Results from the PHOBOS experiment at RHIC

1
Results from the PHOBOS experiment
at RHIC
Christof Roland (MIT) for the PHOBOS
Collaboration
2
Year 1 The Big Picture
AuAu Collisions _at_ ?s 130 GeV
Q Entropy Production Hadro-Chemistry
Thermal Equilibration

• Multiplicity
• Charged Particle Density

• Event Anisotropy
• Flow

• Particle Ratios

3
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

4
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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Multiplicity at h0 vs Energy
First RHIC physics result Back et al., PRL 85
(2000) 3100
RESULT ?s Npart 56 GeV 330 /- 4 10/-15 130
GeV 343 /- 4 7/-14 ?s dN/d h 56 GeV 408 /- 12
/-30 130 GeV 555 /- 12 /-35 ?s 2dN/d
h/Npart 56 GeV 2.47 /-.10 /-.25 130 GeV 3.24
/-.10 /-.25 Ratio 1.31 /-.04 /-.05
6 most central

• AuAu _at_ 130 GeV
• 40 increase from pp to central AuAu
• 70 increase from CERN-SPS

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Multiplicity at h0 vs Npart
Q How does dNch/dh_at_h0 evolve from 2.3 (pp) to
3.25 (AuAu) per participant pair?
Physics run July-Sept 2000 10 centrality bins
for upper 45 of cross section
dNch/dh/(0.5Npart)

• EKRT Saturation disfavored
• Good agreement with previous PHOBOS point
• Neither HIJING nor EKRT describe data well

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Multiplicity in 4p (Hit counting)
Rings
Octagon
Rings
f
h
h

• Occupancy correction (assuming Poisson
  statistics)

Background correction
Acceptance Factor
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dNch/dh vs Centrality
Preliminary
Statistical errors only - 10-20 systematical
uncertainty
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Evolution of dNch/dh vs Npart
Preliminary
Statistical errors only
Data
Npart356
HIJING
Nch(hlt5.4)
Npart215
(dNch/dh)/(½Npart)
Npart103
10 Systematic Uncertainty
h
Npart

• ltNchgt 4100 /- 410 for 3 most central
• Additional particle production near h0
• Wider more particles relative to HIJING

HIJING
(dNch/dh)/(½Npart)
h
10
Collective Effects Elliptic Flow

• Q
• Does the initial space
• anisotropy translate into
• final state momentum
• space anisotropy?

b (reaction plane)
dN/d(f -YR ) N0 (1 2V1cos (f-YR) 2V2cos
(2(f-YR ) ) ... )
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Elliptic Flow Results
Preliminary
Averaged over centrality
h lt 1.0
V2
Hydro. model
SPS
AGS
Normalized Paddle Signal

• Systematic error 0.007
• Large V2 signal compared to lower energy
• Centrality Dependence close to Hydro.
• V2 drops for h gt 1.5

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Hadro Chemistry Particle Ratios
Q Baryo-Chemical Potential, Baryon Stopping
Preliminary
Centrality 12 ltNpartgt 310
-
p



.)
syst
(
.
.)
stat
(
.
.
05
0
01
0
01
1

p
-
K



.)
syst
(
.
.)
stat
(
.
.
1
0
09
0
91
0

K
p



.)
syst
(
.
.)
stat
(
.
.
1
0
05
0
54
0
p

• Thermal Model Estimte mB 50 MeV
• Central region not net 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 4-p
• 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
• Central region not baryon free

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Outlook I 2001

• gt100x statistics
• Physics
• low-pT physics
• Spectra
• HBT
• Resonances (f at low pT)
• Event-by-Event physics
• Both arms completed
• Energy systematics
• Species systematics