Results from the PHOBOS experiment at RHIC

Birger Back/ANLWinter Workshop on Nuclear Dynamics, 2001

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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

UNIVERSITY OF ILLINOIS AT CHICAGO

UNIVERSITY OF MARYLAND

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

Joshua Hamblen , Erik Johnson, Nazim Khan, Steven Manly,Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs

Russell Betts, Clive Halliwell, David Hofman, Burt Holzman,Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter

Richard Bindel, Edmundo Garcia, Alice Mignerey


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Outline

• Apparatus• 2000 run statistics• Physics results

• Charged particle multiplicity • Energy dependence at h=0 • Centrality dependence at h=0 • 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

Ring Counters

Paddle Trigger Counter

Spectrometer

TOF

Octagon+Vertex

• 96000 Silicon Pad channels

• 4 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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• Energy Dependence (vs. AGS/SPS Data)

• System Size (p+p, Npart Dependence )

Results from 2000 run

MEASUREMENT Charged Particle DensityEvent Anisotropy - FlowParticle 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 Au+Au at SsNN = 130 GeV

Negative

Paddles

Positive Paddles

ZDC N

ZDC PAu Au

x

z

PPPN


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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 = 0Commissioning runMay-July 2000

Ss = 56 and 130 GeV

Spectrometer

Vertex

Double Si layer


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First RHIC physics result: Back et al., PRL 85 (2000) 3100About 20 theoretical papers (Feb 15, 2001)

Multiplicity at =0 vs Energy

RESULTSs dN/dh56 GeV 4081230130 GeV 5551235

Ss 2dN/dh/Npart

56 GeV 2.47.10.25130 GeV 3.24.10.25

Ratio: 1.31.04.05

@ 130 GeV 30% higher than p+p70% higher than SPS

SPS

Phobos

p+p


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Some model comparisons

HIJING EKRT (saturation model)

• 40% increase from p+p to central Au+Au• Data constrain model parameter space• Further insight from centrality dependence


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Multiplicity at =0 vs Centrality (130 GeV)

Physics run July-Sept 2000

Complete year 1 setup

Tracklets in spectrometerand vertex

12 centrality bins for upper55% of cross section


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Multiplicity at =0 vs Npart

Npart

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

Yellow band: Systematic uncertainty

PreliminarydN

ch/d/

(0.5

*Npa

rt)


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Multiplicity in 4(Hit counting)

• Determine dNch/d for –5.4 < < 5.4

• Shape• Total multiplicity

• Evolution vs Npart

• Hit counting

Octagon

Rings

Single Si layer


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PHOBOS Multiplicity detectors

Octagon 1 of 6 Rings


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1 Count hits binned in , centrality (b)

2 Calculate acceptance A(ZVTX) for that event

3 Find occupancy in hit pads O(,b) by counting empty to hit channels assuming Poisson statistics

4 Fold in a background correction factor fB(,b)

dNch

d =hits

O(,b) ×fB(,b)

A(ZVTX)

E depositionin multiplicitydetectors for one event.

Rings Octagon Rings


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dNch/d vs Centrality Preliminary

Statistical errors only - 10-20% systematical uncertainty


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Centrality dependence of dN/dh

Phobos HIJING

Central

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

Mid central

Peripheral


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Centrality Dependence of Nch(||<5.4)

Npart

PHOBOS Prelim.

±10% Systematic Uncertainty±10% Systematic Uncertainty

HIJING

Nch

(||

<5.

4)


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Elliptic Flow

dN/d(R ) = N0 (1 + 2V1cos (R) + 2V2cos (2(R) + ... )

b (reaction plane)

Octagon Ring

|| < 1.0


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Hydrodynamic modelV2

Normalized Paddle Signal

Systematic error ~ 0.007

|| < 1.0

SPS

AGS

Preliminary

Large V2 signal compared to lower energy

Preliminary

Centrality Dependence


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V2 PHOBOS STAR (PRL)

Elliptical flow vs

• Averaged over centrality

• V2 drops for || > 1.5

Preliminary

Systematic error ~ 0.007


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Anti-particle / particle ratios

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

+p

K+

+

K-

p

-


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Particle Ratios

Central region not baryon-free

Preliminary

.)syst(..)stat(..

.)syst(..)stat(..

.)syst(..)stat(..

p

p

K

K

10050540

10090910

050010011

Centrality =12 %<Npart> ~ 310


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• dNch/d @=0 per participant• 70% higher than SPS for central Au+Au• 40% higher than p+p at RHIC energy• Npart evolution between HIJING and EKRT

• dNch/d in 4• <Nch> = 4100 +/- 410 (||< 5.4) for 3% central• Additional particle production near =0 for central events• Wider than HIJING

• Elliptic flow• V2 up to 0.06 – close to hydrodynamic limit• larger than SPS• V2 drops for || > 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

Summary


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Outlook I: 2001

• 100x statistics• Both arms completed• Physics:

• low-pT physics

• Spectra• HBT

• Resonances (at low pT

• Event-by-Event physics

• Energy systematics• Species systematics


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Outlook II: 2003 + beyond

Micro-Vertex

Transition Radiation Detector

EM-Calorimeter

• Existing Spectrometer• High rate (> 0.5 kHz)• High Resolution

• Add

• Micro-vertex Detector• Narrow beam-pipe• ALICE prototype TRD

Electron ID• EM-Calorimeter

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


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Results from the PHOBOS experiment at RHIC

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