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

Flow in PHOBOSFlow in PHOBOS

coverage

Data at 19.6, 130 and 200 GeV

1m2m

5m

0 1 2 3 4 512345

coverage for vtx at z=0

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

Poisson occupancy weighting

)1ln(

1),(

unocc

occ

N

N

eOcc

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

RingsN Octagon RingsP

Spec holes

Vtx holes

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