Overview of Results from PHOBOS experiment at RHIC

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Overview of Resultsfrom PHOBOS experiment at RHIC
Andrzej Olszewski Institute of Nuclear Physics,
Kraków, Poland for the PHOBOS Collaboration
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PHOBOS at RHIC
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PHOBOS Collaboration

• ARGONNE NATIONAL LABORATORY
• BROOKHAVEN NATIONAL LABORATORY
• INSTITUTE OF NUCLEAR PHYSICS, KRAKOW
• MASSACHUSETTS INSTITUTE OF TECHNOLOGY
• NATIONAL CENTRAL UNIVERSITY, TAIWAN
• UNIVERSITY OF ROCHESTER

• Birger Back, Nigel George, Alan Wuosmaa
• Mark Baker, Donald Barton, Alan Carroll,
  Joel Corbo, Stephen Gushue, Dale Hicks, Burt
  Holzman,Robert Pak, Marc Rafelski, Louis
  Remsberg, Peter Steinberg, Andrei Sukhanov
• Andrzej Budzanowski, Roman Holynski,
  Jerzy Michalowski, Andrzej Olszewski, Pawel
  Sawicki , Marek Stodulski, Adam Trzupek, Barbara
  Wosiek, Krzysztof Wozniak
• Wit Busza (Spokesperson), Patrick
  Decowski, Kristjan Gulbrandsen, Conor Henderson,
  Jay Kane , Judith Katzy, Piotr Kulinich, Johannes
  Muelmenstaedt, Heinz Pernegger, Michel Rbeiz,
  Corey Reed, Christof Roland, Gunther Roland,
  Leslie Rosenberg, Pradeep Sarin, Stephen
  Steadman, George Stephans, Gerrit van
  Nieuwenhuizen, Carla Vale, Robin Verdier, Bernard
  Wadsworth, Bolek Wyslouch
• Chia Ming Kuo, Willis Lin, Jaw-Luen Tang
• Joshua Hamblen , Erik Johnson, Nazim
  Khan, Steven Manly,Inkyu Park, Wojtek Skulski,
  Ray Teng, Frank Wolfs
• Russell Betts, Edmundo Garcia, Clive
  Halliwell, David Hofman, Richard Hollis, Aneta
  Iordanova, Wojtek Kucewicz, Don McLeod, Rachid
  Nouicer, Michael Reuter, Joe Sagerer
• Richard Bindel, Alice Mignerey

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The PHOBOS Detector (2001)
1m

• 4p Multiplicity Array
• - Octagon, Vertex Ring Counters
• Mid-rapidity Spectrometer
• TOF wall for high-momentum PID
• Triggering
• Scintillator Paddles Counters
• Zero Degree Calorimeter (ZDC)

137000 silicon pad readout channels
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Central Part of the Detector
(not to scale)
0.5m
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PHOBOS in PHOTOS
Octagon Detector
25 cm
Vertex Detector
Ring Counter
Silicon pad sizes Octagon Detector
2.7 x 8.8 mm2 Vertex Detector 0.5 x (12-24)
mm2 Ring Counter (5x5) - (10x10)
mm2 Spectrometer (1x1) - (0.5x19) mm2
Spectrometer
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PHOBOS Running Summary
Year 2001 running
Year 2000 running

• Commissioning (May-July)
• Part of silicon installed
• AuAu collisions at
• ?sNN 56 GeV and 130 GeV
• First published results on
• dNch/d??lt1, ?sNN56 and 130 GeV
• Physics run (July-August)
• 1 spectrometer arm setup
• AuAu collisions at ?sNN
• 130 GeV 3.5 M collisions total

• Commissioning (mid-July)
• Add 2nd spectrometer arm
• AuAu collisions
• ?sNN 130 GeV and 200 GeV
• First published result on
• dNch/d??lt1, ?sNN 200 GeV
• Physics run (mid-August?)
• 2 spectrometer arm setup
• AuAu collisions at ?sNN 200 GeV 3.5 M
  collisions by end of August

Essentially flawless performance of PHOBOS
detector
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Results to Date
_

• Charged particle density
• Versus energy
• Central collisions, ??0
• ?sNN 56 and 130 GeV
• PRL 85 (2000) 3100
• ?sNN 200 GeV,
• submitted to PRL
• Versus centrality
• ?sNN 130 GeV, ??0
• submitted to PRC
• Versus angle and centrality
• ?sNN 130 GeV, ?lt5.4
• PRL 97 (2001) 102303

• Particle ratios p/p, K-/K, ?-/?
• Central collisions
• ? sNN 130 GeV
• PRL 97 (2001) 102301

• Elliptic flow
• Versus angle and centrality
• ?sNN 130 GeV, ?lt5.3
• QM2001, to be submitted soon

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Triggering on Collisions
Positive Paddles
Negative Paddles
ZDC N
ZDC P
Au
Au
Paddle Counter
PN
PP
ZDCCounter
Valid Collision
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Selecting Collision Centrality
3lthlt4.5
Larger signal more central collision.
PN
PP
Peripheral Collision ? Small number of
participating nucleons
Central Collision ? Large Npart
b
side view of colliding nuclei
side view of colliding nuclei
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Centrality Determination
HIJING GEANT
Error on Npart
Npart
(3lt?lt4.5)
Analysis is limited to events with Npart gt 70
?(Paddle mult.) ?(Npart)
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Charged Particle Density
Measurement

• Study
• Energy/entropy density production
• Response to properties of nuclear/partonic
  medium
• Saturation
• Jet quenching
• Importance of hard and soft processes
• Re-scattering effects
• Long-range particle correlations
• Memory of the initial geometry in the final state

• Charged Particle Density
• Event Anisotropy - Flow

Context

• Energy Dependence
• System Size
• Angular Dependence

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Tracklets
Vertex detector

• Tracklet Two-hit combination vertex position
• gt300 tracklets/central event in Vertex, gt100 in
  Spectrometer

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Analog and Digital Hit-Counting
Hits in Octagon, Ring and Vertex for single event
Digital Count hits above energy
threshold, assume Poisson- statistics in
the distribution of hits among the pads
Analog Use deposited energy (dE/dx) in each
pad to estimate number of particles that
crossed the pad
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Charged Particle Density

• Four counting methods
• Tracking detectors - h ? 0 measurements
• Tracklets in Spectrometer
• Tracklets in Vertex detector
• Single layer detectors - 4? measurements
• Use deposited energy (dE/dx) in each Si-pad
• Count hits above threshold, assume
  Poisson-statistics

All four measurements corrected for secondary
particles, feed-down from weak decay, stopping
particles Systematic uncertainty from 4.5
(Tracklets in Spectrometer) to 10 (Hit counting)
All four methods deliver consistent results -
final results averaged
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Charged Particle Density at h?0
PHOBOS first measurements - charged particle
density - in mid-rapidity - for 6 of the most
central events
dNch/d??lt1(56 GeV) 408 ? 12(stat) ?
30(syst) dNch/d??lt1(130 GeV) 555 ? 12(stat)
? 35(syst) dNch/d??lt1(200 GeV) 650 ?
35(syst)
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dNch/d??lt1 vs Energy
PHOBOS 200
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RHIC 130
pp
PHOBOS 56
SPS
nucl-ex/0108009 Submitted to PRL
Preliminary
AGS
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dNch/d??lt1 Ratio 200/130 GeV
nucl-ex/0108009
New results for 200 GeV
dNch/d??lt1 650 ? 35 dNch/d??lt1/?0.5Npart
? 3.78 ? 0.25
R200/130 1.14 /- 0.05(sys)
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Multiplicity at h0 vs Centrality
nucl-ex/0105011
?sNN 130GeV
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Multiplicity in 4? - Centrality Dependence
?sNN 130GeV
PRL 97 (2001) 102303
The width of the distribution changes with
centrality
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Multiplicity in 4? - Centrality Dependence
PRL 97 (2001) 102303
(dNch/d?)/(0.5Npart)
Total Nch(? ? 5.4)
central(0-6)
central(0-6)
peripheral(35-45)

• 3 most central collisions
• ltNchgt 4200 ? 470

• Additional particle
• production near ?0

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Change in dN/dh with Energy

• First attempt to compare
• dN/d? shape for AuAu
• at 130 and 200 GeV
• Limiting fragmentation
• check by plotting dN/d?
• with ? ? ? - Ybeam
• Agreement for AA in the
• fragmentation region
• Different slope when
• compared to pp

200 GeV - 6
130 GeV - 6
UA5 200 GeV (NSD)
Systematic errors not shown
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Azimuthal Angular Distributions
V2 determines to what extent the initial state
spatial/momentum anisotropy is preserved in the
final state.
dN/d(f -YR ) N0 (1 2V1cos (f-YR) 2V2cos
(2(f-YR)) ... )
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Centrality Dependence of V2
Preliminary
h lt 1.0 ?sNN130GeV
V2
SPS
17 GeV
PHOBOS Systematic error 0.007
Normalized Paddle Signal
(STAR Normalized Nch )

• Anisotropy increases for peripheral collisions
• Large V2 signal compared to lower energy

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V2 (elliptical flow) vs h

• Averaged over centrality
• V2 drops for h gt 1.5

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Why Measure Antiparticle/Particle Ratios?

• Microscopic viewpoint
• Antiproton/proton ratio determined by
• Baryon stopping
• Pair production
• Absorption in nuclear medium

• Thermodynamic viewpoint
• Particle ratios can be used to estimate
  hadro-chemical potentials

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Anti-particle / particle Ratios
p
70 cm
K
p
p
K-

• Tracking in the spectrometer
• Alternate 2T magnetic fields
• Energy loss and momentum

p-
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Results for Ratios
PRL 97 (2001) 102301
K-/K vs Energy
p/p vs Energy
p-/p 1.00 0.01 (stat) 0.02 (syst) K-/K
0.91 0.07 (stat) 0.06 (syst) p/p 0.60
0.04 (stat) 0.06 (syst)

• Higher values of K-/K and
• p/p than at lower energies

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Results for Ratios
PRL 97 (2001) 102301

• Results consistent with
• ?B455 MeV, which is
• much lower than that observed
• at SPS (240-270 MeV)

Assumes freezeout temp 170 MeV in statistical
model of Redlich (QM01)
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Summary 1

• Charged Particle Densities
• (Entropy)
• dNch/dh at h?0 per participant
• First look at AuAu at 200 GeV
• - increase in density by 14 compared to 130GeV
• Logarithmic increase with energy from AGS to RHIC
• Npart evolution stronger then linear, indicates
  increasing
• contributions from hard processes
• dNch/dh in 4p
• Additional particle production concentrated
• near h?0 for central events
• Decreasing width with increasing centrality
• On average 4200 particles in central collisions
  at 130GeV

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Summary 2

• Elliptic flow
• Increase of elliptic flow (V2) for more
  peripherial events
• Increase of flow effect with increasing energy
• V2 at mid-rapidity up to 0.06
• V2 drops for h gt 1.5
• Particle ratios
• K-/K and p/p significantly higher than at AGS
  or SPS
• m B 45 MeV vs 270 MeV at SPS
• p/p between HIJING and RQMD predictions
• Central region closer to baryon free state

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Outlook Year 2001?

• 100x statistics
• Physics
• low-pT physics
• Spectra
• HBT
• Resonances (f at low pT)
• Event-by-Event physics
• Energy systematics
• Species systematics

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The End
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Silicon Signals 200 vs 130 GeV
Signal shapes are (almost) identical
Methods developed for 130 GeV good also at 200 GeV
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dNch/d? at mid-rapidity ?lt1
Tracklet counting method (2 Si layers)
Tracklets ? three-point tracks two-hit
combinationsmeasured event vertex

• ?x 450 ?m
• ?y 200 ?m
• ?z 200 ?m

The measurements in the two different Si pad
detectors (different location, granularity,
acceptance, systematics)
Spectrometer
Vertex Detector
D (??2 ??2) 1/2 lt 0.015
?? lt 0.04 , ?? lt 0.3
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Tracklet analysis
?(Zvtx, Nhits)
TOTAL systematic errors 4.5 (Spectrometer)
7.5 (Vertex)
FINAL RESULTS Combined Vertex and Spectrometer
measurements (weighted by the inverse of their
total systematic error)
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Hit Counting method

• Count hits in (??,Npart) bins
  
• Evaluate number of particles per hit pad
  
• Assume Poisson statistics P(N)?Ne-?/N!
• ? - determined by the measured ratio (p)
  of occupied
• to empty pads ? ln(1p)
• Perform a multi-Landau fits to ?E
  (??,Npart) spectra
• (convoluted with gaussian)
• Calculate acceptance
  
• Fold in a final background correction (MC)
  
• FBkg(??,Npart) (dNch/d?)MCTruth/(dNch/d?)MC
  Reconstructed

Nhits(??,Npart) Ntr/hit(??,Npart) A(??,Zvtx)
FBkg(??,Npart)
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Analog method

• Get energy deposited in each Si pad
  ?E(??,Npart)
• Divide this energy by the average energy per
  track lt?E(?)gt
• Correct for the fraction of primaries
  fprim(?)
• lt?Egt and fprim are obtained from HIJINGGEANT
  simulations

Hit Counting and Analog methods agree to within
5 (systematic errors in each method are ?10)
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Corrections to the Raw Numbers

• Secondary particles (2)
• Little material between interaction point and
  sensitive volume
• Antiproton absorption in detector (8)
• From GEANT simulations
• Feed-down from weak decays (-2)
• Reduced by tracking within 10cm of vertex
• Further limit by distance-of-closest-approach cut
  on tracks

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