Charged-particle dN/dh from PHOBOS

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Charged-particle dN/dh from PHOBOS

• A. H. Wuosmaa
• (Argonne National Laboratory)
• for the
• PHOBOS Collaboration

Quark Matter 2001
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The PHOBOS Collaboration
ARGONNE NATIONAL LABORATORY Birger Back, Nigel
George, Alan Wuosmaa BROOKHAVEN NATIONAL
LABORATORY Mark Baker, Donald Barton, Alan
Carroll, Stephen Gushue, George Heintzelman,
Robert Pak, Louis Remsberg, Peter Steinberg,
Andrei Sukhanov INSTITUTE OF NUCLEAR PHYSICS,
KRAKOW Andrzej Budzanowski, Roman Holynski,,
Jerzy Michalowski, Andrzej Olszewski, Pawel
Sawicki , Marek Stodulski, Adam Trzupek, Barbara
Wosiek, Krzysztof Wozniak MASSACHUSETTS INSTITUTE
OF TECHNOLOGY Wit Busza , Patrick Decowski,
Kristjan Gulbrandsen, Conor Henderson, Jay Kane ,
Judith Katzy, Piotr Kulinich, Johannes
Muelmenstaedt, Heinz Pernegger, 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 NATIONAL CENTRAL UNIVERSITY,
TAIWAN Willis Lin, JawLuen Tang UNIVERSITY OF
ROCHESTER Erik Johnson, Josh Hamblen, Nazim Khan,
Steven Manly, Inkyu Park, Wojtek Skulski, R.
Teng, Frank Wolfs UNIVERSITY OF ILLINOIS AT
CHICAGO Russell Betts, Clive Halliwell, David
Hofman, Burt Holzman, Wojtek Kucewicz, Don
McLeod, Rachid Nouicer, Michael Reuter UNIVERSITY
OF MARYLAND Richard Bindel, Edmundo Garcia-Solis,
Alice Mignerey spokesperson
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Why study dNch/dh ?

• dNch/dh is sensitive to all aspects of
  charged-particle production in heavy-ion
  collisions
• Interplay between hard and soft processes
• Effects of shadowing, jet quenching
• Reaction dynamics, re-scattering
• Full distribution reflects a time integral of
  particle production throughout the collision and
  total entropy production
• Lots of existing data for pp, pA, AB,AA
• How do RHIC data fit into this picture?

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Charged-particle Multiplicity in Au-Au at RHIC
Energies
Influence of various physical effects on dNch/dh
in very central collisions
(Wang Gyulassy, Private communication)
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The PHOBOS Detector
Octagon Multiplicity Detector
All multiplicity detectors are silicon pad sensors
Ring Multiplicity Detectors
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Coverage of multiplicity detectors in h
Schematic Multiplicity Detector
h coverage for vertex at Z0.
h
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PHOBOS Multiplicity detectors
Octagon
1 of 6 Rings
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What I will not discuss

• Event Selection
• Vertex Determination
• Centrality Determination
• See Talks by
• R. Pak, Tuesday 320
• J. Katzy, Wednesday 440
• And see Poster by
• P. Decowski

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DE deposition in multiplicity detectors for 1
event.
f
h

• Count hits binned in h, centrality (b)
• Calculate acceptance A(ZVTX) for that event
• Find the occupancy per hit pad O(h,b)
• Fold in a background correction factor fB(h,b)

dNch
O(h,b) fB(h,b)
Shits

dh
A(ZVTX)
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Measuring the occupancy
Method Assume Poisson statistics
0-3
Octagon
(central)
Rings
Ntracks/hit pad
50-55
(peripheral)
h
Nnumber of tracks/pad m mean number of
tracks/pad
The numbers of empty, and occupied,
pads determine the occupancy as a function of h,b
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Discriminating background with DE
Monte Carlo
Data
DE (MIP)
h
h
Not from vertex
Si
DE vs. h in the Octagon
From vertex
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Estimating remaining backgrounds
MC, Occupancy corrected
600
dNch/dh
400
MC truth
fBMCTruth/MCOcc
200
h
Compare PHOBOS Monte Carlo data analyzed
using occupancy corrections to truth - the
difference gives corrections for remaining
background.
fB(h,b)
h
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dNch/dh for different centrality bins
dNch/dh
45-55
35-45
25-35
dNch/dh
15-25
6-15
0-6
h
h
h
Statistical Unc. only
PHOBOS Prelim.
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Centrality Dependence of Nch(hlt5.4)
HIJING
Nch(hlt5.4)
Npart
10 Systematic Uncertainty
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(No Transcript)
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Shapes of dNch/dh for different Npart
Mean Npart
s
HIJING
0-3
Data
354
15-20
216
35-40
dNch/dh
102
dNch/dh
Data
HIJING
(dNch/dh)/(½Npart)
(dNch/dh)/(½Npart)
h
h
Systematic error (10-20)
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Centrality dependence of dNch/dhh
Solid lines HIJING
Symbols
Errors are systematic
h
lt1
2-2.4
(dNch/dh)/(½Npart)
3-3.4
4-4.4
5-5.4
Npart
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Summary

• First multiplicity distributions over 4p now
  available at for
  wide range of impact parameters
• Nch(hlt5.4)4100410 for the 3 most central
  collisions
• Distributions are somewhat wider than predicted
  by some models
• (dNch/dh)/(½Npart) in fragmentation region drops
  by ½ from Npart100 to 350
• Outlook coming analyses EbyE, d2N/dhdf (I.
  Park, Wed. 555)

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The appetizers were truly delightful, and I am
anxiously looking forward to the main course
-N. Khrushchev
Or -
We cant wait for 200 GeV!
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Backup slides to follow
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Data and Monte Carlo
Comparison of DE measured in the Octagon at
h3.0, with spectrum predicted by
Monte-Carlo Conclusion MC is a good predictor
of background.
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(dNch/dh)/(½Npart)
h
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Occupancy reality check
Backup Method Study DE spectra
Can determine from DE relative yields of events
with 1,2,3 hits/pad gives independent
measurement of Occupancy
Good fewer potentially unjustified
assumptions Bad Fitting procedure somewhat
unstable, occasional large errors
Very Good We get the same answer in the end!!
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Comparison of all methods at mid rapidity
MULT ARRAY
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pp data from ISR
W. Thome et al., Nucl. Phys. B129 (1977) 365.
h