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Collision Geometry Scaling of Mid-Rapidity Charged Particle Multiplicity in PHOBOS from
√sNN = 19.6 to 200 GeV
Aneta Iordanova
University of Illinois at Chicago
For the collaboration
DNP04/Chicago
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 (October 2004)
Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Abigail
Bickley,
Richard Bindel, Wit Busza (Spokesperson), Alan Carroll, Zhengwei Chai, Vasundhara Chetluru,
Patrick Decowski, Edmundo García, Tomasz Gburek, Nigel George, Kristjan Gulbrandsen,
Clive Halliwell, Joshua Hamblen, Ian Harnarine, Conor Henderson, David Hofman, Richard Hollis,
Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane, Nazim Khan, Piotr Kulinich,
Chia Ming Kuo, Wei Li, Willis Lin, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen,
Rachid Nouicer, Andrzej Olszewski, Robert Pak, Heinz Pernegger, Corey Reed, Christof Roland,
Gunther Roland, Joe Sagerer, Iouri Sedykh, Wojtek Skulski, Chadd Smith, 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 combination 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
formed by rotating Inner Vertex Detector layers 1800 about the beam pipe
corrects for: azimuthal acceptance of the
detector tracklet reconstruction
efficiency secondary decays
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)
• ‘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.
Measured pseudorapidity density per participant pair as a function of <Npart>
• Model 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>
• 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
• Final 1- systematic and statistical error – Centrality dependent
• central events 3%
• mid-central events 7%
Ratio of the two data sets –systematic and statistical errors
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
Phys. Rev. C70, 021902(R)(2004)
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(