RHIC Physics Through the Eyes of PHOBOS

1
RHIC Physics Through the Eyes of PHOBOS
Moriond, March 2003

• Wit Busza
• MIT

2
Relativistic Heavy Ion Collider
3
Why Collide Heavy Ions?
From Frank Wilczek
time
4
UA1, 900 GeV
anti-proton
proton
5
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

6
What Are the Correct Variables When Looking at AA
Collisions?
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.
Will the following be equivalent to the above?
? inel (R1R2)2 (A11/3 A21/3)2 A2/3 Npart
A2/3(A11/3 A21/3) A Ncoll A2/3(A11/3
A21/3) A4/3
7
pA multiplicities were found to be proportional
to Npart
Busza et al., PRL 41(1978).285
8
In no rest frame is this picture correct
In rest frame of one nucleus
Soft components overlap, gluon saturation
effects, shadowing etc.
In rest frame of the center of mass of the system
The use and relevance of Npart is far from
obvious when the collision is viewed from
different frames of reference
9
Central Collisions
200 GeV
19.6 GeV
130 GeV
PHOBOS
PHOBOS
PHOBOS
dN d?
?
Peripheral Collisions
1. Is there an interesting state created in high
energy hadronic (in particular AA) collisions?
10
Evidence that shortly after the collision a high
energy density is created
Number of Particles Produced at y0
Total energy released 2000GeV
Max. initial overlap volume
Initially released energy density gt5GeV/fm3
Note energy density inside proton 0. 5GeV/fm3
Energy of Collision
Strictly speaking it is the energy released in
the transverse direction per unit volume
11
Evidence that at y0 this high energy density
state has the quantum numbers of the vacuum
AA central collisions
K/K
Ratio of antimatter to matter
p/p
Energy of collision
12
Evidence for interactions with the created state
Peripheral AuAu data
Central AuAu data
D. Hardtke QM 02

• Disappearance of back-to-back correlations in
  central AuAu
• Away-side particles absorbed or scattered in
  medium

• Jets seen in peripheral AuAu and pp
• Azimuthal correlations
• Small angle (Df 0)
• Back-to-Back (Df p)

13
Evidence that the created state has a high
pressure
Phobos data for 130 and 200 GeV
Azimuthal Angular Distributions
Peripheral
Central
head on view of colliding nuclei
Also, PHOBOS sees very few low Pt particles All
this is direct evidence of collective effects
14
Evidence that most of the action ends very
quickly after the collision
Elliptic Flow
v2
PHOBOS AuAu
200 GeV
130 GeV
ltNpartgt190
130 GeV result nucl-ex/0205021, submitted to
PRL
h
15
Evidence that the system may reach some kind of
equilibriuim
Event by Event Fluctuations
NA49, PRL 86 (2001) 1965
NA49, Phys Lett B459 (1999) 679
From Gunther Roland/MIT
16
Further evidence that it may be reaching
statistical equilibrium
Gene Van Buren. QM02
Particle ratios compared to statistical model
STAR Preliminary
17
2. There are remarkable similarities between
ee-, pp AA collisions Is this evidence that
dynamics are dominated by the initial state
interactions?
18
Collision viewed in rest frame of CM
Central Collisions
200 GeV
19.6 GeV
130 GeV
PHOBOS
PHOBOS
PHOBOS
dN d?
?
Peripheral Collisions
Collision viewed in rest frame of one nucleus
Limiting fragmentation
19
ISR data
ProtonAntiproton
Limiting fragmentation
24
31
53
45
63
4
2-4
900 GeV
3
546 GeV
5-9
200 GeV
Total observed multiplicity
2
10-14
53 GeV
1
15-19
UA5
20-24
0
0
-2
-4
-6
h-ybeam
W. Thome et al., Nucl. Phys. B129 (1977) 365.
20
Amazing similarity of AA and ee-
ee-
AuAu
(preliminary)
ee-
Number of Particles Produced
Eskola, QM 01
Energy of Collision
From P. Steinberg
AuAu
dN/dhhlt1
21
3. Some results inconsistent with naïve
expectations
e.g. impact parameter dependence of the number of
particles produced at the center of mass of the
collision
Data inconsistent with the following picture
PHOBOS AuAu
200 GeV
PRC 65 (2002) 061901R
Slow quark
Fast quark
130 GeV
19.6 GeV preliminary
AuAu yields normalized to corresponding pp value
for all three energies
pp
200GeV
130GeV
19.6GeV (PRELIMINARY)
pp
Errors from AuAu only
22
4. Direct study of the properties of the produced
state
X-Ray of Medium Using Jets
Leading Particle
Leading Particle
23
Charged Hadron Spectra
24
Submitted to Phys.Lett.
AuAu 200GeV
Particle Production at high Pt
Fast quark
Ncollscaling
pA
AuAu
Ncollscaling
Relative Yield per participant
PHOBOS
Cronin effect data
25
Quenching of leading partons in pA collisions?
Skupic et al.
Eichten et al. Baron et al.
W. Busza Nucl.Phys. A544 (1992) 49c
26
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?

27
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
LABORATORY INSTITUTE OF NUCLEAR PHYSICS,
KRAKOW MASSACHUSETTS INSTITUTE OF
TECHNOLOGY NATIONAL CENTRAL UNIVERSITY,
TAIWAN UNIVERSITY OF ILLINOIS AT
CHICAGO UNIVERSITY OF MARYLAND UNIVERSITY OF
ROCHESTER