Energy dependence of elliptic flow over a large pseudorapidity

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Energy dependence of elliptic flow over a large
pseudorapidity range in AuAu collisions at RHIC
Steven Manly University of Rochester Representing
the PHOBOS Collaboration
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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
Holynski, 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 Wozniak, Alan
Wuosmaa, Bolek Wyslouch ARGONNE NATIONAL
LABORATORY BROOKHAVEN NATIONAL
LABORATORY INSTITUTE OF NUCLEAR PHYSICS,
KRAKOW MASSACHUSETTS INSTITUTE OF
TECHNOLOGY NATIONAL CENTRAL UNIVERSITY,
TAIWAN UNIVERSITY OF ILLINOIS AT
CHICAGO UNIVERSITY OF MARYLAND UNIVERSITY OF
ROCHESTER
Collaboration meeting, BNL October 2002
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Flow patterns in the energy, momentum, or
particle density distributions that we use to
ferret out clues as to the nature of the
collision/matter
To what extent is the initial geometric asymmetry
mapped into the final state?
View along beamline
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Flow as an experimental probe

• Sensitive to interaction strength

View along beamline

• Sensitive to very early times and particle
  velocities since asymmetry disappears with time

• With sufficient ? coverage, it probes
  longitudinal uniformity of system

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Flow quantified
View along beamline
dN/d(f -YR ) N0 (1 2V1cos (f-YR) 2V2cos
(2(f-YR) ... )
Fourier decomposition of the azimuthal
multiplicity distribution
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Flow quantified
View from above
View along beamline
dN/d(f -YR ) N0 (1 2V1cos (f-YR) 2V2cos
(2(f-YR) ... )
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Flow quantified
View along beamline
dN/d(f -YR ) N0 (1 2V1cos (f-YR) 2V2cos
(2(f-YR) ... )
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Flow quantified
View along beamline
dN/d(f -YR ) N0 (1 2V1cos (f-YR) 2V2cos
(2(f-YR) ... )
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n2, elliptic flow
View along beamline
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Flow at RHIC to date (a few highlights)
Elliptic flow is large near ?0 (relative to
hydro limit)
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Flow at RHIC to date (a few highlights)
Elliptic flow is large near ?0 (relative to
hydro limit)
V2(pT) grows with pT at low pT, consistent with
hydro
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Flow at RHIC to date (a few highlights)
Elliptic flow is large near ?0 (relative to
hydro limit)
V2(pT) grows with pT at low pT, consistent with
hydro
V2(pT) saturates at high pT
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Flow at RHIC to date (a few highlights)
Elliptic flow is large near ?0 (relative to
hydro limit)
V2(pT) grows with pT at low pT, consistent with
hydro
V2(pT) saturates at high pT
nucl-ex/0306007
Xhangbu Xu, Quark Matter 2004
Partonic energy loss plus quark coalescence may
explain saturation and meson-baryon difference
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Flow at RHIC to date (a few highlights)
Elliptic flow is large near ?0 (relative to
hydro limit)
V2(pT) grows with pT at low pT, consistent with
hydro
V2(pT) saturates at high pT
Partonic energy loss plus quark coalescence may
explain saturation and meson-baryon difference
Elliptic flow falls off sharply as a function of
?
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Flow at RHIC to date (a few highlights)
Elliptic flow is large near ?0 (relative to
hydro limit)
V2(pT) grows with pT at low pT, consistent with
hydro
V2(pT) saturates at high pT
Partonic energy loss plus quark coalescence may
explain saturation and meson-baryon difference
Elliptic flow falls off sharply as a function of
?
n?2 terms observed
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Flow at RHIC to date (a few highlights)
Elliptic flow is large near ?0 (relative to
hydro limit)
V2(pT) grows with pT at low pT, consistent with
hydro
V2(pT) saturates at high pT
Partonic energy loss plus quark coalescence may
explain saturation and meson-baryon difference
Elliptic flow falls off sharply as a function of
?
n?2 terms observed
Systematic study of v2(E,?) probes the
longitudinal dynamics of the collision
This work ?
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Flow in PHOBOS
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Large ? coverage
Data at 19.6, 62.4, 130 and 200 GeV
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Flow basic method

• Subevent technique correlate
  event plane in one part of detector to ?
  asymmetry in track pattern in other part of
  detector
• Correct for imperfect reaction plane resolution

? dependence of the multiplicity
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Pixelized detector
Hit saturation, grows with occupancy Sensitivity
to flow reduced
Can correct using analog energy deposition or-
measure of occupied and unoccupied pads in local
region assuming Poisson statistics
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Azimuthally symmetric backgrounds
flow signal
Dilutes the flow signal

• Remove Background
• Estimate from MC and correct

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Background suppression
Demand energy deposition be consistent with angle
Works well in Octagon
Technique does not work in rings because angle of
incidence is 90?
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Hit-based method
Subevents for reaction plane evaluation
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Flow method continued
Determine event plane in each subevent, ?2
Method from Poskanzer and Voloshin, Phys. Rev.
C58 (1998) 1671
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Flow method continued
Correlate ?2 with hits outside of given subevent
to get raw v2
Method from Poskanzer and Voloshin, Phys. Rev.
C58 (1998) 1671
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Flow method continued
Determine event plane resolution by correlating
?2 and ?2-
Method from Poskanzer and Voloshin, Phys. Rev.
C58 (1998) 1671
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Flow method continued
Correct raw v2 by resolution (factor of 1.7 to 3
depending on energy and centrality, well
understood) Correction determined from data
Method from Poskanzer and Voloshin, Phys. Rev.
C58 (1998) 1671
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Flow method continued
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Flow method continued

• Have agreement between
• Two hit-based analyses ? one holy, one not
• Track-based analysis with NO background

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v2 vs. ? (four energies)
Bars are 1? statistical errors, expect some
correlation
(0-40 central AuAu data)
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v2 vs. ? (four energies)
Boxes are 90 C.L. systematic errors
(0-40 central AuAu data)
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v2 vs. ? (four energies)
Shape is triangular at all four energies, no
evidence of plateau
(0-40 central AuAu data)
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v2 vs. ? (four energies)
Drop highest ? points at 19.6 GeV in following
results
(0-40 central AuAu data)
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Systematic errors
Hit definition Beam orbit/alignment Subevent
definition Transverse vertex position cut Bins
for weighting matrix definition Dead channel
correction algorithm Poisson occupancy correction
algorithm Hole filling alogorithm Knowledge of
azimuthally symmetric background dN/d?
shape Symmetry in ?
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v2 vs. ? (four energy overlay)
Preliminary
AuAu data
(0-40 central)
Only statistical errors shown
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Evolution of v2 with energy
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Limiting fragmentation
Take out differing beam boosts by going into
approximate frame of reference of target Look at
? scaling
PHOBOS AuAu results PRL 91, 052303 (2003)
limiting fragmentation ? energy independence in
??-ybeam
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y vs. ?
Boost invariant spectra transform as
Jacobian suppresses spectra at low ?, low pT, and
for large mass
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y vs. ? effect on multiplicity
dN/d?
dN/dy
0
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y vs. ? effect on v2
P. Kolb, Proc. of 17th Winter Workshop on Nuclear
Dynamics (2001)
Near mid-rapidity, integration over pT weights
flow to higher pT due to suppression at low pT ?
v2(?) larger than v2(y)
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y vs. ? effect on v2
V2(?)
V2(y)
0
No change in the qualitative features of the
result (lt15 at ?0)
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Limiting fragmentation and elliptic flow
Preliminary
AuAu data
(0-40 central)
Only statistical errors shown
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Limiting fragmentation and elliptic flow
Preliminary
AuAu data
(0-40 central)
??-ybeam
Only statistical errors shown
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Conclusions
No boost invariant plateau over a broad region of
?
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Conclusions
No boost invariant plateau over a broad region of
?
Linear logarithmic growth with center-of-mass
energy in differing regions of ?
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Conclusions
No boost invariant plateau over a broad region of
?
Linear logarithmic growth with center-of-mass
energy in differing regions of ?
No sharp changes in the dynamics of particle
production in pseudorapidity or beam energy
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Conclusions
No boost invariant plateau over a broad region of
?
Linear logarithmic growth with center-of-mass
energy in differing regions of ?
No sharp changes in the dynamics of particle
production in pseudorapidity or beam energy
Preliminary
AuAu data
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