First Results from the PHOBOS Experiment at RHIC:

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First Results fromthe PHOBOS Experiment at RHIC
Charged Particle Multiplicity Near Mid-Rapidity
in Central AuAu Collisions at ?s56 and 130
AGeV
Judith Katzy, MIT
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PHOBOS Collaboration

• ARGONNE NATIONAL LABORATORY
• Birger Back, Nigel George, Alan Wuosmaa
• BROOKHAVEN NATIONAL LABORATORY
• Mark Baker, Donald Barton, Mathew Ceglia, Alan
  Carroll, Stephen Gushue, George Heintzelman,
  Hobie Kraner ,Robert Pak,Louis Remsberg, Joseph
  Scaduto, Peter Steinberg, Andrei Sukhanov
• INSTITUTE OF NUCLEAR PHYSICS, KRAKOW
• Wojciech Bogucki, Andrzej Budzanowski, Tomir
  Coghen, Bojdan Dabrowski, Marian Despet,
  Kazimierz Galuszka, Jan Godlewski , Jerzy Halik,
  Roman Holynski, W. Kita, Jerzy Kotula, Marian
  Lemler, Jozef Ligocki, Jerzy Michalowski, Andrzej
  Olszewski?, Pawel Sawicki , Andrzej Straczek,
  Marek Stodulski, Mieczylsaw Strek, Z. Stopa, Adam
  Trzupek, Barbara Wosiek, Krzysztof Wozniak, Pawel
  Zychowski
• JAGELLONIAN UNIVERSITY, KRAKOW
• Andrzej Bialas, Wieslaw Czyz, Kacper Zalewski
• MASSACHUSETTS INSTITUTE OF TECHNOLOGY
• Wit Busza, Patrick Decowski, Piotr Fita, J.
  Fitch, C. Gomes, Kristjan Gulbrandsen, P.
  Haridas, Conor Henderson, Jay Kane , Judith Katzy
  , Piotr Kulinich, Clyde Law, Johannes
  Muelmenstaedt, Marjory Neal, P. Patel, Heinz
  Pernegger, Miro Plesko, Corey Reed, Christof
  Roland, Gunther Roland, Dale Ross, Leslie
  Rosenberg, John Ryan, Pradeep Sarin, Stephen
  Steadman, George Stephans, Katarzyna Surowiecka,
  Gerrit van Nieuwenhuizen, Carla Vale, Robin
  Verdier, Bernard Wadsworth, Bolek Wyslouch
• NATIONAL CENTRAL UNIVERSITY, TAIWAN
• Yuan-Hann Chang, Augustine Chen, Willis Lin,
  JawLuen Tang
• UNIVERSITY OF ROCHESTER
• A. Hayes, Erik Johnson, Steven Manly, Robert Pak,
  Inkyu Park, Wojtech Skulski, Teng, Frank Wolfs
• UNIVERSITY OF ILLINOIS AT CHICAGO
• Russell Betts, Christopher Conner, Clive
  Halliwell, Rudi Ganz, Richard Hollis, Burt
  Holzman,, Wojtek Kucewicz, Don McLeod, Rachid
  Nouicer, Michael Reuter
• UNIVERSITY OF MARYLAND
• Richard Baum, Richard Bindel, Jing Shea, Edmundo
  Garcia-Solis, Alice Mignerey

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Relativistic Heavy Ion Collider
species pp , AuAu energies 30-200AGeV

• 13 June 1st Phobos AuAu Collisions _at_ ?s 56
  AGeV
• 24 June 1st Phobos AuAu Collisions _at_ ?s 130
  AGeV
• 2001 AuAu Collisions _at_ ?s
  200 AGeV

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RHIC physics goal
Create high energy density over a large volume
quark gluon plasma deconfinement phase
transition chiral symmetry restoration
Study QCD
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Nucleus - Nucleus Collisions - the scattering
process
Hadronic scattering via p, r, w exchange
AGS ?s 4.8 GeV
SPS ?s 17 GeV
Partonic scattering via gluon exchange?
Rescattering? Minijets?
RHIC ?s 53-200 GeV
d N/ dh is sensitive to the scattering process
(hard vs soft)
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Energy density in Au Au Collisions

Spectator nucleons
Produced (primary) particles
Energy density?
Participating nucleons
d N
d N
y 1/2 ln( EpL/E-pL) h - ln (tan q/2 )
e

d y
d h
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PHOBOS Detector
1m
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Commissioning run set-up
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PHOBOS Trigger
Positive Paddles
Negative Paddles
ZDC P
ZDC N
Au
Au
PN
PP
First Collisions June 13
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Examples of events
Hits in SPEC
Tracks in SPEC
Hits in VTX
130 AGeV
130 AGeV
56 AGeV
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Monte Carlo Simulations
generator HIJING 1.35 simulations Geant 3.21
Silicon Detector Response
Very good under- standing of detector response
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Centrality Determination
Data
6
6 events with highest signal in paddle
counters corresponds to 6 most central events of
inelastic cross section
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Inelastic cross section from spectator neutrons
(not in scale)
B
ZDC
ZDC
B
18 m
Forward ns

• Confirms
• centrality measurement
• paddle counter acceptance
• for periphal collisions

130 AGeV
Multiplicity
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Extracting Npart
MC
Events/bin
b
Npart
Npart

• Select 6 most central events based on paddles
  gives

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Vertex Distribution
Y
X
X
Z
Z
Fiducial volume -25cm lt Z lt 15 cm
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Event Statistics

• 56 AGeV
• Collision Events 6352
• Central Events 382
• Central Events (25 lt z lt 15) 103
• 130 AGeV
• Collision Events 12074
• Central Events 724
• Central Events (25 lt z lt 15) 151

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Tracklets Reconstruction
y
z
z

• Two hit combinations that point to the vertex
• Vertex tracklets dhlt0.1, dh h2 - h2, ,
• Spectrometer tracklets dRlt0.02 , dR ? (dh2
  df2)
• Redundancy essentially eliminates feed-down,
  secondaries, random noise hits

x
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Uncorrected dN/dh
Spectrometer
Vertex
130 GeV 56 GeV

• Final result extracted by integrating over Z

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Final Results for dN/dh
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Comparisons with SPS and pp
Submitted for publication

• dN/dh obtained at RHIC is 70 higher then at
  SPS
• gt increase of energy density by 70
• dN/dh per participating nucleon obtained in AuAu
  significantly
• higher then in pp collisions
• gt AuAu collisions differ from simple
  superposition of pp

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The Central region of PHOBOS

• Reconstruct charged particle multiplicity
• in h lt 5.5 over 4p event by event
• search for events with high multiplicity
  (e.g.excess of entropy in QGP)
• search for fluctuations (e.g. dcc)

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1 arm of the spectormeter

• Spectrometer
• measure pt spectrum down to very low momentum
• -gtsensitive to collective phenomena over large
  volumes and DCC
• reconstruct size of volume with high energy
  density and lifetime (HBT)
• -gt excess of entropy
• reconstruct f mass and width -gt look for unusual
  medium effects

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Status of PHOBOS
Complete Detector Installed July 5 - 13 25000
Collisions Recorded at 130 AGeV more to come
during August 2000