Christof Roland/MITMoriond,March, 2001 -1-

Results from the PHOBOS experiment at RHIC

Christof Roland (MIT)for the

PHOBOS Collaboration

Christof Roland/MITMoriond,March, 2001 -2-

Year 1: The Big Picture

Q: Entropy Production Hadro-Chemistry Thermal

Equilibration Multiplicity Charged Particle Density

Event Anisotropy Flow

Particle Ratios

Au+Au Collisions @ s = 130 GeV

Christof Roland/MITMoriond,March, 2001 -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

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

Christof Roland/MITMoriond,March, 2001 -4-

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

Christof Roland/MITMoriond,March, 2001 -5-

First RHIC physics result: Back et al., PRL 85 (2000) 3100

Multiplicity at =0 vs Energy

RESULTs Npart56 GeV 330 +/- 4 +10/-15

130 GeV 343 +/- 4 +7/-14

s dN/d 56 GeV 408 +/- 12 +/-30130 GeV 555 +/- 12 +/-35

s 2dN/d /Npart

56 GeV 2.47 +/-.10 +/-.25130 GeV 3.24 +/-.10 +/-.25

Ratio: 1.31 +/-.04 +/-.05

Au+Au @ 130 GeV:• 40% increase from p+p to central Au+Au• 70% increase from CERN-SPS

6% most central

Christof Roland/MITMoriond,March, 2001 -6-

Multiplicity at =0 vs Npart

Npart

Yellow band: Systematic uncertainty

Preliminary

dNch

/d/

(0.5

*Npa

rt) Physics run July-Sept 2000

10 centrality bins for upper 45% of cross section

Q: How does dNch/d@=0 evolve from 2.3 (p+p) to 3.25 (Au+Au) per participant pair?

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

Christof Roland/MITMoriond,March, 2001 -7-

Multiplicity in 4(Hit counting)

Occupancy correction (assuming Poisson statistics)

dNch

d =hits

O(,b) ×fB(,b)

A(ZVTX)

Rings Octagon Rings

Acceptance Factor

Backgroundcorrection

Christof Roland/MITMoriond,March, 2001 -8-

dNch/d vs Centrality Preliminary

Statistical errors only - 10-20% systematical uncertainty

Christof Roland/MITMoriond,March, 2001 -9-

Evolution of dNch/d vs Npart

• <Nch> = 4100 +/- 410 for 3% most central• Additional particle production near =0• Wider + more particles relative to HIJING

Preliminary

(dN

ch/d

)/(

½N

pa

rt)

Data

HIJING

Npart=356

Npart=215

Npart=103

(dN

ch/d

)/(

½N

pa

rt)

Statistical errors only

Npart

PHOBOS Prelim.

±10% Systematic Uncertainty±10% Systematic Uncertainty

HIJING

Nch

(||

<5.

4)

Christof Roland/MITMoriond,March, 2001 -10-

Collective Effects: Elliptic Flow

Q:

Does the initial space

anisotropy translate into

final state momentum

space anisotropy?

b (reaction plane)

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

Christof Roland/MITMoriond,March, 2001 -11-

• Systematic error ~ 0.007

• Large V2 signal compared to lower energy

• Centrality Dependence close to Hydro.

• V2 drops for || > 1.5

Elliptic Flow Results

V2 PHOBOS STAR (PRL) Hydro. model

V2

Normalized Paddle Signal

SPSAGS

Preliminary

|| < 1.0 Averaged over centrality

Christof Roland/MITMoriond,March, 2001 -12-

Hadro Chemistry: Particle Ratios

p

K

p-

Preliminary

Thermal Model Estimte : B ~ 50 MeV Central region not net baryon-free!

Q: Baryo-Chemical Potential, Baryon Stopping

.)syst(..)stat(..

.)syst(..)stat(..

ppK

K

10050540

10090910

.)syst(..)stat(.. 050010011

Centrality =12 %<Npart> ~ 310

Christof Roland/MITMoriond,March, 2001 -13-

• 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

• Central region not baryon free

Summary

Christof Roland/MITMoriond,March, 2001 -14-

Outlook I: 2001

• >100x statistics• Physics:

• low-pT physics

• Spectra• HBT

• Resonances (at low pT

• Event-by-Event physics

• Both arms completed • Energy systematics• Species systematics