The particle dependence of v 2 at moderate p T in Au+Au collisions

The particle dependence of vThe particle dependence of v22 at moderate pat moderate pTT in Au+Au in Au+Au

collisionscollisions

SQM2003–Atlantic Beach, North Carolina March 11–17, 2003

— — Paul Sorensen —Paul Sorensen —University of California–Los AngelesUniversity of California–Los Angeles

OutlineI. Analysis method overview.II. Theories related to our measurements.

III. The minimum-bias v2(pT) for KS and Λ to pT ~ 6.0 GeV/c.

IV. The v2 for KS and Λ from 0–5, 5–30, 30–70% of the collisions cross-sections.

V. Discussion.Paul SorensenPaul Sorensen

ObjectiveMeasurements of the event–wise azimuthal anisotropy of charged particle production indicate that the elliptic anisotropy parameter (v2) is large and nearly pT independent for 2.0 < pT < 8.0 GeV/c.

I will present v2(pT) for KS and Λ from three centrality intervals and for a minimum–bias data set reaching pT of nearly 6 GeV/c.

2

March 11–17, 2003SQM2003–Atlantic Beach, North Carolina

Paul SorensenPaul Sorensen

The products of the decays KSπ+π- (i/ 69%) and Λpπ- (i/ 64%) are detected in the TPC. Topology cuts are made on the vertices and the yield is extracted from the invariant mass distributions.

3Analysis overviewAnalysis overview


Centrality classes and the event plane


The centrality is estimated from the number of charged tracks.

The reaction-plane is estimated using the event-plane defined by the anisotropy in the azimuthal distribution of tracks.

The event-plane is an imperfect estimator of the reaction-plane so the v2 is corrected for the resolution estimated from a sub-events analysis.

4Analysis overviewAnalysis overview

The azimuthal anisotropy parameters

r

nrn

TT

v

xy

xy

nvdydpp

Nd

pd

NdE

2cosby best zedcharacteri anisotropy

momentuma toleads anisotropy spatial The

cos212

1

2

22

22

1

2

3

3

Paul SorensenPaul Sorensen 5Analysis overviewAnalysis overview

Azimuthal anisotropy from energy loss

6

In an energy loss (dE/dx) scenario, high pT partons lose energy as they traverse the hot nuclear matter.

As such, high pT hadron production in central collisions may be suppressed relative to binary nucleon-nucleon collision scaling of peripheral collisions (RAA). The dE/dx may also lead to the loss of away–side correlations from jet fragmentation.

more opaque

more opaque:fewer high pT particles

In this picture, dE/dx can also lead to non-

zero v2 at high pT.

less opaque

The very large dE/dx limit:Emission is only seen from near the

surface. Surface emission should lead to a pT and √sNN independent v2:

E. V. Shuryak, Phys.Rev. C66 (2002)

less opaque:more high pT particles

Physics scenariosPhysics scenarios

A particle dependence from hadronization

quarkbaryonquarkmeson

quarkmesonquarkbaryon

vvvv

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pnd

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dependenceparticle RAA

7

(from parton transport)

(Molnar & Voloshin: nucl-th/0302014)

(Bass et al.: nucl-th/0301018)

Physics ScenariosPhysics Scenarios

A particle dependence from the pT scale

This sketch illustrates the possible effects of pQCD overtaking non-pQCD effects (baryon junctions, hydro, etc.) at different pT scales for protons and pions. In this scenario, a particle type dependence will be seen in v2 with proton v2 higher than pion v2 and the saturation of charged particle v2 is caused by the superposition of the two. What will the particle dependence be for RAA?

p T sc

ale

8Physics scenariosPhysics scenarios

KS and Λ v2 results at 130 GeV:

“Azimuthal Anisotropy of KS and Lambda + Anti-lambda Production at Midrapidity from Au+Au Collisions at sNN = 130 GeV”

Published September 23, 2002, Phys. Rev. Lett. 132301-1.

Preliminary sNN = 200 GeV

QM 2002

QM 2002

Transverse Momentum pT (GeV/c)

Paul SorensenPaul Sorensen 9BackgroundBackground


The particle dependence of v2v2 appears to saturate at approximately 0.16 for K0.16 for KSS and

0.23 for 0.23 for ΛΛ (v2 Λ =v2 Λ-bar).

The mass dependence is hydro-like at low pT (a smaller mass gives a larger v2).

The pT onset of the saturation in v2 is different for KS and Λ. The phenomenology seems better described in mT – m0 than pT ; Why (kinetic energy)?

What’s drives the different pT scales for KS and Λ v2? 10

Centrality dependence of v2 for KS and Λ


The v2 as a function of particle type and event centrality.

The mT – m0 plot makes it easier to study the deviations between particle type.

The approximate maximum v2 from surface emission for this centrality is well below v2 for both particles but, the systematic errors must be considered.

11

ST

AR

preliminary (A

u+A

u; 200 GeV

; |y|<1.0)

Particle/centrality dependence of v2 for KS and Λ


Is the Λ v2 to KS v2 ratio smaller in central collisions? The errors–systematic and statistical–must be studied further and reduced, particularly for 0–5% central.

12

ST

AR

preliminary (A

u+A

u; 200 GeV

; |y|<1.0)

Au+Au collision geometry; Scaling by eccentricity ε

Paul SorensenPaul Sorensen 13

40mbσ ;fmGeV0.159ρ 0.535fm;a ;A1.12R

e1Te1Tds

Nd ;ρdzT ;

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model nucleon a wounded and ondistributi Saxon-Woods A

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σTB

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A

rA

Centrality dependence of v2/ε for KS and Λ


v2/ε is increasing monotonically from peripheral to central collisions for both particles.

The accuracy of our estimates of ε need to be considered.

14

ST

AR

preliminary (A

u+A

u; 200 GeV

; |y|<1.0)

v2/ε; central/peripheral (species and mT dependence)

•While the increase in v2/ε with √s happens at low pT, the increase in v2/ε with centrality happens primarily at high pT.•v2/ε is independent of centrality for hydrodynamics with a constant speed of sound.•v2/ε demonstrates how effective the system is at converting geometric to momentum anisotropy. Central collisions are more effective than peri. in creating anisotropy in high pT.

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charged hadron v2: Kirill Filimonov QM2002

C. Adler et.al. Phys. Rev. C66, 034904

From v2 to RAA

Preliminary sNN = 200 GeV

16Paul SorensenPaul Sorensen

RAA is an observable that may be intimately related to v2. See Hui Longs talk for the particle dependence of RAA.

For the first time the particle dependence of v2 has been measured for high and low pT (and vs. centrality).

Centralbin

Peripheralbin

TPeripheral

TCentral

AA

TNN

AA

TAA

AA

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N

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dydpndR

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section-x collision from derived R

factor onmodificatiNuclear

Charged hadron RAA


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The pT scale of KS and Λ production

What physics could be behind the pT scales of the saturation in v2 and the suppression in RAA?

How does the particle type influence the pT scale? • pT

meson≈2·pTparton?

• pTbaryon≈3·pT

parton?

The saturation of v2 and the drop of RAA seem to be correlated.

Summary• v2 and RAA for KS and Λ show a strong particle type dependence:

– Compared to KS, Λ production shows a larger azimuthal anisotropy but a smaller suppression for 2.0 < pT < 4.0 GeV/c.

• v2/ε rises monotonically from peripheral to central collisions:

– Suggestion of saturation is seen in all centralities.

– Central collisions are more effective than peripheral at converting spatial anisotropy to momentum anisotropy in high pT KS and Λ and production.

• For central collisions RAA for Λ is larger than for KS and coincides with our estimate of Nbin scaling expectations for 1.8 < pT < 3.5 GeV/c.

• While an extreme energy loss scenario predicts a maximum v2 of ~15%, Λ v2 reaches as high as ~24% and KS v2 reaches as high as ~16%.



Outlook•In a dE/dx scenario–where opacity from partonic dE/dx leads to a finite, pT independent v2, an RAA below one, and an absence of back-to-back, jet-like correlations–the larger Λ v2 (for 2 < pT < 4) contradicts the smaller Λ suppression manifested in RAA.

•What are the origins of the particle type dependence;–A particle type dependence in the pT scale of a non-pQCD (hydro, baryon-junctions, etc.) to pQCD transition? → Below 4 GeV/c Λ’s are not from jets?

–A mass dependence (from hydro, cronin or fragmentation)?

–A 2 vs. 3-quark dependence (coalescence after a partonic v2 is established?

•For moderate to high pT, the hadronization process may be a crucial component to understanding heavy ion collisions (see, for example, the following slides on coalescence).

•As we continue the systematic study of identified particles (φ, Ξ, Ω, etc.) STAR willwill resolve many of these open issues. See for example Hui Long’s work presented at this meeting.


In this picture v2 of a hadrons at pT is the

partonic v2 at pT/n scaled by the # of quarks (n).

20

STAR preliminary (Au+Au; 200 GeV; |y|<1.0)

In this scenario we must assume that some v2 is built up during a partonic stage.

• We can then infer the value of the quark v2 in

the relevant pT region (~8%).

A comparison of vA comparison of v22 for K for KSS and Λ to and Λ to qualitative predictions from coalescencequalitative predictions from coalescence

This This parton coalescenceparton coalescence rescaling seems rescaling seems to work for each of our centrality intervalsto work for each of our centrality intervals


Supplementary Slides

-Lambda vs. Anti-lambda v2-

-Reaction plane distribution-

-Nbin and Npart scaling-

-v2 for 130 GeV identified particles-

-Modified blast wave fits-

-And much more-Paul SorensenPaul Sorensen

for the STARSTAR Collaboration 22

Paul SorensenPaul Sorensen23


v2/ε vs. charged particle density (1/S dN/dy)


S is the area of the overlap region; dN/dy is the rapidity density.

The most central events approach the hydro limit (a mean free path much smaller than the collision overlap region).

v2/ε is increasing with charged particle density. What is the detailed √s and centrality behavior?

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charged hadron v2 √s ratio: Kirill Filimonov QM2002


v2/ε; mid-central to peripheral


charged hadron v2: Kirill Filimonov QM2002


v2(pT) particle dependence


At low mT-m0 the Λ and KS follow a hydro like mass dependence.

For mT-m0 ≥ 1.0 GeV/c Λ v2 continues to increase while KS v2 begins to saturate; so that the ratio deviates from hydro model expectations.

26

STAR preliminary (Au+Au; 200 GeV; |y|<1.0)

Monte Carlo Glauber Calculation sNN = 200 GeV

%geo <Npart> <Nbin> <Nbin/Npart>

0-5 352 (67) 990.0 (69

77) 2.85 (0.17)

5-10 298 (1010) 783.7 (71

74) 2.65 (0.17)

10-20 232 (1110) 563.2 (64

59) 2.43 (0.17)

20-30 165 (1312) 355.0 (53

49) 2.14 (0.17)

30-40 114 (1312) 213.9 (41

36) 1.86 (0.16)

40-60 61 (1010) 91.8 (22

23) 1.44 (0.14)

60-80 19.8 (56) 20.0 (7

9) 0.96 (0.10)

Woods-Saxon nuclear geometry parameters:

0 = .16935 nucl./fm3 r0 = 6.38 0.06 fm c0 = 0.535 0.027 fm

Cross-sections:

NN = 42 1 mb geo = 7.2 0.4 b27

v2 vs mt – m0 for 130 GeV identified particles

C. Adler et al. Phys. Rev. Lett. 87, 182301 (2001).

28

Modified blast wave fits

A simultaneous fit to both particles at all pT fails.

A simultaneous fit to both particles for pT less than 1.0 GeV/c gives results similar to those reported by STAR for 130 GeV identified particle v2

*.* C. Adler et al. Phys. Rev. Lett. 87, 182301 (2001). 29

How do we maintain an azimuthal anisotropy in the number of particles produced at a given pT while the azimuthal anisotropy of <pT> disappears?

0.8

1.2

1.6

30

Surface width=1.6 fm

1.0

1.0

1.0

Surface Binary Scaling

RAA

pT (GeV/c)

STAR Preliminary Au+Au 200 GeV

• Woods-Saxon distribution.

• A binary collision occurs if dmin (/).

• Pythia is used to handle particle production for each binary collision with sqrt(s) > 4 GeV, otherwise, particles are produced through resonance,eg NN N.

Au+Au at sNN=200 GeV

Centrality<5%

Nbin_v=1016,Npart=347

5%<Centrality<10%


10%<Centrality<20%


30%<Centrality<40%


Surface N-binary Scaling for High pT (Model Calculations by An Tai)

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v2 for mT – m0 between 1.5 – 4.0 GeV/c2


The relative probability to produce mesons or baryons depends on the density of the quarks.

2 ;

3

3

3

3

3

3

3

3

3

dp

nd

dp

nd

dp

nd

dp

nd quarkmesonquarkbaryon

Hadronization Schemes:

• Parton-hadron duality

• Independent fragmentation

• Whatever works?

The hadronization scheme may be important for understanding the particle dependence of RAA and v2.

Coalescence is qualitatively consistent with the particle dependence of v2 and RAA.

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The spectra for KS and Λ + Λ


ST

AR

preliminary (A

u+A

u; 200 GeV

; |y|<1.0)


RAA for KS and Λ

For pT from 1.8-3.5 GeV/c Λ+ Λ production coincides with Nbin scaling while KS and h+/- production is suppressed in this region (RAA Λ =RAA Λ-bar).

At pT ~ 5.5 GeV/c KS, Λ+Λ and h+/- production are suppressed by a similar amount.

The pT scales for KS and Λ RAA are different.

34

35

ST

AR

preliminary (A

u+A

u; 200 GeV

; |y|<1.0)

36

The pT scale of KS and Λ production

What physics is behind the pT scales of the saturation in v2 and the suppression in RAA?

How does the type influence the pT scale? • pT

meson≈2·pTparton?

• pTbaryon≈3·pT

parton?

The saturation of v2 and the drop of RAA seem to be correlated.

37

The pT scale of v2 & RAA for KS, Λ & h+/-

What physics is behind the pT scales of the saturation in v2 and the suppression in RAA?

How does the particle type influence the pT scale?

The v2 saturation and the the decrease in RAA appear to be loosely correlated for both KS and Λ.

200 GeV130 GeV

Lambda v2 (130 GeV) : 0.58 KS v2 (130 GeV) : 0.68Lambda v2 (200 GeV) : 0.80 KS v2 (200 GeV) : 0.80

• Nhits > 15• Nhits/ Nmax > 0.52• |eta| < 1.5

• 0.1 < pT < 2.0 GeV/c• DCA < 2.0 cm• z–vertex < 25 cm

Track–wise and event–wise cuts

Maximum resolution correction factor from random sub-events

Event plane resolution correction factor

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RFF B

RFF C FFF C

FFF B


The particle dependence of v 2 at moderate p T in Au+Au collisions

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