondary tracks remove tracks
with common points with ? daughter candidates
(avoid autocorrelation)
extraction of the azimuthal angle of the event
plane X2 Q2 cos(2?2 EP) ?i pTi
cos(2?ilab) - ltcos(2?lab)gt Y2 Q2 sin(2?2
EP) ?i pTi sin(2?ilab) - ltsin(2?lab)gt
?2 EP (tan-1 Y2 / X2 ) / 2
9
Acceptance Correction for the Event Plane

• Acceptance correction by recentering the
  distribution
• ltcos(2?lab)gt, ltsin(2?lab) gt 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

10
Acceptance Correction for the Event Plane
additional acceptance correction by artificial
mixed-events dN/d(flab-F2 EP)
dNreal/d(flab-F2 EP) / dNmix/d(flab-F2 EP)
11
Event Plane Resolution
determined from two random subevents (equal
multiplicity) ?F2 F2A F2B Fit 1 2
ltcos 2?F2gt cos(2?F2) ltcos 2(FEP-FRP)gt 2
ltcos 2?F21/2
12
Selection of ? candidates

• selection by decay topology ??pp-
• preselection of daughter candidates by dE/dx
• suppression of random background by geometrical
  cuts

13
Selection of ? candidates
invariant-mass cut (1.108 1.124
GeV) bin-by-bin background subtraction acceptance
range-1.5 lt y lt 1.0 0.4 GeV lt pt lt 4 GeV Large
anisotropy in azimuthal distribution
14
Extraction of v2
?/?TOT 12.5 - 23.5
Fit of dN/d(flab-F2 EP) 1 2v2'
cos2(flab-F2 EP) 2v4' cos4(flab-F2
EP) Correction for event plane resolution v2
v2' / ltcos(2(?EP - ?RP))gt
?/?TOT 5 - 12.5
15
Rapidity Dependence
no significant dependence of ? v2 on rapidity
observed similar to protons, but in contrast to
pions
16
Rapidity Dependence (p, p)
pion v2
proton v2
C.Alt et al., Phys. Rev. C 68 (2003) 034903
17
pt Dependence of ? v2
increase with pt from 0.5 GeV on stronger
increase for more peripheral collisions
18
Comparison with CERES
Good agreement with CERES for comparable event
selections
19
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)
20
Ellipitic Flow for Different Species
linear increase with pt for all particle
species mass hierarchy p gt p gt ? for pt lt 2
GeV data well reproduced by blast-wave fit (T
92 MeV, lt?0gt 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
21
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 ? v2 (pT) data
• but cannot reproduce pT spectra

Hydrodynamic calculations by P.Huovinen 1-st
order phase transition, Tc165 MeV
22
Naive Coalescence Model

• v2 of protons and ? 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

23
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