Latest Results from the PHOBOS Experiment

Latest Results from the PHOBOS Experiment

Barbara WosiekThe Henryk Niewodniczański Institute of Nuclear Physics

Polish Academy of Sciences

Kraków, [email protected]

for the Collaboration

Barbara Wosiek for PHOBOS Quark Matter'08 - Jaipur, India, Feb.4 1

Collaboration

Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard Bindel, Wit Busza (Spokesperson), Vasundhara Chetluru,

Edmundo García, Tomasz Gburek, Joshua Hamblen, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova,

Chia Ming Kuo, Wei Li, Willis Lin, Constantin Loizides, Steven Manly, Alice Mignerey,Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak,

Corey Reed, Christof Roland, Gunther Roland, Joe Sagerer, Peter Steinberg, George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek,

Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Bolek Wysłouch

ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORYINSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY

NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGOUNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER

9 Ph.D. StudentsBarbara Wosiek for PHOBOS Quark Matter'08 - Jaipur, India, Feb.4 2

PHOBOS Detector

Unique low-pT measurements

Large acceptance for Nch-5.4 < η < 5.4 (0.5o < θ < 179.5o )

0 < φ < 2π

-5 -3 0 3 5η

2π

φ

RingsN Octagon RingsPVertex+Spec fill the Octagon holes

0


PHOBOS Research Program

I. Systematic study of charged hadron production inp+p, d+Au, Cu+Cu, Au+Au

II. Comprehensive study of correlations and fluctuations

PHOBOS

Analyses are based on data setscollected during the first 5 RHIC runs,

exploiting the large coverage in η and φ.


Systematic StudiesThe study of global properties of charged particle production

in different collision systems is essentially completed.arXiv:0709.4008 [nucl-ex]

η-4 -2 0 2 4

η

/dch

dN

0

50

100 22.4 GeV Cu+Cu

η

/dch

dN

50

100

150 62.4 GeV Cu+Cu

η

/dch

dN

50

100

150

200200 GeV Cu+Cu

0- 6%

6-15%

15-25%

25-35%

35-45%

45-55%

PHOBOS ‘fingerprints’:LARGE η coverage

BROAD centrality range

Cu-Cu, 62.4 GeVh±, 0-40%

Cu-Cu, 200 GeV, h± , 0-40%

Phys. Rev. Lett. 98 (2007) 242302


Systematic Studies: New Results

Charged particle production

arXiv:0709.4008 [nucl-ex]

TotchN


Poster by Gabor Veres

1 10 210 3100

5

10

15

20

25

30

22.4 GeV (Cu)

19.6 GeV (Au)

200 GeV

62.4 GeV

⟩/2

p

art

N⟨/chT

ot

N

⟩ part

N⟨

d + Au

) + p Inelasticpp(

Cu + Cu

Au + Au

Total charged particle multiplicityscales with Npart

Antiparticle to particle ratiossubmitted

At most weak dependence on system size

Poster by Vasu Chetluru

Systematic Studies: New ResultsUNIQUE PHOBOS measurements on

energy and centrality dependence of the low-pT spectra

[GeV/c]Tp

-110 1

]2/G

eV2

[c

TN

/dyd

p2

d-1 ) Tpπ

(2

1

10

210

310

)-π++π(

)-+K+(K

)p(p+

PHOBOSPHENIX

π K p

Au+Au 200 GeV , 0-6% Au+Au 62.4 GeVPHOBOS

Phys. Rev. C75, 024910 (2007) New data

No anomalous enhancementRadial flow effects breaking mT scaling

Au+Au 200 GeV 0-6% PHOBOS preliminary

Talk by Tomasz GburekBarbara Wosiek for PHOBOS Quark Matter'08 - Jaipur, India, Feb.4 7

Systematic Studies: Summary

Particle production in HI collisions is controlled by simple scaling rulesNpart scaling of total Nch; extended longitudinal scaling in the nucleus rest frame; factorization of the energy and centrality dependencies

− The collision geometry determines the dynamical evolution of the system

− To date no theory/model can consistently explain the scaling rules

− The observations provide a tool for extrapolating RHIC data to the LHC collisions


Correlation & Fluctuation Studies

Exploit large η- φ coverage of the PHOBOS detector

PHOBOS Acceptance – by far the largest of all RHIC experiments

-5 -3 0 3 5η

2π

φ

RingsN Octagon RingsPVertex+Spec fill the Octagon holes

0


Correlation & Fluctuation StudiesInsight into different stages of the system evolution

Initial state Hydrodynamical evolution Freeze-out

Hadronizationtwo-particle correlations

Fluctuations of the

initial source geometry

Elliptic flowfluctuations

Medium response tohigh-pT partons

pT-triggered correlations


Correlation & Fluctuation Studies

New results from PHOBOS:

Fluctuations of the initial source geometry from the GlauberMonte Carlo model

Elliptic flow fluctuations corrected for non-flow effects

Two-particle correlations with high-pT trigger for Au+Au

Two-particle correlations in (∆η,∆φ) for pp, Cu+Cu, Au+Au


Initial Source EccentricityMonte Carlo Glauber (MCG) approach

arXiv:0711.3724 to be published in Phys. Rev. C

Robustness of <εpart>:

Cu+CuAu+Au

200 GeV

Phys. Rev. Lett. 98 (2007) 242302

Participant eccentricity

Choice of the MCG parameters• inter-nucleon separation• nuclear radius• nuclear skin depth• σNN

MCG model assumptions• binary collisions vs. participants• local matter distribution(point-like/Gaussian/hard-sphere)

Event-by-Event Fluctuations of εpartCalculation of the higher order cumulants

εpart{2}, εpart{4}

<εpart> is robust


Poster by Richard Hollis

v2 Fluctuations and Initial Geometry Fluctuations

Quark Matter 2006 Highlight:PHOBOS measured

large dynamical fluctuations of v2

>ε<

εσ≈

><σ

part

part

2

2 )(v

)v(

Quark Matter 2008:- Non-flow contribution determined from data

- Relative v2 fluctuationscorrected for non-flow contribution


This result was corrected for non-flowcontribution estimated from the HIJING model

(negligible, <2%)

Au+Au 200 GeV nucl-ex/0702036HIJING based non-flow correction

Au+Au 200 GeV PHOBOS preliminaryData based non-flow correction

Determination of the Non-Flow Term δNon-flow term: δ = <cos(2∆φ)>non-flow, ∆φ=φ1-φ2

• Non-flow component is dominated by short-range correlations in ∆η• Flow component depends on η and is present at all ∆η - all particles

are correlated with the reaction planeThis difference and the large PHOBOS acceptance in ηallow us to disentangle the two effects

Au-Au 200GeV, 40-45%22v

η2

η 1

For each η1 and η2 measure the two-particle correlations in ∆φ:

)2cos(),(v2),,(R 212221n φ∆ηη∝φ∆ηη

),()(v)(v),(v 2122122122 ηηδ+η⋅η=ηη

flow component non-flow termflow ⊕ non-flow


Au-Au 200GeV, 40-45%δ

η2

η 1

Determination of the Non-Flow Term δ• Assume that the non-flow component is small at large ∆η

• Fit at large ∆η (∆η > 2) to find the flow component of

For large ∆η:

• Subtract the flow component from to get δ(η1,η2)

• Average over dN/dη1, dN/dη2<δ>

),(v)(v)(v 21222212 ηη≈η⋅η

• Right-side was corrected for a small non-flow effect at ∆η>2, estimated from HIJING

),(v 2122 ηη

),(v 2122 ηη

non-flow

)(v)(v),(v),( 2212212221 η⋅η−ηη=ηηδ



Determination of the Non-Flow Term δ1. Determine v2(η1,η2)2 from the

correlations in (η1,η2,∆φ)

2. Fit v2(η1,η2)2 at ∆η>2 to get theflow component v2(η1)v2(η2)

3. Subtract flow componentfrom v2(η1,η2)2 to get δ(η1,η2)

4. Average over dN/dη1, dN/dη2

flow ⊕ non-flow non-flow

<δ>flow

Au-Au 200GeV, 40-45%22v

η2 η 1

Au-Au 200GeV, 40-45%δ

η2 η 1

η2 η 1

v2(η1)v2(η2)PHOBOS preliminaryAu-Au 200GeV, 40-45%

Au+Au 200GeV

Elliptic Flow Fluctuations

σ(v2) – measured elliptic flow fluctuations(flow ⊕ non-flow)

<δ> – non-flow term determined fromthe data

Contribution from non-flow to σ(v2)σδ(v2) = √<δ>/2 (arXiv:07080800)

Relative flow fluctuations corrected for non flow contribution

and comparedto the eccentricity fluctuations

Within systematic errors the magnitudeof v2 fluctuations

is in agreement with εpart fluctuations


Au-Au 200GeV

Au+Au 200GeV

Talk by Burak Alver

pT - Triggered Two-Particle Correlations

Au+Au 200 GeV, 0 - 30%PHOBOS preliminary

η∆φ∆ ddNd

N1 ch

2

trig

33cutnop

5.10c/GeV5.2passocassoc

T

trigtrigT

<η<−⋅−

<η<>

Motivation: medium response to high-pT partonstests the presence of long-range ∆η correlationsat near- and away-sides

For pions pT > 35 MeV/c at η~0pT > 4 MeV/c at η~4-5


pT - Triggered Two-Particle Correlations

Talk by Ed Wenger

Short-range|∆η| <1

Long-range–4< ∆η <–2

Talk by Ed Wenger

Central collisions Peripheral collisions


-6

6

PHOBOS p+p@200GeV

6

Cu+Cu@200GeV

-6

0-10%

Two-Particle CorrelationsHow are hadrons produced at freeze-out?

Phys. Rev. C75(2007)054913 J. Phys. G34(2007)s1005

New

⎥⎦

⎤⎢⎣

⎡−

φ∆η∆φ∆η∆

−=φ∆η∆ 1),(B),(F)1n(),(R

n

n

Multiplicity independent two-particle correlation function:

6

-6

Au+Au@200GeV0-10%


Two-Particle Correlations

Au+Au 200 GeV 0-10%

Keff : effective cluster size√2 δ: cluster decay width

After averaging over ∆φ, R(∆η) showsshort-range correlations, which can be

explained in terms of clusters:

p+p

NpartK

effEffective cluster size in Cu+Cu and Au+Au

• Cluster size decreases with centrality• All cluster sizes are larger than extractedfrom models with resonances (~1.7)

• For the same Npart: Au+Au clusters > Cu+Cu clusters


Two-Particle CorrelationsComparison of Au+Au and Cu+ Cu

for the same fraction of the inelastic cross section σ/σ0

Near- and away-side clusters


Size of the clusters in Au+Auis similar to that in Cu+Cu

Away-side clusters are smaller and depend more strongly oncentrality than near-side ones.

Kef

f

1-σ/σ0

near-side

away-side

Kef

f

1-σ/σ0

Talk by Wei Li

Correlation & Fluctuation Studies: Conclusions

Fluctuations of the initial sourcegeometry are imprinted inthe final distributions of particles

system thermalizes very rapidly

Ridge effect persists up to ∆η = 4in central Au+Au collisions

Effective cluster size shows intriguing system size dependence


-4< ∆η <-2

Kef

f

1-σ/σ0

PHOBOS at Quark Matter 2008• TALKS:

Ed Wenger – High-pT triggered correlations in Au+Au collisionsTuesday, parallel session VIII, 16:10

Burak Alver – Measurement of non-flow correlations and elliptic flow fluctuationsin Au+Au collisions at RHICFriday, parallel session XII, 14:00

Tomasz Gburek – Energy and centrality dependence of particle productionat very low pT

Saturday, parallel session XVII, 14:40Wei Li – System size dependence of two-particle correlations in p+p, d+Au,

Cu+Cu and Au+AuSaturday, parallel session XIX, 15:20

• POSTERS (Wednesday) :Gabor Veres – System size, energy, centrality and pseudorapidity dependence of

charged-particle density in Au+Au and Cu+Cu collisions at RHICVasu Chetluru – Antiparticle to particle ratios using the PHOBOS detectorRichard Hollis – The importance of correlations and fluctuations on the initial

source eccentricity in high energy nucleus-nucleus collisions.



Backups

Initial Source EccentricityMonte Carlo Glauber (MCG) approach

arXiv:0711.3724 to be published in Phys. Rev. C

Study of the robustness of <εpart>:

Binary collisions vs. participants Local matter distributions

Number of participants0 50 100 150 200 250 300 350

⟩ pa

rt∈ ⟨

0

0.2

0.4

0.6

0.8

1Binaries (x=1)

Mixture (x=0.13)

Participants (x=0)


1

1.2

1.4

Ratio to Participants


⟩ pa

rt∈ ⟨

0

0.2

0.4

0.6

0.8

1= 0.6fm)

hsHard-sphere (R

= 0.3fm)gσGaussian (

Participants (point-like)


0.7

0.8

0.9

1Ratio to Participants

Au+AuCu+Cu

Au+AuCu+Cu

Participant eccentricity is robust to the Glauber model assumptionsBarbara Wosiek for PHOBOS Quark Matter'08 - Jaipur, India, Feb.4 26

Fluctuations of the Initial Source EccentricityFull MCG approach: includes spatial correlations among participantsarXiv:0711.3724 to be published in Phys. Rev. C

><= 222 v}EP{v

R0 0.5 1

α

1

1.5

2

= 0bkg

100, f≥ 2N

1≥ bkg

100, f≥ 2N

0 ≥ bkg

= 50, f2N

0≥ bkg

25, f≤ 2N

}EP{vv 2/1

2 =>><< αα

Event Plane Resolution, R

PHOBOS R: 0.13 – 0.55

higher order cumulants


Ecc

entr

icity

0

0.2

0.4

0.6

0.8

1

⟩ part

∈ ⟨{2}part∈{4}part∈

⟩ RP

∈ ⟨s∈

Full MCG

Au+Au

Number of participants0 20 40 60 80 100

Ecc

entr

icity

0

0.2

0.4

0.6

0.8

1

⟩ part

∈ ⟨{2}part∈{4}part∈

⟩ RP

∈ ⟨s∈

Full MCG

Cu+Cu

Poster by Richard HollisBarbara Wosiek for PHOBOS Quark Matter'08 - Jaipur, India, Feb.4 27

δ as a function of centrality

• Average δ(η1,η2) over all hit pairs

• Non-flow in data islarger than in HIJING• These values are valid for PHOBOS geometry

Au+Au200GeV


Model Comparison

Au+Au200GeV

• Results are in agreement with both Glauber and CGC calculations within errors

CGC: arXiv:0707.0249


Comparison to Total Fluctuations


Comparison to STAR

Error bars go down to 0 for red points


PYTHIA p+p Reference


• PHOBOS is limited by statistics in p+p • Compare our Au+Au results to PYTHIA,

which reasonably reproduces STAR p+p data

Ridge Extent in ∆η

0-10% central |∆φ|<1rad


Comparison to Predictions

0-10% central |∆φ|<1rad

C.Y. Wong, private communication

C.Y. Wong PRC76, 054908 (2007)


• Cluster size decreases with Npart in A+A, but not monotonically.• Enhancement of cluster from p+p to peripheral A+A.• No strong dependence on Npart for cluster decay width.

p+p p+p

Clusters in Cu+Cu and Au+Au


Near- and Away-Side Clusters


K. Eggert et al.,Nucl. Phys. B 86:201, 1975

Cluster Model

R(∆η) = α Γ(∆η)B(∆η)

−1⎡

⎣ ⎢

⎤

⎦ ⎥

Γ(∆η) ∝ exp −(∆η)2

4δ 2

⎛

⎝ ⎜

⎞

⎠ ⎟

Keff = α +1=< k(k −1) >

< k >+1=< k > +

σ k2

< k >

B(∆η)

Decay width: δ

: background distribution

Two-particle rapidity correlation function:

Keff : effective cluster size

2

correlations between particlesfrom one cluster

k: cluster size


Latest Results from the PHOBOS Experiment

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