Results from the PHOBOS experiment at RHIC

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Results from the PHOBOS experiment
at RHIC
Birger Back Argonne National Laboratory for the
PHOBOS Collaboration
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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, Stephen
  Gushue,
• George Heintzelman, Robert Pak, 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, 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
• Willis Lin, Jaw-Luen Tang

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Outline

• Apparatus
• 2000 run statistics
• Physics results
• Charged particle multiplicity
• Energy dependence at h0
• Centrality dependence at h0
• Pseudo-rapidity distributions
• Total charged particle multiplicity
• Flow of charged particles
• Antiparticle / particle ratios
• Summary
• 2001 run plan
• Future upgrade plans

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PHOBOS Detector

• 96000 Silicon Pad channels
• 4p Multiplicity Array
• Mid-rapidity Spectrometer
• Scintillator Paddles Zero Degree
• Calorimeter for triggering
• TOF wall for high-momentum PID

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Year 2000 running

• Commissioning run (May-July)
• Sacrificial detectors installed
• 56 GeV 6352 collisions
• 130 GeV 12074 collisions
• Physics run (July-August)
• Full complement of detectors
• 130 GeV 3.5 M collisions
• Well over 1 TB of data written to tape
• Essentially flawless performance of PHOBOS
  detector

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Results from 2000 run
MEASUREMENT Charged Particle Density Event
Anisotropy - Flow Particle Ratios
CONTEXT
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Selecting Collisions

• Coincidence between Paddle counters
• Paddle ZDC timing reject background
• Sensitive to 97 of inelastic cross-section for
  AuAu at SsNN 130 GeV

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Multiplicity of charged particles

• Physics motivation
• Time integral of particle production during the
  collision
• Total entropy production
• Lots of existing data for pp, pA, AB,AA
• How do RHIC data fit into this picture?
• Methods of analysis
• 2 Si layers Tracklet counting
• 1 Si layer Hit counting Poisson statistics
• Check with energy
  deposition

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Multiplicity from tracklet counting

• Select 6 most central events
• Count Tracklets near h 0

Commissioning run May-July 2000 Ss 56 and 130
GeV
Vertex
Double Si layer
Spectrometer
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Multiplicity at h0 vs Energy
RESULT Ss dN/dh 56 GeV 408612630 130
GeV 555612635 Ss 2dN/dh/Npart 56
GeV 2.476.106.25 130 GeV 3.246.106.25 Ratio
1.316.046.05 _at_ 130 GeV 30 higher than pp 70
higher than SPS
pp
Phobos
SPS
First RHIC physics result Back et al., PRL 85
(2000) 3100 About 20 theoretical papers (Feb 15,
2001)
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Some model comparisons
HIJING
EKRT (saturation model)

• 40 increase from pp to central AuAu
• Data constrain model parameter space
• Further insight from centrality dependence

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Multiplicity at h0 vs Centrality (130 GeV)
Physics run July-Sept 2000 Complete year 1
setup Tracklets in spectrometer and vertex 12
centrality bins for upper 55 of cross section
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Multiplicity at h0 vs Npart
Preliminary
Yellow band Systematic uncertainty
dNch/dh/(0.5Npart)
Npart

• Good agreement with previous PHOBOS point
• Good agreement with recent PHENIX data
• Neither HIJING nor EKRT describe data well

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Multiplicity in 4p (Hit counting)
Single Si layer
Rings
Octagon

• Determine dNch/dh for 5.4 lt h lt 5.4
• Shape
• Total multiplicity
• Evolution vs Npart
• Hit counting

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PHOBOS Multiplicity detectors
Octagon
1 of 6 Rings
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Rings
Octagon
Rings
DE deposition in multiplicity detectors for one
event.
f
h

• Count hits binned in h, centrality (b)
• Calculate acceptance A(ZVTX) for that event
• Find occupancy in hit pads O(h,b) by counting
  empty to hit channels assuming Poisson statistics
  
• Fold in a background correction factor fB(h,b)

O(h,b) fB(h,b)
dNch
Shits

dh
A(ZVTX)
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dNch/dh vs Centrality
Preliminary
Statistical errors only - 10-20 systematical
uncertainty
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Centrality dependence of dN/dh
Phobos
HIJING
Central
Mid central
Peripheral

• Increased particle production at mid-rapidity
• Differences in shapes

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Centrality Dependence of Nch(hlt5.4)
HIJING
Nch(hlt5.4)
10 Systematic Uncertainty
Npart
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Elliptic Flow
Octagon
Ring
b (reaction plane)
h lt 1.0
dN/d(f -YR ) N0 (1 2V1cos (f-YR) 2V2cos
(2(f-YR) ... )
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Centrality Dependence
Preliminary
h lt 1.0
V2
Hydrodynamic model
Preliminary
SPS
AGS
Systematic error 0.007
Normalized Paddle Signal
Large V2 signal compared to lower energy
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Elliptical flow vs h
Preliminary
Systematic error 0.007

• Averaged over centrality
• V2 drops for h gt 1.5

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Anti-particle / particle ratios


p
K
p
p
K-

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

p-
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Particle Ratios
Preliminary
Centrality 12 ltNpartgt 310
Central region not baryon-free
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Summary

• dNch/dh _at_h0 per participant
• 70 higher than SPS for central AuAu
• 40 higher than pp at RHIC energy
• Npart evolution between HIJING and EKRT
• dNch/dh in 4p
• ltNchgt 4100 /- 410 (hlt 5.4) for 3 central
• Additional particle production near h0 for
  central events
• Wider than HIJING
• Elliptic flow
• V2 up to 0.06 close to hydrodynamic limit
• larger than SPS
• V2 drops for h gt 1.5
• Particle ratios
• p/p ratio 0.54 /- 0.1
• between HIJING and RQMD
• Central region not yet baryon free maybe at 200
  GeV

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Outlook I 2001

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

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Outlook II 2003 beyond

• Charm Production at RHIC
• Measure single electrons from displaced vertices

EM-Calorimeter
Transition Radiation Detector

• Existing Spectrometer
• High rate (gt 0.5 kHz)
• High Resolution
• Add
• Micro-vertex Detector
• Narrow beam-pipe
• ALICE prototype TRD Electron ID
• EM-Calorimeter

Micro-Vertex
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The End