qjlQM02

1
Dissipation and Fragmentation of low-Q2 scattered
partons at RHIC R. L. Ray University of Texas
at Austin (STAR Collaboration)
Transverse Momentum Fluctuations Correlations
Correlations in 200 GeV pp as a Reference for AA
Conventional pQCD Parton Energy Loss
large ?pt? fluctuations in the STAR acceptance
How are low-Q2 partons affected by the A-A medium?
high-pt partons probe a thermalized medium
A-A collision with single hard scattering

• point interactions
• binary collisions
• gluon bremsstrahlung
• RAA, away-side jet
• high gluon density

excess width
high-pt partons
Q/2 1-5 GeV
data
B
width increased by 13.71.3(syst).
?
hard
soft
?
A
Gamma dist. reference
?
trigger
STAR preliminary
?
centrality dependence
fragmentation
color energy loss
jet quenching
but
data - reference
a calibrated probe
?
correlations
what happens to the medium?
In units of expected rms from CLT
color-string fragmentation, charge
ordering, local momentum conservation which may
be distorted by the medium
Centered at 0
semi-hard parton scattering, parton
fragmentation, minijets probe the medium

• Two-Component Model
• Longitudinal color string fragmentation
• low pt particle production
• Transverse semi-hard parton scattering and
• fragmentation (i.e. breakup into hadrons)
• Local charge and momentum conservation

AuAu at 130 GeV nonstatistical ?pt? fluctuations
what happens to low-pt partons?
?
A
B
fluctuations on bin size source revealed
J. Adams et al. (STAR), Phys. Rev. C 71, 064906
(2005).
AuAu at 200 GeV
inclusive pt spectrum
angular correlation
Parton dissipation is studied with low-Q2 partons
to probe non-pQCD medium effects
?pt? fluctuations and corresponding pt
correlations from low-Q2 partons provide
information on the velocity structure of the
parton-medium system.
Number correlations on (h,f) reveal strong
variation of the fragmentation process with A-A
centrality, depending on the state of the
underlying medium.
(h,f) Correlations Subtract cos(fD) and cos(2fD),
Centrality Dependence of Same-Side Peak Structure
Number Correlations for 130 GeV Au-Au on
(h1-h2,f1-f2)
2D autocorrelations retain all angular
correlation info.
(flat on hD) to reveal minimum-bias parton
fragments
strong variation of amplitude and widths with
centrality

• Notable Results
• Absence of away-side soft
• string fragmentation structure
• beginning in most peripheral bin
• studied here.
• Strong elongation along hD
• Narrowing along fD

STAR preliminary
All Charges
near-side peak amplitude scales linearly with
nucleon path length peak volume however
increases faster than linear
central
Central
p-p lowest Q2
(correlation amplitude per final-state hadron)
amplitude volume
widths
amplitude
sh
Relative widths reversed for central Au-Au compare
d to p-p
peak amplitude
p-p minbias
Features peak at small relative angles
cos(fD) momentum conser-
vation at low pt cos(2fD) elliptic anisotropy
J. Adams et al. (STAR), nucl-ex/0411003
J. Adams et al. (STAR), nucl-ex/0411003
STAR preliminary
11 aspect ratio
Peripheral
sf
peak volume
Linear ampl. increase
Factor 2.7 increase
300k events 0.15 lt ptlt2 GeV/c hlt1.3, full
f2p merging HBT cuts applied
p-p 200 GeV reference
Au-Au mid-central
peripheral
(h,f) Correlations for 62 GeV Au-Au
Minijet Deformation Possible Interpretations
Multiple Minijets in A-A Collisions
comparisons with pQCD dissipation models
evolution of parton and string fragments,
elliptic flow
autocorrelation technique combines multiple
sources
p-p 200 GeV
STAR preliminary
Rather than studying conditional distributions
using one high pt trigger particle, we sum all
pairs into an autocorrelation which integrates
multiple event-wise sources into a single
structure, thus increasing statistical
sensitivity to e.g. minijets.
pQCD flowing medium
phenomenology
Au
0.15 lt ptlt2 GeV/c hlt1.0, full f2p merging
HBT cuts applied
Correlation structure evolves strongly but
smoothly from p-p to central Au-Au
Soft, away-side recoil, cos(fD)
Gluon bremsstrahlung/ medium dragging
calculations (Armesto, Salgado, Wiedemann,
hep-ph/0405301)
No trigger particle
minijet minimum-bias jet
Peripheral
minijet
Construct two-particle autocorrelations using
all particle pairs
100 GeV jet
Au
Interaction with longitudinally expanding color
fluid drags pre-hadronic matter associated with
semi-hard partonic scattering along
pseudorapidity.
F?
F?
F?
F?
F?
??
??
??
??
??
we estimate 20-30 minijets per central Au-Au
collision in hlt1 compared to 1 per 50 p-p
collisions
STAR preliminary
min-bias Hijing
Central
pQCD models underpredict h broadening
Hijing-1.37 Au-Au 200 GeV with without jet
quenching same-side peak widths increase (Liu,
Prindle, Trainor hep-ph/0410180)
F?
F?
F?
F?
F?
??
??
??
??
??
?pt? Fluctuations and pt Correlations on (hD,fD)
Compare with p-p pt Autocorrelations
Data and Monte Carlo
peak amplitudes
peak widths
fluctuation inversion reveals physical mechanisms
cross checks with p-p direct pair counting
confirm method
pt correlations from pQCD models do not
reproduce structures in Au-Au collisions
p-p 200 GeV minbias
p-p 200 GeV nch gt 9
full STAR acceptance
partons and velocity correlations
CILSUS
STAR preliminary
STAR preliminary
70-80
direct from pair ratios
autocorrelation
variance excess
centrality
Hijing dotted, dash-dot lines
?pt? fluctuations
Hijing Monte Carlo
RHIC data
200 GeV
Hijing Au-Au 200 GeV
f
70-80
h
20-30
70-80
data Au-Au 200 GeV
quench on
quench off
CDLS-US
fluctuation inversion
subtract multipoles
fluctuation scaling
centrality
0-5
0-10
0-10

• Minijets velocity/temperature correlation
  structures on (h,f)
• Strong elongation on h and new negative same-side
  structure

fast
from fluctuation inversion
pair counting is costly 5-10? slower for minbias
Au-Au
STAR preliminary
?pt? Fluctuations SPS ? RHIC
Summary
What might a central Au-Au collision look like?
strong energy dependence reflects partonic origins
The bulk medium is locally equilibrated, highly
dissipative, strongly self-interacting, with very
high gluon density (gluon dominated) but the
events appear to be peppered with
strongly-modified minijets

• We study how the measured correlation structures
  in p-p collisions evolve with centrality in Au-Au
• The origin of ?pt? fluctuations is revealed using
  pt ? pt correlations and is driven by pt gt 0.5
  GeV/c components
• Same-side charge-averaged number and pt
  correlations strongly elongate on h and narrow on
  f
• Observations are consistent with a picture in
  which
• semi-hard parton scattering transfers pt to the
  bulk, inducing
• local temp/velocity correlations
• correlated particles are carried by the
  longitudinally
• expanding medium.

central collisions scale dependence
full acceptance
A possible view of the deconfined system at
fixed time in the lab of about 5 fm/c after
initial impact
0-5
STAR
STAR
centrality
CERES NP A 727, 97 (2003)
STAR preliminary
dramatic increase of ?pt? fluctuations with
increasing ?sNN
STAR
20-30 minijets based on correlations per
final-state particle in Au-Au assuming 2-3
hadrons per minijet from p-p colors indicate
fast color-charge carriers, mainly
gluons... gluonic liquid
Hijing
CERES
Poster xxx, Quark Matter 2005
Supported in part by the Office of Science of the
U. S. Department of Energy