Recent Results from PHOBOS at RHIC

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Recent Results from PHOBOS at RHIC

• Robert Pak
• Brookhaven National Laboratory
• for the PHOBOS Collaboration
• PANIC 2002
• Osaka, Japan
• Sept. 30th, 2002

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Collaboration
ARGONNE NATIONAL LABORATORY Birger Back, Alan
Wuosmaa BROOKHAVEN NATIONAL LABORATORY Mark
Baker, Donald Barton, Alan Carroll, Nigel George,
Stephen Gushue, George Heintzelman, Burt
Holzman, Robert Pak, Louis Remsberg, Peter
Steinberg, Andrei Sukhanov INSTITUTE OF NUCLEAR
PHYSICS, KRAKOW Andrzej Budzanowski, Roman
Holynski, Jerzy Michalowski, Andrzej
Olszewski, Pawel Sawicki, Marek Stodulski,
Adam Trzupek, Barbara Wosiek, Krzysztof
Wozniak MASSACHUSETTS INSTITUTE OF
TECHNOLOGY Maartin Ballintijn, Wit Busza
(Spokesperson), Patrick Decowski, Kristjan
Gulbrandsen, Conor Henderson, Jay Kane, Judith
Katzy, Piotr Kulinich, Jang Woo Lee, Heinz
Pernegger, Corey Reed, Christof Roland,
Gunther Roland, Leslie Rosenberg, Pradeep
Sarin, Stephen Steadman, George Stephans,
Carla Vale, Gerrit van Nieuwenhuizen, Gábor
Veres, Robin Verdier, Bernard Wadsworth, Bolek
Wyslouch NATIONAL CENTRAL UNIVERSITY,
TAIWAN Chia Ming Kuo, Willis Lin, Jaw-Luen
Tang UNIVERSITY OF ILLINOIS AT CHICAGO Russell
Betts, Edmundo Garcia, Clive Halliwell, David
Hofman, Richard Hollis, Aneta Iordanova, Wojtek
Kucewicz, Don McLeod, Rachid Nouicer, Michael
Reuter, Joe Sagerer UNIVERSITY OF
MARYLAND Abigail Bickley, Richard Bindel, Alice
Mignerey, Marguerite Belt Tonjes UNIVERSITY
OF ROCHESTER Joshua Hamblen, Erik Johnson, Nazim
Khan, Steven Manly, Inkyu Park, Wojtek
Skulski, Ray Teng, Frank Wolfs
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PHOBOS Apparatus
PID from Spec Arms
p
K
?
4p Multiplicity Detector
500
f
0 keV
h
5.4
-5.4
Unrolled single event display
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Kinematics many ways to slice pz
Rapidity Generalized velocity
Feynman x scaled pz
Pseudorapidity y easier to measure
Away from mid-rapidity
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Centrality Determination
more central
Counts

• HIJING GEANT
• Glauber Calculation
• Model Paddle Response

Paddle signal (a.u.)
Counts
Number of participants is number of nucleons that
interact
Npart
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Particle Spectra
15 central
0.2ltyp lt1.4
PHOBOS niche at low p
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Why charged particle production?

• Fundamental handle on the initial state
• Upper bound on initial entropy density
• Entropy does NOT require equilibrium
• Initial gluon density
• Constrains subsequent evolution
• Freezeout entropy density

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Pseudorapidity Density Distributions

• AuAu collisions at ?s 19.6, 130, 200 GeV
• dN/dh for h lt 5.4 over full azimuth
• Top 50 of total cross section (Npart 65 - 360)
• Data at 56 GeV only for central density at h lt 1

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Particle Density near Midrapidity
(2001)
(2000)
(2000)
Theory 1999
Theory 2000
Constraining the Models
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Centrality Dependence of Central Density
Two Component Fit
PHOBOS AuAu
200 GeV
130 GeV
19.6 GeV preliminary
?sNN x ------
-------------

• 0.110.04
• 0.110.04
• 19.6 0.150.07

Data from PRC 65 061901R (2002)
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Limiting Fragmentation
PHOBOS AuAu
200 GeV
130 GeV

• Universal limiting distribution
• Extent of limiting fragmentation region grows
  with beam energy
• No broad boost invariant region

dN/dh?/?Npart/2?
19.6 GeV
h? h - ybeam
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Centrality Dependence of ?Nch?
Error band due to high-h extrapolation
AuAu
Total charged particle multiplicity per
participant is centrality independent
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Remove the Leading Proton Effect
?s
?seff
Basile et al (1980-1984)
?Nch?
pQCD ee- Calculation
(A. Mueller, 1983)
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Comparison of áNchñ vs. Energy
Different systems converge at high energy
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Total Multiplicity vs. Beam Energy
pp?pX
ee- Fit
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Azimuthal Angular Distributions
head on view of colliding nuclei
Look at emission patterns using Fourier
expansion extract v2 components from the fits.
N0 (1 2v1cos (f-YR) 2v2cos
(2(f-YR)) ... )
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v2 vs. centrality
v2
200
h lt 1
130
PHOBOS AuAu
ltNpartgt
Suggests early thermalization
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v2 vs. transverse momentum
v2
PHOBOS preliminary h h- 200 GeV AuAu
track-weighted centrality averaging
0lthlt1.5
(top 55)
17 scale error
v2 appears to saturate for pT gt 2 Similar to
results from STAR and PHENIX
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v2 vs. pseudorapidity
v2
PHOBOS AuAu
ltNpartgt190

No boost invariance over a broad region
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v2 vs. ? cf. Hydro Model
v2
130 GeV result nucl-ex/0205021, accepted in PRL

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Conclusions

• Not boost invariant over broad h range
• Limiting fragmentation confirmed for AA
• Total yields in AA scale primarily with Npart
• Surprising similarity between AA and ee-