Measuring Mid-Rapidity Multiplicity in PHOBOS

Aneta Iordanova

University of Illinois at Chicago

For the collaboration

Outline

• Multiplicity Analysis Technique – Vertex Tracklet reconstruction method

• Results– Mid-rapidity charged-particle multiplicity and

its centrality dependence for 19.6 and 200GeV

– Compare the results with model predictions

• Conclusions

Collaboration

Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Abigail Bickley,

Richard Bindel, Wit Busza (Spokesperson), Alan Carroll, Zhengwei Chai, Patrick Decowski,

Edmundo García, Tomasz Gburek, Nigel George, Kristjan Gulbrandsen, Clive Halliwell,

Joshua Hamblen, Adam Harrington, Michael Hauer, Conor Henderson, David Hofman,

Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane, Nazim Khan,

Piotr Kulinich, Chia Ming Kuo, Willis Lin, Steven Manly, Alice Mignerey,

Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Inkyu Park,

Heinz Pernegger, Corey Reed, Christof Roland, Gunther Roland, Joe Sagerer,

Helen Seals, Iouri Sedykh, Wojtek Skulski, Chadd Smith, Maciej Stankiewicz, Peter Steinberg,

George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek, Carla Vale,

Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger,

Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Alan Wuosmaa, Bolek Wysłouch

ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORYINSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY

NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGOUNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER

Multiplicity measurement at mid-rapidity (||<1)

Vertex Detector

Top

Bottom

62.1mm

50.4mm

Z,

Beam pipe

1 channel

Y

X

8192 silicon channels • Outer Layer: 2 × 2048 channels, 0.47mm × 24.1mm• Inner Layer: 2 × 2048 channels, 0.47mm × 12.0mm

Inner Layer

Outer Layer

Inner Layer

Outer Layer

ReconstructedVertex

hit

hit

Top VertexTop Vertex

Tracklet Reconstruction

• Tracklet

Two-hit combinations from Outer and Inner Vertex (Top or Bottom), pointing to the reconstructed vertex.

• Reconstructed vertex– from Spectrometer

Detector

19.6GeV

x,y,z=0.3,0.3,0.4 mm (central)

x,y,z=0.6,0.5,0.8 mm (mid-central)

First Pass

Second Pass

Seed Layer

Search Layer

Reconstructed Vertex

hit

hit

Search ,Search

Extrapolate Seed ,Seed

• || = |Search – Seed| < 0.3

• || =|Search – Seed| < 0.1

• smallest combination.

Tracklets with a common hit in the “Search Layer”

•smallest combination.

Top VertexTop Vertex

Tracklet Reconstruction

etsted_tracklreconstruc1||charged 1

1N

d

dN

Acceptance + Efficiency Correction

Factor

Combinatorial Background

Multiplicity Determination

Acceptance and Efficiency Correction Factor

depends on:– Z-vertex position– multiplicity in detector

(hits)

Hits in Outer Vertex Layer / 20

19.6 GeV200 GeV

corrects for:– azimuthal acceptance of

the detector– tracklet reconstruction

efficiency– secondary decays

Combinatorial Background Correction Factor

• Combinatorial background:– formed by rotating Inner

Vertex Detector layers 1800 about the beam pipe

Z,

Beam pipe

Combinatorial Background Correction Factor

• Tracklets/Background for 80 to 100 Hits in Outer Vertex Layer, 19.6 GeV

• = Nbg_tracklets/Nreconstructed

– =0.76

Data

MC

Cou

nts

Cou

nts

Results

Centrality Determination

• Select the “same” regions at 200 and 19.6 GeV

• Have two centrality methods at each energy– One at mid-rapidity– One away from mid-rapidity

• Mechanism for comparing ‘like’ regions to see systematic effects

• Results presented here– for a) and c)

Regions are ‘matched’ according to the ratio of beam rapidities

(a) with (c) (b) with (d)

Measured pseudorapidity density per participant pair as a function of <Npart>

• ‘Geometry-normalized’ multiplicity in Au-Au collisions higher than corresponding values for inelastic

• Percentile cross-section– 0-50% for 200 GeV– 0-40% for 19.6 GeV

p)pp(

200 GeV (measured UA5)

19.6 GeV (interpolated ISR)pp

pp

90 % C.L.

• Models predictions– Hijing

• does not follow data trend

– Saturation Model (KLN)Phys.Lett.B523 79 (2001)

arXiv:hep-ph/0111315

• better agreement

90 % C.L.

Measured pseudorapidity density per participant pair as a function of <Npart>

Ratio of the two data sets – systematic errors

• Most of the systematic errors on the individual measurements at the two energies will cancel in the ratio– Analyses performed with the same method– Detector– Centrality determination

• Percentile cross-section used in ratio– top 40%

• Errors are estimated as 1-.

Ratio of the two data sets – systematic errors

• R– Most of geometry/efficiency

effects cancel in the ratio– Contribution from

secondary decays

• R– is found to be the same for

Data/MC for the two data sets– Uncertainty from measured y-

beam position

• RNpart– Nucleon-nucleon inelastic

cross-section– MC simulations of the detector

response– Glauber model calculations

R R19.6 R200 RNpart

2% 0.4% 0.4%

Ratio of the two data sets –systematic and statistical errors

• RNrec

– Counting statistics– Uncertainty in trigger efficiency

(centrality bin position)• central events 0%• mid-central events 6%

• Final 1- systematic and statistical error – Centrality dependent

• central events 3%

• mid-central events 7%R R19.6 R200 RNpart RNrec Rcentral

2% 0.4% 0.4% 2.2% 3.0%

Ratio for the data sets

• Data ratio– Au+Au1 (fixed fraction of

cross-section)

1- errors

Ratio for the data sets

• Data ratio– Au+Au1 (fixed fraction of

cross-section)• No centrality (geometry)

dependence

• R = 2.03 ± 0.02 ± 0.05 (simple scale-factor between 19.6 and 200GeV)

1- errors

Ratio for the data sets

• Data ratio– Au+Au1 (fixed fraction of

cross-section)• No centrality dependence

• R = 2.03 ± 0.02 ± 0.05

– Au+Au2 (fixed <Npart>)

• No centrality dependence

1- errors

Ratio for the data sets

• Models– Hijing

• increase in mid-rapidity with centrality

– Saturation Model (KLN)• flat centrality dependence

as in data

1- errors

http://xxx.lanl.gov/nucl-ex/0405027

Other ‘Geometry Scaling’

observations in

• Multiplicity– 200/130 GeV mid-rapidity

ratioPhys.Rev.C65 061901(R) (2002)

– 19.6-200GeV Nch/<Npart/2>

• Plot from QM 2002 talks

• Charged hadron pT spectra– Ratio of yield for 200 and 62.4 GeV is centrality independent for all

measured pT bins

Other ‘Geometry Scaling’

observations in

Conclusions

• We measured charged-particle pseudorapidity density at mid-rapidity for Au-Au collisions at 200 and 19.6GeV – Centrality, derived from different -regions for each of the two

Au-Au collision energies, yield consistent results

– An increase in particle production per participant pair for Au-Au compared to the corresponding values for collisions

– The ratio of the measured yields for the top 40% in the cross section gives a simple scaling factor between the two energies

p)pp(