RHIC Physics Through the Eyes of PHOBOS

Wit Busza

MIT

Moriond, March 2003

Relativistic Heavy Ion Collider

time

From Frank Wilczek

Why Collide Heavy Ions?

proton anti-proton

UA1, 900 GeV

Gold Gold

sNN = 130 GeV

Goal of Relativistic Heavy Ion Physics is to Obtain a Better Understanding of the Solutions of the QCD Lagrangian:

• QCD Phase Diagram• Properties of QGP• Mechanism of Particle Production • Structure and Interactions of Relativistic

Hadrons & Nuclei

Spectator Nucleons

Participating Nucleons

Npart= 7

Ncoll.= 10

Nquarks +gluons = ?

Ninelastic= 1

In calculating Npart or Ncoll

taken to be nucleon-nucleon inelastic cross-section. A priori no reason for this choice other than that it seems to give a useful parameterization. inel ~ (R1+R2)2 ~ (A1

1/3 + A21/3)2 ~ A2/3

Npart ~ A2/3(A11/3+ A2

1/3) ~ A

Ncoll ~ A2/3(A11/3 * A2

1/3) ~ A4/3

What Are the Correct Variables When Looking at AA Collisions?

Will the following be equivalent to the above?

pA multiplicities were found to be proportional to Npart

Busza et al., PRL 41(1978).285

In no rest frame is this picture correct

In rest frame of one nucleus:

Soft components overlap, “gluon saturation effects”, shadowing etc.

The use and relevance of Npart is far from obvious when the collision is viewed from different frames of reference

In rest frame of the center of mass of the system:

19.6 GeV 130 GeV 200 GeVPHOBOS PHOBOS PHOBOS

Central Collisions

Peripheral Collisions

dNd

1. Is there an interesting state created in high energy hadronic (in particular AA) collisions?

Initially released energy density >5GeV/fm3

Note: energy density inside proton ≈ 0. 5GeV/fm3

1=1−=

o45

1000~all

d

dN⎟⎟⎠

⎞⎜⎜⎝

⎛

GeVE 1~

32 200~)1(~ fmfmRπ

Total energy released ~2000GeVMax. initial overlap volume

Evidence that shortly after the collision a high energy density* is created

Nu

mb e

r o f

Pa r

t icl

es P

rod

u ce d

at

y=0

Energy of Collision

* Strictly speaking it is the energy released in the transverse direction per unit volume

K–/K+

p/p

Rati

o o

f an

tim

att

er

to m

att

er

A+A central collisions

Energy of collision

Evidence that at y≈0 this high energy density state has the quantum numbers of the vacuum

• Jets seen in peripheral Au+Au and p+p

• Azimuthal correlations– Small angle ( ~ 0)– Back-to-Back ( ~ p)

Peripheral Au+Au data

Evidence for interactions with the created state

Central Au+Au data

• Disappearance of back-to-back correlations in central Au+Au

• Away-side particles absorbed or scattered in medium

D. Hardtke

QM ‘02

Azimuthal Angular Distributions

“head on” view of colliding nuclei

Also, PHOBOS sees very few low Pt particles

All this is direct evidence of collective effects

Peripheral Central

Phobos data for 130 and 200 GeV

Evidence that the created state has a high pressure

Preliminary v2200

Final v2130

PHOBOS Au+Au

v 2

200 GeV

130 GeV

<Npart>~190 130 GeV result: nucl-ex/0205021, submitted to PRL

PHOBOS preliminary

h+ + h-

200 GeV Au+Au

0<<1.5

(top 55%)

v 2

17% scale error

Elliptic Flow

Evidence that most of the action ends very quickly after the collision

Evidence that the system may reach some kind of equilibriuim

NA49, Phys Lett B459 (1999) 679NA49, PRL 86 (2001) 1965

From Gunther Roland/MIT

Event by Event Fluctuations

Further evidence that it may be reaching statistical equilibrium

STAR Preliminary

Gene Van Buren.QM’02

Particle ratios compared to statistical model

2. There are remarkable similarities between e+e-, pp & AA collisions

Is this evidence that dynamics are dominated by the initial state interactions?

19.6 GeV 130 GeV 200 GeVPHOBOS PHOBOS PHOBOS

Central Collisions

Peripheral Collisions

dNd

Collision viewed in rest frame of CM:

Limiting fragmentation

Collision viewed in rest frame of one nucleus:

)GeV(s24 31 5345 63

2-4

5-9

10-

14

15-

19

20-

24

Tot

al o

bser

ved

mul

tiplic

ity

d

d n

n

1

W. Thome et al.,Nucl. Phys. B129(1977) 365.

ISR data Proton+Antiproton

UA5

900 GeV

546 GeV

200 GeV

53 GeV

0-2-4-6-ybeam

0

1

2

3

4

Limiting fragmentation:

Au+Au

(preliminary)

)/exp( sBsch CAN αα=

Nu

mb e

r o f

Pa r

t icl

es P

rod

u ce d

Energy of Collision

e+e-

Au+Au

Eskola, QM ’01

dN/d||

e+e-Amazing similarity of AA and e+e-

From P. Steinberg

200GeV 130GeV

19.6GeV (PRELIMINARY)

Au+Au yields normalized to corresponding pp value for all three

energies

pp Errors from Au+Au only

PHOBOS Au+Au

19.6 GeVpreliminary

130 GeV

200 GeV

e.g. impact parameter dependence of the number of particles produced at the center of mass of the collision

PRC 65 (2002) 061901R

pp

Slow quark

Fast quark

3. Some results inconsistent with naïve expectations:

Data inconsistent with the following picture:

“X-Ray” of Medium Using Jets

Leading Particle

Hadrons

q

q

Hadrons

Leading Particle

Hadrons

q

q

Hadrons

Leading Particle

Leading Particle

4. Direct study of the properties of the produced state

Charged Hadron Spectra

Preliminary sNN = 200 GeV

Preliminary sNN = 200 GeV

Rel

ativ

e Y

ield

per

par

tici

pant

Submitted to Phys.Lett.

Fast quark

Particle Production at high Pt

Cronin effect data

PHOBOS

pA

AuAu 200GeV

AuAuNcollscaling

Ncollscaling

“Quenching” of leading partons in pA collisions?

W. Busza Nucl.Phys. A544 (1992) 49c

Eichten et al.

Baron et al.

Skupic et al.

Summary• pp, pA, AA collisions are magnificent laboratories

for the study of QCD

• No doubt a very high energy density creates a fascinating medium. If it equilibrates, it does so quickly. If it is the QGP, the transition is almost certainly a cross-over

• Main difficulty in interpretation of data is the separation of the initial and final state interactions

• Data continues to surprise us– Smoothness of data with energy – Jet quenching– Similarity of AA with e+ e-

– Why approx. Nparticipant scaling, even at high Pt?

Collaboration (Jan 2003)

Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Abigail Bickley,

Richard Bindel, Andrzej Budzanowski, Wit Busza (Spokesperson), Alan Carroll, Patrick Decowski,

Edmundo Garcia, Nigel George, Kristjan Gulbrandsen, Stephen Gushue, Clive Halliwell,

Joshua Hamblen, George Heintzelman, Conor Henderson, David Hofman, Richard Hollis,

Roman Holynski, Burt Holzman, Aneta Iordanova, Erik Johnson, Jay Kane, Judith Katzy, Nazim

Khan, Wojtek Kucewicz, Piotr Kulinich, Chia Ming Kuo, Jang Woo Lee, Willis Lin, Steven Manly,

Don McLeod, Jerzy Michalowski, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer,

Andrzej Olszewski, Robert Pak, Inkyu Park, Heinz Pernegger, Corey Reed, Louis Remsberg,

Michael Reuter,

Christof Roland, Gunther Roland, Leslie Rosenberg, Joe Sagerer, Pradeep Sarin, Pawel Sawicki,

Wojtek Skulski, Stephen Steadman, Peter Steinberg, George Stephans, Marek Stodulski,

Andrei Sukhanov, Jaw-Luen Tang, Ray Teng, Marguerite Belt Tonjes, Adam Trzupek, Carla Vale,

Gábor Veres, Robin Verdier, Bernard Wadsworth, Frank Wolfs, Barbara Wosiek, Krzysztof

Wozniak, Alan Wuosmaa, Bolek Wyslouch

ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORYINSTITUTE OF NUCLEAR PHYSICS, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGYNATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGO

UNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER