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Update on flow studies with PHOBOS
S. Manly
University of Rochester
Representing the PHOBOS collaboration
Flow Workshop
BNL, November 2003
The Phobos CollaborationThe Phobos Collaboration
Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Bruce Becker, Russell Betts,
Abigail Bickley, Richard Bindel, Andrzej Budzanowski, Wit Busza (Spokesperson), Alan Carroll,
Zhengwei Chai, Patrick Decowski, Edmundo Garcia, Tomasz Gburek, Nigel George,
Kristjan Gulbrandsen, Stephen Gushue, Clive Halliwell, Joshua Hamblen, Adam Harrington,
Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova,
Erik Johnson, 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, Michael Ricci, 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, Siarhei Vaurynovich,
Robin Verdier, Gábor Veres, Edward Wenger, Frank Wolfs, Barbara Wosiek,
Krzysztof Wožniak, Alan Wuosmaa, Bolek Wysłouch, Jinlong Zhang
ARGONNE NATIONAL LABORATORYARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORYBROOKHAVEN NATIONAL LABORATORYINSTITUTE OF NUCLEAR PHYSICS, KRAKOWINSTITUTE OF NUCLEAR PHYSICS, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGYMASSACHUSETTS INSTITUTE OF TECHNOLOGY
NATIONAL CENTRAL UNIVERSITY, TAIWANNATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGOUNIVERSITY OF ILLINOIS AT CHICAGOUNIVERSITY OF MARYLANDUNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTERUNIVERSITY OF ROCHESTER
Pixelized detector
Hit saturation, grows with occupancy
Sensitivity to flow reduced
Can correct using analogue energy deposition
–or-
measure of occupied and unoccupied pads in local region assuming Poisson statistics
Poisson occupancy Poisson occupancy correctioncorrection
Acceptance (phase space) weighting
Octagonal detector
Require circular symmetry for equal phase space per pixel
Pixel’s azimuthal phase space coverage depends on location
Relative phase space weight in annular rings = <Nocc>-1
z
Dilutes the flow signal
Remove Background
Estimate from MC and correct
flow signal
Non-flow background
+
Non-flow Backgrounds
Background suppression
Works well in Octagon
dE
(keV)
cosh
Background!
Technique does not work in rings because angle of incidence is ~90
Beampipe
Detector
Demand energy deposition be consistent with angle
Determining the collision point
High Resolution
extrapolate spectrometer tracks
Low Resolution
octagon hit density peaks at vertex z
position
RingsN Octagon RingsP
Spec holes
Vtx holes
Detector symmetry issues where SPEC vertex efficiency highest
Most data taken with trigger in place to enhance tracking efficiency
Strategies:
Avoid the holes – Offset vtx method
PHOBOS flow analyses based on subevent technique
Poskanzer and Voloshin, Phys. Rev. C58 (1998) 1671.
Azimuthal symmetry is critical
Use the holes – Full acceptance method
Use a different type of analysis, such as cumulants
Track-based Track-based analysis:analysis:
Avoids holes for reaction plane determination
Uses tracks passing into spectrometer
Hit-based analysesHit-based analyses
RingsN Octagon RingsP
Offset vtx method
Limited vertex range along z
Subevents for reaction plane evaluation
Good azimuthal symmetry
Fewer events, no 19.6 GeV data
Gap between subevents relatively small
Technique used for published 130 GeV data
RingsN Octagon RingsP
Full acceptance method
Vertex range -10<z<10
Subevents for reaction plane evaluation vary with analysis
Good statistics, 19.6 GeV data in hand
Gap between subevents large
Requires “hole filling”
Dealing with the holes
RingsN Octagon RingsP
Inner layer of vertex detector fills holes in top and bottom. Must map hits from Si with different pad pattern and radius onto a “virtual” octagon Si layer
Dealing with the holes
RingsN Octagon RingsP
Fill spectrometer holes by extrapolating hit density from adjoining detectors onto a virtual Si layer. (Actual spec layer 1 is much smaller than the hole in the octagon.)
RingsN Octagon RingsP
Track-based method
Vertex range -8<z<10
Subevents for reaction plane
Momentum analysis
200 GeV data
Gap between tracks and subevents large
Little/no background
Vertex measurement
Reaction plane determined by hits in widely separated subevent regions, symmetric in ,
Track-based method – detector space
Correlate tracks in spectrometer to reaction plane to determine v2
Track-based method – detector space
A question to this workshop:
Are there non-flow correlations that stretch across 3-6 units of ?
vz(cm)
Full acceptance v1: sep=6
Full acceptance v2: sep=5.2
Offset vertex v2: sep=0.2-1.0
Track-based analysis
v2 vs. centrality and energy
Preliminary v2200
Final v2130
PHOBOS Au-Au
130 GeV result: PRL 89:222301, 2002
||<1
200
130
v 2
<Npart>
v2200 (hit)
v2200 (track)
PHOBOS Preliminary
200 GeV Au-Au
v2 vs. centrality, method comparison
||<1
trackhit
v 2
<Npart>
PHOBOS preliminary
h+ + h-
200 GeV Au-Au
track-weighted centrality averaging
0<<1.5
(top 55%)
v 2
17% scale error
v2 vs. pT
v2 vs. and energy
Preliminary v2200
Final v2130
Hit-based result
v2200 & v2
130 similar
PHOBOS Au-Auv 2
200 130
<Npart>~190
130 GeV result: PRL 89:222301, 2002
A. Poskanser showed in his talk that STAR agrees with the PHOBOS v2(). It will be interesting to see if it is possible to deconvolute the STAR and BRAHMS results in the forward region to determine what fraction of the drop in v2() comes from dN/dpT and what fraction comes from v2(pT).
Directed flow: MC analysis, resolution and background corrected, used event
plane from 1st harmonic
Input flow
A little Quark Matter preview
Preliminary directed flow sensitivity
PHOBOS preliminary
h+ + h- Au-Au data
A little Quark Matter preview
Flow at PHOBOS: What’s new?Flow at PHOBOS: What’s new?
200 GeV analyses200 GeV analyses
Finalizing systematics
Plan to release soon final results in 3 bins of centrality
Directed flow (vDirected flow (v11))
Still optimizing analysis and working to understand fine points of data analysis using full acceptance technique
Goal is to release preliminary v1() at 19.6, 130 and 200 GeV for Quark Matter