Recent Results from PHOBOS experiment at RHIC

VI Simposio Latinoamericano de Física Nuclear

Iguazú, Argentina Octubre 2005

Edmundo García, UIC

Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard

Bindel,

Wit Busza (Spokesperson), Zhengwei Chai, Vasundhara Chetluru, Edmundo García, Tomasz

Gburek, Kristjan Gulbrandsen, Clive Halliwell, Joshua Hamblen, Ian Harnarine, Conor Henderson,

David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane,Piotr

Kulinich, Chia Ming Kuo,

Wei Li, Willis Lin, Constantin Loizides, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen,

Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed, Eric Richardson, Christof Roland,

Gunther Roland, Joe Sagerer, Iouri Sedykh, Chadd Smith, Maciej Stankiewicz, Peter Steinberg,

George Stephans, Andrei Sukhanov, Artur Szostak, Marguerite Belt Tonjes, Adam Trzupek,

Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Donald

Willhelm,

Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Shaun Wyngaardt, 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

PHOBOS Collaboration

Outline

• Matter phases in Heavy Ion Reactions

• Experimental characterization of RHIC Collisions

• Global observables Scaling behaviors Limiting fragmentation

Description of a heavy-ion collision

M. Gyulassy, L. McLerran Nucl. Phys. A 750 (2005)30

McLerran, hep-ph/0311028

ZEUS data for gluon distributionIn partons

Gluon density grows

low x -> s<<1At a given Q(x,A) -> gluon density saturation -> saturation models for heavy ions

Color Glass Condensate (CGC)

Saturation Models Au+Au

Saturation Model (KLN) Phys.Lett.B523 79 (2001)

dN/d c

h /

(<N

part>/

2)

<Npart>

PHOBOS 130 GeV

PHOBOS

centrality

Saturation Models d + Au

PHOBOS

CGC not valid in the Au fragmentation region

Assume dN/d=NpartAu dNpp/d in

the Au fragmentation region

Saturation Model (KLN) hep-ph/0212316

PHOBOS

Quark Gluon Matter/Plasma

particles

QGM

energy/density

Thermalization

Estimation of the energy density reached at RHIC

Nucleon transparency at RHIC larger than for AGS and SPS. However the energy density achieved ~ 5 GeV/fm at mid rapidity: high energy density system formed during collisions

Bearden et al., BRAHMS Collaboration, Nucl. Phys. A757 (2005) 1-27

Baryon free medium created at RHIC

Evidence of thermalization

Evidence of successful description of data by hydrodynamics,high energy density system is to certain degree thermalized. From PHOBOS particle rations T~120 MeV, B~ 27.2 MeV

STAR, Nucl. Phys. A 757 (2005) 102 BRAHMS, Nucl. Phys. A757 (2005) 1-27

Another characteristic of the high energy density matter formed at RHIC: Flow

Reaction plane

(R)

x

z

y

x

x (defines R)

y

zy

Initial spatial anisotropy

px

py

Final momentum anisotropy dN/d(R ) = N0 (1 + 2v1cos (R) +

2v2cos (2(R)) + ... )

Elliptic Flow: v2

Strongly interactive system I

centrality

Evidence of large elliptic component of flow found,the high density system interacts more like a fluid than like a gas.

PHOBOS

PHOBOS

Suppression of high-pT particles: Medium is highly interactive and parton density is high.

AuAu 200 GeV

PHOBOS dAu 200 GeV

PRL 91, 072302 (2003)

From Glauber(HIJING 1.383)

41mb (same as for Glauber)

From UA1, usingPythia to go from|| < 2.5 to 0.2 < < 1.4

PHOBOS data

Strongly interacting system II

Strongly interacting system II cont.

Suppression of high pt particles

New on dijet supression: Emergence of dijets with increasing pT(trig)

Au+Au, 0-5%

correlations (not background subtracted)

• Hint of narrow back-to-back peak for higher pT(trig)

• Jets do exist but they are highly suppressed

pT(trig)

pT(assoc) > 2 GeV/c

preliminary

RHIC

• High energy density medium

• Baryon Free medium

• Evidence of Thermalization

• Medium created at RHIC strongly interacts with high pt partons.

What other global observables define RHIC collisions so far as measured by PHOBOS?

1. Scaling behaviors

2. Limiting fragmentation

“Participant” Scaling

Ncoll= # of NN collisions: ~N4/3

L~N1/3

Npart/2 = # of participating nucleons: N

“Collision” Scaling

Npart/2 = # of participating

pairs of nucleons: = 1

Ncoll = # of NN collisions: = 1

collpartch Nf

Nf

d

dN 21 2

Wang,Gulassy Phys. Rev. Lett 86(2001) Kharzeev,Nardi Phys. Lett. B 501(2001)

Au+Au centrality dependence allows only about 10% Ncoll scaling at mid rapidity

Au+Au

0 200 4000

10

20dN

/d/

(Np

art/

2)

<Npart>

200 GeV - ||<1

peripheral central

p+p

Binary Collision(Ncoll) Scaling

Participant(Npart) Scaling

PHOBOS

All RHIC energies show a similar Npart dependence

19.6 GeV preliminary

130 GeV200 GeV

Data is normalized by p+p value for each energy.

Au+Au

p + p

peripheral central

Participant (Npart) scaling

Binary collision scaling

PHOBOS

Npart scaling for asymmetric collisions:

arXiv:nucl-ex/0403033

dN/d in Cu+Cu vs. Au+Au Scaling Laws

PHOBOS62.4 GeV 200 GeV

Au+Au35-40%,Npart = 98

Cu+CuPreliminary

3-6%, Npart = 96

Au+Au35-40%, Npart = 99

Cu+CuPreliminary

15-25%, Npart = 61

Au+Au45-50%,Npart = 62

Cu+CuPreliminary

15-25%, Npart = 60

Au+Au45-55%, Npart = 56

Cu+CuPreliminary

3-6%, Npart = 100

Scaling LawsYields vs. Npart, 200 GeV

Au+Au: PRL 94, 082304 (2005), PLB 578, 297 (2004)

Cu+Cupreliminary

Au+Au

PHOBOS

Limiting Fragmentation

UA5, Z.Phys.C33, 1 (1986)

dN

/d

beamy

selected systematic errors

0 – 6 % central

35 – 40 % central

PHOBOS Au + Au

PRL 91, 052303 (2003)

Rest frame of A Rest frame of p or d

Elliptic flow:

PRL 91, 052303 (2003)

PHOBOS

Final Notes

RHIC collisions• Produce a high energy density medium• Baryon Free medium• Evidence of thermalization • Medium created at RHIC strongly interacts with high pt

partons.

Global observables (not clearly understood yet) that define RHIC so far:

1. <Npart> scaling behaviors2. Limiting fragmentation…• LHC

Backup Transparencies

PHOBOS Experiment

Triggering on Collisions & Centrality

• Coincidence between Paddle counters at t = 0 defines a valid collision

• Paddle + ZDC timing reject background

PP

Negative

Paddles

Positive Paddles

Aux

z

PN

Positive ZDC

Negative ZDC

NegativeCerenkov

PositiveCerenkov

Au

Central Peripheral

HIJING +GEANT Glauber calculation Model of paddle trigger

Data Data+MC

Initially released Energy per Unit Volume 5 GeV/fm3

Note: Energy Density inside

Proton ≈ 0.5 GeV/fm3

11

45

1000~all

d

dN

Therefore total energy released in

|| < 1 is ~2000GeV

Nu

mb

er o

f P

arti

c les

Pro

du

ced

at

y=0

Energy of Collision

“Relevant” Initial Volume ~ R2 ( 1 fm) 2

<E> ~ 0.7 GeV

Data from: PRL 85, 3100 (2000); PRL 88, 22302 (2002); PRL 91, 052303 (2003); arXiv:nucl-ex/0405027

dN

ch/d

Energy per Unit Volume Detail

12

1)/exp(1

21 BET

TT

TmAdydm

Ndm

mT = pT2+mh

2

No enhancement in low-pT yields for pions is observed flattening of (p+pbar) spectra down to very low pT, consistent with strong radial flow of the systm:

T= 229 MeV for (++-) 293 MeV for (K++ K

-)

392 MeV for (p + pbar)

Strongly interacting system III

Au+Au at sNN = 200 GeV

Baryon puzzle at RHIC

Elliptic Flow in Cu+Cu vs Au+AuScaling Laws

preliminary

PHOBOS 200 GeV h±

PHOBOS 200 GeV h±

Statistical errors only

Cu+Cupreliminary

PHOBOS 200 GeV Statistical errors only

Au+Au

0-40% centrality

Total charged multiplicity scaling with Npart

Shaded band is uncertainty on extrapolation procedureErrors include contributions from Nch and Npart scaling

Open symbols are UA5 data at 200 GeV and results from an interpolation at lower energies

PHOBOS: nucl-ex/0301017

Eccentricity CalculationScaling Laws

Standard Eccentricity

x

y Nucleus 2Nucleus 1

ParticipantRegion

b

Au+AuCu+Cu

Au+Au

Nucleus 1

Nucleus 2

Participant Region

x

y

Participant Eccentricity

b

Au+AuCu+Cu

Au+Au

Npart scalling of Flow

Standard Eccentricity

Cu+Cupreliminary

Au+Au

PHOBOS 200 GeV

“Participant Eccentricity” allows v2 scaling from Cu+Cu to Au+Au

Participant Eccentricity PHOBOS 200 GeV

Au+AuCu+Cupreliminary

Standard Eccentricity

Cu+Cu

Au+Au

Participant Eccentricity

Cu+Cu

Au+Au

<dN/dy> / <S> scaling:STAR, PRC 66 034904 (2002)Voloshin, Poskanzer, PLB 474 27 (2000)Heiselberg, Levy, PRC 59 2716, (1999)

<dN/dy> / <S> scaling

Emergence of dijets w/ increasing pT(assoc)

correlations (not background subtracted)

• Narrow peak emerges cleanly above vanishing background

8 < pT(trig) < 15 GeV/c

pT(assoc) > 2 GeV/cpT(assoc) > 3 GeV/cpT(assoc) > 4 GeV/cpT(assoc) > 5 GeV/cpT(assoc) > 6 GeV/cpT(assoc) > 7 GeV/cpT(assoc) > 8 GeV/c