Russell Betts (UIC)for the

PHOBOS Collaboration

Multiplicity Measurementswith

The PHOBOS Detector

18th Winter Workshop on Nuclear Dynamics

Nassau, Jan 20th-27th,2002

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, Joel Corbo, Stephen Gushue, George Heintzelman, 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

Richard Bindel, Alice Mignerey

The PHOBOS Collaboration

Completed Spring 2001 •4 Multiplicity Array

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

-Scintillator Paddles- Zero Degree Calorimeter

137000 Silicon Pad channels

Outline of Talk• Centrality Determination Nparticipant and Ncollision

• Techniques for Multiplicity Measurements

1. Tracklets

2. Hit Counting

3. Energy Deposition

• Results

1. Energy Dependence for 1

2. Centrality Dependence

3. dN/d Shapes

• Summary and Taster of Future Delights

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

• Paddle + ZDC timing reject background.

• Sensitive to 97±3 % of inelastic cross section for Au+Au.

t (ns)

Eve

nts

Triggering on CollisionsNegativ

e Paddles

Positive Paddles

ZDC N

ZDC PAu Au

x

z

PPPNPaddle Counter

ValidCollision

ZDC Counter

Trigger Selection - ZDC vs Paddles

Peripheral

b

Central

b

10344 partN

Determining Centrality

Npart

• HIJING + GEANT• Glauber Calculation• Model of Paddle Response

Paddle signal (a.u.)

Co

unt

sC

ou

nts

• Estimating 97% when really 94% overestimates Npart

Uncertainty on Npart

• Measurement sensitive to trigger bias – “Minimum-bias” still has bias

– Affects most peripheral events

Paddle signal (a.u.)

Co

unt

s

Octagon

Rings

Hits in One Layer of Silicon

Vertex

Energy Spectrum (E) in Si pads

1 hit

2 hits

DataMC

Multiplicity Distributions

Au+Au Collision Event Display

Event Vertex Finding

+z

Vertex Resolution:x ~ 450 my ~ z ~ 200 m

Vertex Tracklet Reconstruction

= 1 – 2

= 1 – 2

Tracklets are two point tracks

that are constrained by

the event vertex.

|| < 0.04 || < 0.3

Combinatorial Background

Outer Hit Bin 10 (Data)

All Pairs of Hits

“Background Flip”

Backgrounds

Weak Decays Electrons

Vertex Tracklet Systematic Error

• Reconstruction: Vertex selection, Tracklet algorithm etc. 1.8%

• Weak Decays: Mostly Ks and 2%

• Background: Combinatorial, -electrons - 1.5%

• MC Generators: Different particle production, background etc. - 5%

• Total: 7.5%

Analog and Digital Hit-Counting

0 +3-3 +5.5-5.5

Octagon, Ring and Vertex Detectors (unrolled)

Count Hits or Deposited Energy

Discriminating Background with dE E

(“M

IP”)

20 64-2-6 -4

04

81

2

20 64-2-6 -4

E (

“MIP

”)0

48

12

Data Monte Carlo

Si E vs. in the OctagonFrom vertex

Not from vertex

1 Count hits binned in , centrality (b)

2 Calculate acceptance A(ZVTX) for that event

3 Find the occupancy per hit pad O(,b)

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

E depositionin multiplicitydetectors for 1 event.

dNch

d =hits

O(,b) ×fB(,b)

A(ZVTX)

“Measuring” the Occupancy

!)(

N

eNP

N

N=number of tracks/pad=mean number of tracks/pad

The numbers of empty, and occupied, padsdetermine the occupancy as a function of ,b

Method: Assume Poisson statistics

Ntr

acks

/hit

pad

0-3%

50-55%

Octagon

Rings(central)

(peripheral)

MC, Occupancy Corrected

MC “truth”

Compare PHOBOS Monte Carlo “data” analyzed usingoccupancy corrections to “truth” - the difference gives corrections for remaining background.

f B(

,b)

fB=MCTruth/MCOcc

dNch

/d

Estimating remaining

backgrounds

-6 -4 -2 0 2 4 6

-6 -4 -2 0 2 4 6

0.2

0.4

0.6

0.8

1.0

200

400

600

Energy Loss Multiplicity

300 m Si

PRIM

TOTAL

i

NNiM

Measured S/N = 10 - 20 << Landau Width

Use Non-Hit pads - forCommon-Mode Noise Suppression

M = 240 ± 15 ± 5 ± CMN for one sensor (120 channels) at = 0

NoiseCommonNoiseRandomMLandauM 2i

2i

0.30 - 0.40

Energy deposited in ith pad (truncated)corrected for angle of incidence

Mean energy loss for oneparticle traversing pad RATIO OF TOTAL TRACKS

TO PRIMARY TRACKS

Uncertainty in Theoretical Predictions

Constraining the Models

Ratio 200/130 GeV

Phobos Measurement

Ratio 200/130averaged for four PHOBOS methods

R200/130 = 1.14 +/- 0.05 Moderate Increase in Energy Density?Systematic Uncertainty

Hard and Soft Processes

• Soft processes (pT < 1 GeV)

– Color exchange excites baryons

– Baryons decay to soft particles

– Varies with number of struck nucleons

• “Wounded Nucleon Model”

• Hard processes (pT > 1 GeV)

– Gluon exchange in a binary collision creates jets

– Jets fragment into hadrons, dominantly at mid-rapidity

(mini)jet

(mini)jet

Multiple Collisions with Nuclei• Nuclei are extended

– RAu ~ 6.4 fm (10-15 m)

– cf. Rp ~ .8 fm

• Geometrical model

– Binary collisions (Ncoll)

– Participants (Npart)

• Nucleons that interact inelastically

– Spectators (2A – Npart)

• p+A: Npart = Ncoll + 1

(Npart ~ 6 for Au)

• A+A: Ncoll Npart4/3

Participants

Spectators

Spectators

b(fm)

b

Ncoll

Npart

pp collisions

pA collisions

0 189

1200

400

Hard & Soft

collpppart

pp NxnN

nxd

dN

2)1(

What about non-central events?

We already expect that charged particleproduction can have two components:

We can tune the relative contribution byvarying the collision centrality

proton-proton multiplicity

Fraction from hard processes

Is this Description unique ?

• Gluons recombine at a critical density characterized by “saturation” scale Qs

2

• Below this scale, the nucleus looks “black” to a probe

Parton Saturation• Gluons below x~1/(2mR)

overlap in transverse plane with size 1/Q

3/1222 , AQxNQQ sgsss

Scale depends on volume(controlled by centrality!)

t

“Colored Glass Condensate”McLerran, Venugopalan,

Kharzeev, Dumitru, Schaffner-Bielich…

Data and Models for 130 GeV

Yellow band: Systematic

Error

Data and Models for 200 GeV

Yellow band: Systematic

Error

Shapes of dN/d Distributions at 130 GeV - Hit Counting

• Shapes only weakly dependent on centrality

• Differ in details

(0-6%)

(35-45%) (p-p)

HIJING

AMPT

Most of “new” behavior is at mid-rapidity – detailed comparison with pp and pA required.

130 GeV

Energy Dependence and Comparison to pp•Width increases with Ecm

•Increase = ybeam

•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

Summary

Energy and Centrality Dependence of Mid-Rapidity Multiplicity has Constrained Models and given Insight into Interplay of Different Processes

Shapes of Multiplicity Distributions show Scaling in Fragmentation Region illustrating Common Mechanism for Particle Production which Evolves to Features Unique to HI Situation at Mid-Rapidity

To Come:

Shapes versus Centrality at 200 GeV

Multiplicity at 20 GeV

pp Data with PHOBOS at 200 GeV