a first look at au+au collisions at rhic using the phobos detector

Birger Back/ANLICPAQGP, Jaipur, November 2001 -1-

A first look at Au+Au collisions at RHIC using the PHOBOS

detector

Birger Back Argonne National Laboratory

for the PHOBOS Collaboration


PHOBOS Collaboration (Nov 2001)

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, Alan Wuosmaa

Mark Baker, Donald Barton, Alan Carroll, Joel Corbo, Nigel George, 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

Abigail Bickley, Richard Bindel, Alice Mignerey


Outline

• Apparatus • Triggering – centrality determination• Charged particle multiplicity

• Energy dependence at h=0 , h = - ln{tan(/2)}• Centrality dependence at h=0 • Pseudo-rapidity distributions – widths• Total charged particle multiplicity

• Antiparticle / particle ratios• Summary


The full PHOBOS Detector

Mid-rapidity Spectrometer

~4 Multiplicity Array

TOF

135,000 Silicon Pad channels: spectrometer + multiplicity

CerenkovTrigger Paddles


PHOBOS in PHOTOS

Octagon Detector

Ring Counter

Sample Silicon pad sizesOctagon Detector: 2.7 x 8.8 mm2

Vertex Detector: 0.4 x 12 mm2

Ring Counter: 20 -106mm2

Spectrometer: 1 x 1 mm2

~ 25

cm

Spectrometer

Vertex Detector


Phobos acceptance (zvtx=0)

Spectrometer

Spectrometer

Spectrometer

Vertex

Vertex


t (ns)

Eve

nts

Triggering on Collisions

Negative

Paddles

Positive Paddles

ZDC N

ZDC PAu Au

x

z

PPPN

• Coincidence between Paddle counters at t = 0 defines a valid collision

• Paddle + ZDC timing reject background

• Sensitive to 97% of inelastic cross section for Au+Au at sNN = 130 GeV

Paddle Counter

ValidCollision

ZDCCounter


Determination of Npart

Simulation Measurement

Peripheral

b

Central

Npart

~Npart

~ N

spect

ato

r=2

A-N

part

b


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?

• Phobos multiplicity data• 56 GeV dN/dh(0) • 130 GeV dN/dh(0), dN/dh(0, cent), dN/dh(h)• 200 GeV dN/dh(0), dN/dh(0, cent), dN/dh(h)


Multiplicity Measurements in 4

Single-event display

Vertex “tracklets” – 3 point tracks

~0

-5.4<<5.4


RESULTS

Ss dN/dh56 GeV 4081230130 GeV 5551235200 GeV 65035

Ss 2dN/dh/Npart

56 GeV 2.47.10.25130 GeV 3.24.10.25200 GeV 3.78 .25

130 GeV PRL 85 (2000) 3100200 GeV PRL (Accepted)

Multiplicity at =0 vs Energy (0-6%)

R200/130=1.140.05


Comparison to model predictions

Observed increase smaller than predicted by HIJING with Jet Quenching

Experimental


dN/dh|h=0 comparison of data

Grey band:Systematic uncertainty

Reasonable agreementbetween experiments

Up to 50% larger than pp


dN/dh|h=0 comparison to models

Grey band:Systematic uncertainty HIJING: PRL 86, 3496 (2001)

EKRT: hep-ph/0106330

KN scaling PLB 507,121 (2001)

dN/d=(1-x)nppNpart/2+xnppNcoll

“soft” “hard”

Energy npp x

130 2.23 0.090.04200 2.42 0.110.04


dNch/d shapes @ 130 GeV

Phys. Rev. Lett. 87, 102303 (2001)


Energy dependence and comparison to pp

•Width increases with Ecm

•Increase Dh = Dybeam

•Scaling in fragmentation region

HI part. Production is increased at mid-rapidity

7-10% syst error

7-10% syst error


Scaling in the fragmentation region

FragmentationFragmentation

UA5: Alner et al., Z. Phys. C33,1 (1986) PHOBOS 2000/2001

7-10% syst error


Centrality Dependence of Nchtot (||<5.4)

CentralPeripheral

PRL 87, 102303 (2001)

•Nchtot scales with Npart

•Variance decr. with Npart

•HIJING too low

Ntot=(1-x)nppNpart/2+xnppNcoll

“soft” “hard”

200 GeV x=0.037


Comparison to models

PRL 87 (2001) 102303 AMPT – hadronic re-scattering

130 GeV


Anti-particle / particle ratios

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

+p

K+

+

K-

p

-


Anti-particle/particle ratios

Central region not baryon-free

Centrality =12 %<Npart> ~ 310

06.004.060.0

06.007.091.0

02.001.000.1

p

p

K

K

Phys.Rev.Lett. 87(2001) 102301

Stat. Syst.

K. Redlich, Quark Matter 2001


Compare to Lower Energy

Antiproton/proton ratio Increasing towards unity (baryon free)

Phys. Rev. Lett. 87, 102301 (2001)

PHOBOS


• dNch/d at =0 per participant• ~55% higher than pp at RHIC energies

• Npart evolution between HIJING and EKRT

• 9-11% contribution from hard scattering at =0

• 3.7% for Ntot

• dNch/d in 4• 130 GeV: <Nch> = 4200 +/- 470 (||< 5.4) for 3% central

• Additional particle production near =0 for central events• Wider than HIJING – hadronic re-scattering (AMPT)

• Particle ratios• p/p ratio 0.60 +/- 0.04(stat) +/- 0.06(syst) • Central region not yet baryon free – maybe at 200 GeV??

Summary


Some references

• PHOBOS web-site: www.phobos.bnl.gov• Physics Results

• Charged-particle pseudorapidity density distributions from Au+Au collisions at 130 GeV Phys. Rev. Lett. 87, 102303 (2001)

• Ratios of charged particles to antiparticles near mid-rapidity in Au+Au collisions at 130 GeV Phys. Rev. Lett. 87, 102301 (2001)

• Charged particle multiplicity near mid-rapidity in central Au+Au collisions at 56 and 130 GeV Phys. Rev. Lett. 85, 3100 (2000)

• Energy dependence of particle multiplicities near mid-rapidity in central Au+Au collisions Accepted Phys. Rev. Lett (Aug 2001)

• Centrality Dependence of Charged Particle Multiplicity at h=0 in Au+Au Collisions at 130 GeV Submitted to Phys. Rev. C (August 2001)

• Technical• Array of Scintillator Counters for PHOBOS at RHIC

Nucl. Instr. Meth. A474, 38-45 (2001)

• Silicon Pad Detectors for the PHOBOS Experiment at RHICNucl. Instr. Meth. A461, 143-149 (2001)

• Development of a double metal, AC-coupled silicon pad detectorThe silicon detector for the PHOBOS experiment at RHICNucl. Instr. Meth. A389, 415 (1997)

a first look at au+au collisions at rhic using the phobos detector

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