-1- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

Charged-Particle Multiplicity and Particle Ratiosin Au + Au Collisions at RHIC Energies

Using the PHOBOS Detector

The Latest Results from PHOBOS

Rachid Nouicer

University of Illinois at Chicago

for the Collaboration

-2- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

The PHOBOS Collaboration

Argonne National LaboratoryBrookhaven National Laboratory

Institute of Nuclear Physics KrakowMassachusetts Institute of TechnologyNational Central University, Taiwan

University of Rochester University of Illinois at Chicago

University of Maryland

~ 70 collaborators

-3- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

Completed Spring 2001 •4 Multiplicity Array

- Octagon, Vertex & Ring Counters• Two Mid-rapidity Spectrometers• TOF wall for High-Momentum PID• Triggering

-Scintillator Paddles + Cerenkov Counters- Zero Degree Calorimeter 135000 Silicon Pad channels

PHOBOS Apparatus

-4- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

Collisions at 200 GeV!

Single Event in PHOBOS

-5- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

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

Ring Counter

Octagon Detector

Vertex Detector

Spectrometer

PHOBOS Silicon Detector

-6- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

Silicon Detector Performance

dE/dx resolution = 6.9%

K

p

d• Signal is well reproduced by MC• Signal/Noise ~ 14 to 20 • Particles are well identified by spectrometer within 2T magnetic field

-7- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

Triggering on Collisions & Centrality Negativ

e Paddles

Positive Paddles

Au Aux

z

PPPN

Positive ZDC

Negative ZDC

NegativeCerenkov

PositiveCerenkov

Paddle signal (a.u.)

ZD

C S

ign

al (

a.u

.)

Npart

Nsp

ect

ato

r=2

A-N

part

Central

Peripheral

200 GeV Data • Coincidence between Paddle counters

at t = 0 defines a valid collision

• Paddle + ZDC timing reject background

• Trigger sensitive to 97% of inelastic cross section

Measurement

-8- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

Single-event display

Vertex “tracklets” – 3 point tracks

1

-5.4<<5.4

Multiplicity Measurements in 4

-9- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

56 & 130 GeV : PRL 85, 3100(2000) 200 GeV : PRL 88, 22302(2002)

Charged Particle Multiplicity near Mid-rapidity vs Energy

Results :

Central

Extrapolation to LHC : E(LHC)=30 E(RHIC)

2200) dN/dη LHC ( - KN Saturation Model :

1300 ) dN/dη ( LHC - A + A Fit :

Model prediction :

• no unexpected increase• 200 GeV AuAu 55% higher than pp

• dN/d = 650 +/- 35

for AuAu at 200 GeV (6% central)

hep-ph/0111315

PHOBOS : AuAu at RHIC

-10- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

PHOBOS : 130 GeV : PRC 65, 031901(2002)PHOBOS : 200 GeV submitted to PRC Saturation Model : nucl-th/0108006

Central Peripheral

Centrality Dependence & Comparison to Models Results

Two-component Fit :

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

“soft” “hard”

0.02 1 0.1 2.42 GeV 200

0.02 0.09 2.23 GeV 130

x n Energy pp

|| < 1

PHOBOS : AuAu at RHIC

1.34(130) hard(200) hard

1.07 (130) soft(200) soft

-11- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

dN/d

dN/d

dN/d

dN

/d

dN/d

dN/d

Results : Charged Particle Production vs Au+Au at 200 GeV : latest results  

AuAu at 130 GeV : PHOBOS PRL 87, 102303 (2001)  

200 GeV130 GeV

Central

Peripheral

Systematic Errors

-12- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

Energy Dependence & Comparison to pp

Most central

Width Increases with Ecm

Au+Au & pp at 200 GeV A+A

Results :

HI Particle Production is Increased at Mid-Rapidity

RHIC : PHOBOS AuAu s = 200 GeV

SPS : EMU-13 PbPb s = 17 GeV RHIC : PHOBOS AuAu s = 130 GeV

Systematic errors not included

-13- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

Results : Limiting Fragmentation

Fragmentation

A+A

p + p UA5, Z.Phys.C33, 1 (1986)

AuAu and pp show the same behavior, the extent of the fragmentation region grows with collision energy

Fragmentation

Systematic errors not included

-14- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

Results : Limiting Fragmentation vs Centrality

• The extent of the limiting fragmentation region is independent of centrality

but the shape in this region depends on centrality !

200 GeV 200 GeV

200 GeV Data 130 GeV Data

130 GeV130 GeV

Fragmentation Fragmentation

-15- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

Anti-particle / particle ratios

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

+

K-

p

-

p

K+

+

Positive

Charge

Negative

Charge

-16- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

AuAu130 GeV

Central region not Baryon-Free

Centrality =12 %<Npart> ~ 310

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

Stat. Syst.

K. Redlich, Quark Matter 2001

Results : Anti-particle/particle ratios

06.004.060.0

06.007.091.0

02.001.000.1

p

p

K

K

T= 170 MeV

-17- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

Compare to Lower Energy

PHOBOS : 130 GeV Phys. Rev. Lett. 87, 102301 (2001)

/KK - p/p

Results :

Antiproton/proton ratio Increasing towards unity (baryon free)

PHOBOS

-18- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

• dNch/d @ | per participant• AuAu 55% higher yield than pp at maximum RHIC energy• Npart >100 the R200/130 approximately constant at 1.14 +/- 0.01+/- 0.06

• AuAu data at 200 & 130 GeV are in good agreement with parton saturation model predictions and the two-component fits to the data (hard + soft)

• Using the two-component fits the hard processes increase by 34 % and the soft by 7 % from 130 to 200 GeV in AuAu

• dNch/d in 4• 200 GeV : <Nch> = 4960 +/- 250 (||< 5.4) for 6% central

• Additional particle production near | for central events• Central AuAu has very similar shape to pp • Extent of fragmentation region grows with collision energy

• Particle ratios• p/p ratio 0.60 +/- 0.04(stat) +/- 0.06(syst) • Central region not yet baryon free

Summary

-19- Rachid Nouicer/UICQCD and HEHI, Moriond, March 2002

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

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

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

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

• Energy dependence of particle multiplicities near mid-rapidity in central Au+Au collisions Phys. Rev. Lett 88, 22302(2002)

• Centrality Dependence of Charged Particle Multiplicity at | in Au+Au Collisions at 130 GeV Phys. Rev. C65, 031901(2002)

• Centrality Dependence of Charged Particle Multiplicity at |<1 in Au+Au Collisions at 130 and 200 GeV Submitted to Phys. Rev. C (2002)

• Technical• Silicon Pad Detectors for the PHOBOS Experiment at RHIC

Nucl. Instr. Meth. A461, 143-149 (2001) • Array of Scintillator Counters for PHOBOS at RHIC

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