elliptic flow at rhic

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Elliptic flow at RHIC Raimond Snellings

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Elliptic flow at RHIC. Raimond Snellings. y. x. p y. p x. Why is elliptic flow interesting?. coordinate space. Coordinate space configuration anisotropic (almond shape) however, initial momentum distribution isotropic (spherically symmetric) - PowerPoint PPT Presentation

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Page 1: Elliptic flow at RHIC

Elliptic flow at RHIC

Raimond Snellings

Page 2: Elliptic flow at RHIC

Why is elliptic flow interesting?• Coordinate space configuration anisotropic (almond

shape) however, initial momentum distribution isotropic (spherically symmetric)

• Only interactions among constituents generate a pressure gradient, which transforms the initial coordinate space anisotropy into a momentum space anisotropy (no analogy in pp)

• Multiple interactions lead to thermalization -> limiting behavior ideal hydrodynamic flow

y

x

py

px

coordinate space

Momentum space

12 , tan ( )cos 2( ) y

xrv

p

p

3 2

31

11 2 cos

2 n rnt t

d N d NE v nd p p dp dy

Page 3: Elliptic flow at RHIC

Time evolution in a ideal hydrodynamic model calculation

• Elliptic Flow reduces spatial anisotropy -> shuts itself off

Page 4: Elliptic flow at RHIC

Main contribution to elliptic flow early in the collision

Zhang, Gyulassy, Ko, Phys. Lett. B455 (1999) 45

Page 5: Elliptic flow at RHIC

v2 versus centrality

STAR PRL 86, (2001) 402

|| < 1.3

0.1 < pt < 2.0

First time in Heavy-Ion Collisions a system created which at low pt is in quantitative agreement with hydrodynamic model predictions for v2 up to mid-central collisions

PHOBOS

PHENIX

Page 6: Elliptic flow at RHIC

Identified particle v2 • Typical pt dependence• Heavy particles more

sensitive to velocity distribution (less effected by thermal smearing) therefore put better constrained on EOS

Fluid cells expand with collective velocity v, different mass particles get different p

Page 7: Elliptic flow at RHIC

Identified particle v2 (130 GeV)

dashed solid

T (MeV) 135 20 100 24

0(c) 0.52 0.02 0.54 0.03

a (c) 0.09 0.02 0.04 0.01

S2 0.0 0.04 0.01

The STAR Collaboration, Phys. Rev. Lett. 87 (2001) 182301

Source not spherical in coordinate space at freeze-out!

Page 8: Elliptic flow at RHIC

v2(pt,mass) 130 vs. 200 GeV

• Identified particle v2 at 130 and 200 GeV very close

Preliminary

Page 9: Elliptic flow at RHIC

PHENIX and STAR

PHENIX

STAR

Preliminary

Page 10: Elliptic flow at RHIC

PHOBOS v2()

Preliminary v2200

Final v2130

200

130

Only a little increase

average over

all centrality

(Npart ~200)

Centrality dependence study is limited by statistics

Inkyu Park Talk

Page 11: Elliptic flow at RHIC

v2(pt) for high pt particlesM. Gyulassy, I. Vitev and X.N. Wang

http://www.lbl.gov/nsd/annual/rbf/nsd1998/rnc/RNC.htmR17. Event Anisotropy as a Probe of Jet QuenchingR.S and X.-N. Wang R.S, A.M. Poskanzer, S.A. Voloshin, STAR note, nucl-ex/9904003

Page 12: Elliptic flow at RHIC

Charged particle v2 at high-pt

STAR preliminary

STAR preliminary

Above 6 GeV we do not have a reliable answer (yet) what the real flow contribution is

PHENIX preliminary

Page 13: Elliptic flow at RHIC

What have we learned from elliptic flow at RHIC

– L. McLerran: one needs very strong interactions amongst the quark and gluons at very early times in the collision (hep-ph/0202025).

– U. Heinz: resulting in a well-developed quark-gluon plasma with almost ideal fluid-dynamical collective behavior and a lifetime of several fm/c (hep-ph/0109006).

– E. Shuryak: probably the most direct signature of QGP plasma formation, observed at RHIC (nucl-th/0112042).

– QGP conclusions are model dependent and in my opinion these models are not sufficiently constrained yet. (RS)

Page 14: Elliptic flow at RHIC

Excitation FunctionsT

th [G

eV

]<

r>

[c]

Page 15: Elliptic flow at RHIC

Elliptic flow; excitation function

NA49nucl-ex/0303001

preliminary

Page 16: Elliptic flow at RHIC

v2(pt) SPS-RHIC

• Surprisingly close!

• <pt> pions 158 A GeV ≈ 300 MeV

• <pt> charged particles 200 GeV ≈ 500 MeV

• Integrated v2 mainly driven by <pt>

Preliminary

Page 17: Elliptic flow at RHIC

Why does hydro describe v2 at RHIC and not at the SPS?

Page 18: Elliptic flow at RHIC

The “Reaction Plane”

• Anisotropic flow ≡ azimuthal correlation with the reaction plane

• Experimentally the reaction plane r is unknown

• Can introduce “non-flow” contributions

Page 19: Elliptic flow at RHIC

Event plane resolution• Event plane resolution

N * v22

• Most non flow contributions v2 1/N

• Kovchegov and Tuchin: N = Nwounded

• Non flow contribution will be constant in this variable. Dashed red line estimate of non-flow in first STAR flow paper

iii

iii

1B,A2 2cosw

2sinwTan

2

1

STAR, PRL 86, (2001) 402, Nucl. Phys. A698 (2002) 193

Page 20: Elliptic flow at RHIC

Elliptic flow as a function of centrality

STAR Nucl. Phys. A698 (2002) 193

Non-flow considerable for central and peripheral events

Page 21: Elliptic flow at RHIC

Calculating flow using multi particle correlations

)ψ( r)(cos inrn env

2 1 2 1 21 ( ) ( ) (( ) 2) ( ) ( {2})r r r rinn

in in in ine e e ee v Assumption all correlations between particles due to flow

Non flow correlation contribute order (1/N), problem if vn≈1/√N

1 2 3 4 3 4 3 21 2 1 4( ) ( ) ( )( ) ( ) 4( {4})in in inin in

n ve e e e e

Non flow correlation contribute order (1/N3), problem if vn≈1/N¾

N. Borghini, P.M. Dinh and J.-Y Ollitrault, Phys. Rev. C63 (2001) 054906

Page 22: Elliptic flow at RHIC

Integrated v2 from cumulants

STAR, PRC 66,(2002) 034904

Page 23: Elliptic flow at RHIC

Non-flow estimate from pp

See Aihong’s talk

At low-pt non-flow estimate from pp 5-10% of observed v2 in AA

STAR preliminary

Page 24: Elliptic flow at RHIC

Yet another view on non-flow2 2

2 2

y x

y x

v Fluctuation probably too large; estimate of maximum effect on v2

Page 25: Elliptic flow at RHIC

Comparing optical to MC Glauber

Page 26: Elliptic flow at RHIC

Eccentricity Fluctuations

Page 27: Elliptic flow at RHIC

Fluctuation and their effect on cumulant calculations

1/ 422 4{4} 2v v v v

Page 28: Elliptic flow at RHIC

Fluctuation contribution to extracted v2

v2{2} at mid-central collisions 10% higher than real v2

v2{4} at mid-central collisions 10% lower than real v2

Page 29: Elliptic flow at RHIC

Conclusion

• Comparable measurements of elliptic flow from PHENIX, PHOBOS and STAR

• Elliptic flow well described by boosted thermal particle distributions

• Flow is large; indicative of strong parton interactions at early stage of the collision

• Up to pt = 6 GeV/c sizable elliptic flow• Elliptic flow measurement at low to intermediate pt is real

correlation with the reaction plane• Fluctuation could be main contribution to non-flow; At

mid-central collisions the maximum effect is 10%

Page 30: Elliptic flow at RHIC

Identified particle v2 at high-pt

• See Huan’s talk

• Double splitting of v2(pt) for the different particles

• parton coalescence at intermediate pt? Voloshin QM2002

Preliminary