System Size Dependence of Strange Hadron Elliptic Flow from 200 GeV Au +
Au and Cu + Cu collisions
Yan Lu
for STAR collaboration
Department of Modern Physics University of Science and Technology of China
Quark Matter 2008The 20th International Conference on Ultra-relativistic Nucleus-Nucleus CollisionsFebruary 4–10, Jaipur, India
Yan Lu Quark Matter 2008, Feb. 4 – 10, Jaipur, India 2
Outline
Motivation
Data analysis
Results and Discussion
Summary and Outlook
Yan Lu Quark Matter 2008, Feb. 4 – 10, Jaipur, India 3
Thermalization?
Data: S. Voloshin, nucl-ex/ 0701038Hydro: Kolb, Sollfrank, Heinz, PRC 62 (2000) 054909
21 dNS
vdy
2 2S x y
Low density limit:
2
In case of thermalization:
v2 22
22
yx
xy
part is for the participant zone.
Possible thermalization only for the most central collisions.
‘We argue that the centrality dependence of elliptic flow should be a good indicator of the degree of equilibration reached in the action. ’ S. Voloshin, A. Poskanzer, PLB 474, 27, 2000.
Yan Lu Quark Matter 2008, Feb. 4 – 10, Jaipur, India 4
Partonic Collectivity at RHIC
‘They are made via coalescence of seemingly thermalized quarks in central Au+Au collisions, the observations imply hot and dense matter with partonic collectivity has been formed at RHIC’ STAR: Phys. Rev. Lett., 99, 112301(07), nucl-ex/0703033
minimum bias
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Why System Size and Centrality Dependence?
versus
- Systematic study for identified hadrons will gain information on interplay of collectivity, NQ scaling and thermalization as a function of collision system size and centrality.- Determine parameters for partonic EoS.
Collectivity Local Thermalization nq scaling
System SizeCentrality
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Dataset and Cutsmultiplicity collision centrality
• System: Au+Au collision • Energy : sNN = 200 GeV• Event sample: ~22 M events• Centrality: 0 – 80 %, 0 – 10%,
10 – 40%, 40 – 80%
• System: Cu+Cu collision • Energy : sNN = 200 GeV• Event sample: ~21 M events• Centrality: 0 – 60 %, 0 – 20%, 20
– 60%
• Event cuts: |Vertex z| < 30cm• FTPC track cuts: 6 =< nFitHits < 12, nHitsFit/nHitsPoss > 0.52 pT > 0.1, 2.5 < || < 4.0, Dca < 2.0 cm
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PID and v2 Methods
0S
() p (p )
)()( )()(
KK
v2 vs. minv method (background subtraction technique): - used for K0S, Λ , Ξ, , and analysis and for Au+Au and Cu+Cu datasets flow method - Event Plane method used for Au+Au and Cu+Cu datasets - non-flow systematic: Lee-Yang Zero method for Au+Au and Event Plane method with FTPC reaction plane for Cu+Cu
Lee-Yang Zero method: R. S. Bhalerao, N. Borghini, J.-Y.Ollitrault, Nucl. Phy. A 727 (2003) 373-426Event Plane method: A. M. Poskanzer, S. A. Voloshin, Phys. Rev. C58, 1671 (1998)
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Centrality Dependence ofIdentified v2
All centralities: mass ordering at low pT mT-m scaling at low mT-m. All centralities: baryon v2 > meson v2 at intermediate pT or mT-m.
STAR Preliminary Transverse kinetic energy: mT – m = (pT)2 + m2 - mTransverse momentum
Hydro: P. Huovinen, private communications, 2007
200 GeV Au+Au
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Centrality Dependence of Number-of-Quark Scaling
nq scaling is observed at all centrality bins.
200 GeV Au+Au
STAR Preliminarynq: number of quark within a hadron
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Eccentricity Scaling(?)
STAR Preliminary
Not observe the part scaling claimed by PHENIX.
Divided v2 by <v2>ch instead of p
art, it appears that the scaling works better.
Scale by part to remove initial geometry effect.
Larger v2/part indicates stronger flow in more central collisions.
PHENIX: Phys. Rev. Lett 98, 162301 (2007)Phys. Lett. B 503, 58 (2001)
200 GeV Au+Au200 GeV Au+Au
(c)
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pT-integrated v2/part
STAR Preliminary
Charge particle data: STAR, Phys. Rev. C 72, 014904, 2005.
v2/part versus Npart data: increasing trend indicates stronger flow in more central collisions. hydro: little sensitivity to the collision centrality as expected in equilibrium scenario.
v2/part for a given centrality hydro: a clear hadron mass dependence. data: not clear due to large errors.
Above Npart ~ 170, integrated v2 consistent with hydro prediction indicates local thermalization.
200 GeV Au+Au
p
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System Size DependenceSTAR Preliminary
A given colliding system: clear scaling with nq. System size dependence: - No part scaling claimed by PHENIX - v2 seems to fall at lower pT in Cu+Cu than in Au+Au
PHENIX: Phys. Rev. Lett 98, 162301 (2007)See S. S. Shi’s poster
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Summary We report azimuthal anisotropy parameter v2 of strange and multi-strange hadrons from Au+Au and Cu+Cu collisions at 200 GeV. Detailed centrality dependence is presented up to pT ~ 6 GeV/c.
At low pT region, collectivity is observed at all centrality bins under study. it follows a mT-m scaling.
Not observe the part scaling. larger v2/part indicates stronger flow in more central collisions and larger system.
v2/part increase with Npart, reaching hydro prediction in central Au+Au collisions. indicate thermalization in central Au+Au collisions.
Future:
1) More systematic measurements for , : partonic EoS parameters
2) Heavy quark collectivity: early thermalization of light quarks
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Backup slides!
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Elliptic flow
xz
y
px
pyy
x
y 2 x 2y 2 x 2
v2 cos2 , tan 1(pypx)
Reaction plane: z-x plane
impact parameter
beam
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v2 vs minv method• analysis method: v2 vs. minv • motivated by
Borghini et al. [nucl-th/0407041]
• advantages over standard method:– only one fit per pT bin- smaller systematic uncertainties
• method used for K0S, Λ , Ξ, , and analysis• standard method and v2 vs. minv method give consistent results
SIG + BG
BG
BG(SIG+BG)
SIG (SIG+BG)
v2TOT(minv)
)()()(
)(2cos()(
22
2
invinvBG
invSIG
invTOT
mBGSIG
BGmvmBGSIG
SIGv
mv
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Lee-Yang Zero method: - measure flow using genuine correlations among all particles. - flow determined from first minimum r0
of generating function.
Systematic Errors Evaluation Standard method (Event Plane method): - flow vector - event plane angle
iiixQ )2cos(2
i
iiyQ )2sin(2 2/tan
2
212
x
y
Sensitive to non-flow. FTPC Reaction Plane: - Similar to Event Plane method - Correlate particles in TPC region with FTPC reaction plane.
TPCFTPC FTPC
A. M. Poskanzer, S. A. Voloshin, Phys. Rev. C58, 1671 (1998)
R. S. Bhalerao, N. Borghini, J.-Y.Ollitrault, Nucl. Phy. A 727 (2003) 373-426
Suppress non-flow.
all particles
arbitrary variable
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Non-flow systematics
200 GeV Au+Au 10-40%. h± (yellow bands): decreasing trend. PID: no clear pT and PID dependence. v2{LYZ} < v2{EP}: ~10% effect.
200 GeV Cu+Cu 0-60%. v2{EP, FTPC} < v2{EP, TPC}. larger effect at higher pT. v2 from two methods consistent for within large errors.
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System Size and Centrality DependenceSTAR Preliminary PHENIX: Phys. Rev. Lett 98, 162301 (2007)
A given centrality of a colliding system: clear scaling with nq. A given system: stronger flow in more central collisions. Not observe the part scaling claimed by PHENIX, which is independent on colliding system .