The Latest Results from PHOBOS @ RHIC

1
The Latest Results from PHOBOS _at_ RHIC
Evidence of final-state suppression of high-pT
hadrons in Au Au collisions using d Au
measurements
Rachid NOUICERUniversity of Illinois at
Chicago and Brookhaven National Laboratory
for the Collaboration

International Europhysics
Conference
on High Energy
Physics
July 17, 2003
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PHOBOS Collaboration (May 2003)
Birger Back, Mark Baker, Maarten Ballintijn,
Donald Barton, Bruce Becker, Russell Betts,
Abigail Bickley, Richard Bindel, Andrzej
Budzanowski, Wit Busza (Spokesperson), Alan
Carroll, Patrick Decowski, Edmundo Garcia, Tomasz
Gburek, Nigel George, Kristjan Gulbrandsen,
Stephen Gushue, Clive Halliwell, Joshua Hamblen,
Adam Harrington, Conor Henderson, David Hofman,
Richard Hollis, Roman Holynski, Burt Holzman,
Aneta Iordanova, Erik Johnson, Jay Kane, Nazim
Khan, Piotr Kulinich, Chia Ming Kuo, Jang Woo
Lee, Willis Lin, Steven Manly, Alice Mignerey,
Gerrit van Nieuwenhuizen, Aaron Noell, Rachid
Nouicer, Andrzej Olszewski, Robert Pak, Inkyu
Park, Heinz Pernegger, Corey Reed, Louis
Remsberg, Christof Roland, Gunther Roland, Joe
Sagerer, Pradeep Sarin, Pawel Sawicki, Iouri
Sedykh, Wojtek Skulski, Chadd Smith, Peter
Steinberg, George Stephans, Andrei Sukhanov, Ray
Teng, Marguerite Belt Tonjes, Adam Trzupek, Carla
Vale, Robin Verdier, Gábor Veres, Bernard
Wadsworth, Frank Wolfs, Barbara Wosiek,
Krzysztof Wozniak, Alan Wuosmaa, Bolek Wyslouch,
Jinlong Zhang 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
68 Participants 8 Institutions 3 Countries
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Hard-scattered Partons are built-in QCD Probes
within RHI Collisions
A main goal of relativistic heavy ion physics is
to investigate high-temperature, high-density
QCD, by creating and then studying the
highly-excited medium produced in high-energy
nuclear collisions.
One method of diagnosing a QCD medium is to look
for any modifications of the probes due to the
medium.
t - few fm/c
t 0 fm/c
t few more fm/c
t few fm/c
Nucleus
Nucleus
Central Collision
Parton
Parton
Hard scattering between partons
Partons within initial nuclei
Scattered partons traverse created hot QCD medium
Hadronization fragmentation

• We measure high-pT leading hadrons.
• The basic thing we look for
• If scattered partons lose energy, the number of
  leading
• hadrons at high-pT will be depleted (suppressed).
  This is the main goal of this talk

Hadrons
Leading Hadron
Detector
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PHOBOS Detector for AuAu at 200 GeV

• 4p Multiplicity Array
• Two Mid-rapidity Spectrometers
• TOF wall for High-Momentum PID
• Triggering Scintillator Paddles , Zero Degree
  Calorimeter

ZDC
TOF
1m
Spectrometer
Ring Counters
Octagon
ZDC
Triggering
135000 Silicon Pad channels
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Triggering on Collisions Centrality

• Coincidence between Paddle counters at Dt 0
  defines a valid collision
• Paddle ZDC timing reject background

Data
DataMC
HIJING GEANT Glauber calculation Model of
paddle trigger
Central
Peripheral
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Definition of Ncoll and Npart
Peripheral Collision
Central Collision
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pT Distribution of Charged ParticlesAu Au at
200 GeV
PHOBOS pT coverage pT 0.030 5 GeV/c
AuAu
Systematic Errors not shown
Can we use this tiny tail to probe the QCD medium
?
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Transverse Momentum Distributions vs Centrality
AuAu at 200 GeV
Data PHOBOS, nucl-ex/0302015 Submitted to Phys.
Lett. B
syst. uncertainties for Ncoll 10-15 .
Centrality
Npart
NColl
0-6
344 12
1050
6-15
276 9
780
500
15-25
200 8
300
25-35
138 6
175
93 5
35-45
65 4
45-50
107
Centrality range ltbgt from 3 to 10 fm lt?gt from
6 to 3
0.2ltyp lt1.4
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Ratio of AuAu and pp Spectra at 200 GeV
Data PHOBOS, nucl-ex/0302015 Submitted to Phys.
Lett . B
Relative to UA1 pp
mid-peripheral Npart 65 4

• We observe a significant change in the spectral
  shape between pp and AuAu collisions already
  for mid-peripheral events.
• Central AuAu collisions show a strong violation
  of (expected) collision scaling at high-pT.

central Npart 344 12
High-pT suppression
Is this a signature of the final
state jet quenching ?
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Is final state jet quenching the only
explanation ?
Au Au at 200 GeV
Data PHOBOS, nucl-ex/0302015 Submitted to Phys .
Lett. B
PHOBOS, nucl-ex/0302015
Expectation for Ncoll-scaling

• Particle production scales approximately
• with Npart at high-pT
• Similar centrality dependence at
• pT 0.5 and 4 GeV/c !

Saturation model Kharzeev, Levin, McLerran hep
-ph/0210332
Expectation for Ncoll-scaling
Saturation model prediction
Particle Yield/ltNpart/2gt

• Initial state parton saturation works
  qualitatively too

Rachid Nouicer
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Conclusions of part I
The observed high-pT suppression suggest two
possibilities

• Strong final state suppression at high-pT?
• Indication of high density, strongly interacting
  matter!
• Strong initial state suppression persisting to
  high-pT?
• Indication of multipartonic effects in the
  nuclear wavefunction!

We need a simpler system such as d Au in order
to understand a complex system Au Au
RHIC Accelerator response no problem!
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Predictions for dAu
pQCD (final state)
Parton Saturation (initial state)
Vitev, nucl-th/0302002, Phys.Lett.B in press
Vitev and M.Gyulassy, Phys.Rev.Lett. 89 (2002)
Kharzeev, Levin, McLerran, hep-ph/021332
30 suppression of high-pT particles (central
vs peripheral)
Nuclear Modification Factor RdAu
Central
Peripheral
16 increase central vs peripheral
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This Year PHOBOS Detector 2003 dAu at 200 GeV
mini-pCal
SPECTRIG
? DAQ upgrade (x10)
? Moved TOF walls back 5 m from interaction
point
? Installed new spectrometer trigger detector
that selects on high pT tracks
pCal
? Installed new time-zero (T0) Cerenkov
detectors to provide triggering and on-line
vertexing as well as a start time for our TOF
walls.
? Proton calorimeter on Gold and Deuteron going
sides for dA run
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Centrality Determination in dAu at 200 GeV
HIJING Simulation
Centrality cuts (4 bins)
0.2lthlt1.4
dN/dh
3.0lthlt5.4
Counts
Multiplicity distribution
Pseudorapidity

• Glauber Calculation
• Hijing 1.383
• Hulthen w.f.
• 41mb inelastic NN cross-section
• Full GEANT Simulation

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Invariant Yields for Charged Hadrons vs pT
dAu at 200 GeV
Data PHOBOS, nucl-ex/0306025 Submitted to Phys.
Rev. Lett.

• Systematic uncertainties
• tracking efficiency 5-10
• malfunctioning channels 5

Deuteron direction
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Nuclear Modification factor (RdAu) vs pT
Data PHOBOS, nucl-ex/0306025 Submitted to Phys.
Rev. Lett.
central AuAu

• Clear evidence of absence of high-pT
  suppression in d Au

• The observation and the absence of high-pT
  suppression in AuAu and dAu respectively,
  can be an indication of creation of highly
  interacting medium in AuAu.

All syst. uncertainties 90 C.L.
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Nuclear Modification Factor (RdAu) vs Ncoll
Data PHOBOS, nucl-ex/0306025 Submitted to Phys.
Rev. Lett.
syst. uncertainties 15-20 _at_ 90 C.L.
Parton saturation predicts 30 suppression
of high-pT particles (central vs
peripheral)

• Data disfavor initial state
• interpretation of AuAu
• high-pT suppression

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Summary

• Clear Evidence of high-pT suppression in more
  central collisions of Au Au at 200 GeV
• Clear Evidence of absence of high-pT suppression
  in more central collisions of d Au at 200
  GeV
• Slight enhancement at high-pT (pT 4 GeV/c) of
  nuclear modification factor of dAu vs Ncoll

• The latest news from PHOBOS
• We have compared central AuAu to central dAu.
• This data strongly disfavors the initial state
  parton saturation interpretation of high-pT
  hadron suppression.
• More to come !