Charged particle multiplicity studies with PHOBOS

Birger Back

Argonne National Laboratory

for the

PHOBOS Collaboration

2

PHOBOS 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

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PHOBOS experiment: June 2000 – June 2005

-5.4 < < 5.40.5o < < 179.5o

Main emphasis:

4 multiplicity and

low pT particles

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PHOBOS multiplicity measurements

System s1/2 (GeV)

Au+Au 200, 130, 62.4, 19.6

Cu+Cu 200, 62.4, 22.4

d+Au 200

p+p 200, 410

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dN/d basics

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What would an isotropic source look like?

Isotropic emission: 1/cosh2

Only 22% emittedwith pT > pL

Theseparticles carry information aboutthe densest regionformed in the collisions

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What happens to the original protons?

AGS

SPS

RHIC 62

RHIC 200

LHC 5500

(BRAHMS preliminary)

dN

/dy

I.Bearden (BRAHMS), QM2006

Net (original) protons move away from mid-rapidity region with increasing collision energy

Mid-rapidity region begins to look like a pure energy-density region reminiscent of the early universe

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dN/d @ =0

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Pre-RHIC theoretical predictions

PHOBOS, Nucl. Phys. A747, 28 (2005)

First RHIC

results

New dataPRC 74, 021901

(2006)

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Factorization of centrality and energy dependence

)N(g)s(fd

dNpart0

ch ||<1

Data: PHOBOS, PRL 97, 012301 (2006); PRC70, 021902(R) (2004); PRC65, 061901(R) (2002)

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Energy density

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Energy density estimates?

Absolute maximum:Total available energy:

Volume at instant of overlap:

Instantaneous energy density:Not equilibrated matter

Birger’s estimate:Isotropic energy:

Volume after 0 =1 fm/c:

Equilibrated energy density:

35,000GeV200/2350/2sNE NNparttot

3333 fm 14fm /100734

r34

V

/

33max0 GeV/fm 2,500GeV/fm 1435,000 /

GeV 1,2150.6/0.631275/fm NE chtisoch

isotot

30

2 fm 300249c2rV

33iso0 GeV/fm 4GeV/fm 3001,215 /

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Bjorken estimate of energy density

Phenix: ET measurement at 130 GeV

0 = 4.6 [GeV/fm3] PRL 87, 052301 (2001)

NA49: ET measurement at 17 GeV

0 = 3 [GeV/fm3] PRL 75, 3814 (1995)

Brahms

Conclusion: All reasonable estimates are substantially larger than thepredicted transition density of0 = 0.7-1.0 GeV/fm3

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CompletedN/d

distributions

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PHOBOS Au+Au Data

* PHOBOS PRL 91,52303 (2003)

PHOBOS, PRL 91, 052303 (2003); PRC 74, 021901 (2006)  

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Cu+Cu data

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Centrality (%)

Npart

(total)

Npart

(Au)

Npart

(d)

0-20 15.5 13.5 2.0

20-40 10.8 8.9 1.9

40-60 7.2 5.4 1.7

60-80 4.2 2.9 1.4

80-100 2.7 1.6 1.1

d+Au centrality dependence

PHOBOS, Phys. Rev. C72, 031901(R) (2005)

d Au

Central: asymmetric

Peripheral: symmetric

Momentum conservation?

Peripheral

Central

>

~

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System size:Au+Au

vs.Cu+Cu

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Unscaled dN/d very similar for Au+Au and

Cu+Cu at same Npart

Scaling Laws

Cu+CuPreliminary

3-6%, Npart = 100

PHOBOS PHOBOS

62.4 GeV 200 GeV

Au+Au35-40%,Npart = 98

Cu+CuPreliminary

3-6%, Npart = 96Au+Au35-40%, Npart = 99

See poster by Richard Hollis

dN/d in Cu+Cu vs Au+Au for Npart ~ 100

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Unscaled dN/d very similar for Au+Au and Cu+Cu

at same Npart

Scaling Laws

Cu+CuPreliminary

15-25%, Npart = 61

PHOBOS PHOBOS

62.4 GeV 200 GeV

Au+Au45-50%,Npart = 62

Cu+CuPreliminary

15-25%, Npart = 60

Au+Au45-55%, Npart = 56

Also true for mid-central Cu+Cu vs peripheral Au+Au

dN/d in Cu+Cu vs Au+Au for Npart ~ 60

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Extended longitudinal scaling

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Extended longitudinal scaling – Au+Au

PHOBOS Phys. Rev. Lett. 91, 052303 (2003) / Nucl. Phys. A757, 28 (2005)

beamyydy

dN independent of energy

Works also for dN/d because:

)m/pln(y tt

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Scaling Laws

19.6 GeV 62.4 GeV 130 GeV 200 GeV

PHOBOS preliminary

preliminary preliminary preliminary preliminary

“Extended Longitudinal Scaling” of all longitudinal distributions

- ybeam

preliminary

PHOBOSAu+Au0-6%

Au+Au0-40%

Au+Au0-40%

200GeV130GeV62.4 GeV (prel)19.6 GeV

Extended longitudinal scaling – Au+Au

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Scaling Laws

Same for Cu+Cu

preliminarypreliminary

preliminary preliminaryPHOBOS

62.4 GeV 200 GeV

‘Extended Longitudinal Scaling’ also seen in Cu+CuPersists from p+p to Au+Au over large range in ’

preliminary

preliminary

PHOBOS

- ybeam

Cu+Cu0-6%

200GeV

62.4GeV

Cu+Cu0-40%

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Extended longitudinal scaling in p-A and d-A

PHOBOS, Phys. Rev. C72, 031901(R) (2005)

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Total charged particle multiplicity

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Total charged particle multiplicities in Au+Au

Nch Q Npart

Width x height = const

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Total multiplicity as a function of energyNch= dN/dx(2ybeam+0.3-dN/d/195)

height x width

width

hei

ght

PHOBOSPRL 91,52303 (2003)

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Energy dependence of Nch

0.31s0.77lndη

dNN

2

where

dηdN

1951

0.32ydη

dNN

2N2N

ch

part

chbeam

ch

partpart

ch

,

0-6% Central AuAu collisions

Npart=344

PHOBOS, PRL 91,52303 (2003)

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Universality – comparing AA to pp and e+e-

PHOBOS, PRC 74, 021902(R) (2006)

Shapes: Au+Au and e+e- “similar”Total Nch: Au+Au same as e+e-

p+p: leading hadron removes 50% of energy

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Evolution of Nch/Npp ratio vs Npart

Nch/(Npart/2) constant with centrality

d+Au also lower than Au+Au

d+Au data similar to low-energy p+A

NchdAu=0.5 Npart Nch

pp

(1 Deuteron = 2 protons)

d+Au: Centrality dependence of total Nch

PHOBOS, Phys. Rev. C72, 031901(R) (2005)

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Other new PHOBOS results (QM2006)

New data on antiparticle/particle ratios Identified particle spectra for 62.4 GeV Au+Au Event-by-event v2 measurement and flow fluctuations

Two-particle correlations and cluster size

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New Data: antiparticle/particle ratios

PHOBOS, QM2006

200, 62.4 GeV Cu+Cu 200 GeV Cu+Cu and Au+Au

Energy dependence for Cu+Cu System dependence at 200 GeV

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Identified particle spectra for 62.4 GeV Au+Au

First published identified spectra for 62.4 GeV Au+Au at RHIC(down to very low pT, a unique PHOBOS measurement)

blast-wave fits

PHOBOS, nucl-ex/0610001Accepted for PRC

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New Analysis: event-by-event v2 measurement

Measure v2 on an event-by-event basis

Average and compare to our standard analysis

Agreement with both hit and track based PHOBOS results

200 GeV Au+Au

PHOBOS, QM2006; arXiv:nucl-ex/0608025Submit to PRL this week

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Event-by-event flow: fluctuations

PHOBOS, QM2006Submit to PRL this week

v2 fluctuations

mirror

part fluctuations

(v2)/<v2> and (part)/<part> in 200 GeV Au+Au Collisions

PHOBOS part prediction

PHOBOS v2 result

90% CL

MC with nofluctuations

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Effective cluster size analysis

On average, particles produced in clusters with a size of 2-3.

Interesting centrality dependence – compare to other systems

p+p

scale error

2

Ke

ff =

eff

ecti

ve c

lust

er s

ize

PHOBOS, QM2006

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Summary and conclusion

Multiplicity

– PHOBOS have performed complete charged particle multiplicity measurements for Au+Au, Cu+Cu, d+Au, and p+p collisions

– Systen size dependence

– ‘Complete’ pseudorapidity distributions Midrapidity multiplicity

– Factorization of centrality and energy dependencies Limiting fragmentation – extended longitudinal scaling

– Seen for Au+Au, Cu+Cu, and d+Au

– Also observed in flow observables Total charged particle multiplicity

– Nch/Npart constant with centrality

– ‘Universality’ – compared to elementary e+e- collisions Future: Finish up many analysis and reviews

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Why mid-rapidity?

Emphasize producedor scatteredparticles

Triple Gaussianfit function

200 GeV Au+Au 0-6% central

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System size dependence - Au+Au vs. Cu+Cu

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Centrality dependence 200 GeV

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Cu+Cu elliptical flow – eccentricity scaling

PHOBOS, QM2006