Anomalous centrality variation of minijet angular correlations in Au-Au collisions at 62 and 200 GeV from STAR

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Anomalous centrality variation of minijet angular
correlations in Au-Au collisions at 62 and 200
GeV from STAR

• Michael Daugherity
• University of Texas at Austin
• for the STAR Collaboration

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Overview

• We report a survey of minimum-bias two-particle
  correlations in AuAu collisions
• These are sensitive to minijets, elliptic flow,
  resonances, HBT, etc. allowing a novel comparison
  of correlation amplitudes and ranges
• Each correlation source has a unique distribution
  on relative (?,f) making decomposition possible
• We observe a surprising trend in minijet
  correlations

minijet Same-side jet-like correlations with no
trigger particle
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Correlation Measure

• We use the correlation definition

• and measure pair density ?
• Covariance object - uncorrelated reference
  (mixed-event pairs)
• The denominator provides per-particle
  normalization
• Correlations are directly comparable regardless
  of event multiplicity
• This is done with all possible pairs
• No trigger particle is specified.

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ProtonProton Components
J of Phys Conf 27 (2005) 98
axial correlations
f?
??
STAR Preliminary
These structures define minijet correlations
Longitudinal Fragmentation (strings) 1D
Gaussian, US pairs HBT peak at origin, LS pairs
Minijets 2D Gaussian at origin, away-side peak
actually cos(f?)
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Fit Function (in 5 Easy Pieces)
Proton-Proton fit function
STAR Preliminary


f?
f?
f?
??
??
??
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200 GeV Data
proton-proton
Analyzed 1.2M minbias 200 GeV AuAu events, and
13M 62 GeV minbias events (not shown) Included
all tracks with pT gt 0.15 GeV/c, ? lt 1, full f
note 38-46 not shown
f?
??
STAR Preliminary
f?
??
We see the evolution of correlation structures
from peripheral to central AuAu
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200 GeV Model
Fit model
f?
??
STAR Preliminary
f?
??
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200 GeV Residual
Fit residual data - model
f?
??
STAR Preliminary
f?
??
We have a good fit with the simplest possible fit
function. Other than adding the cos(2f?)
quadrupole term, no other modification was
necessary. Residuals at 62 GeV are comparable.
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Quadrupole Component
Instead of removing a background, we can make a
measurement
Data
cos(2f?) component
Amplitudes
200 GeV 62 GeV

• 62 and 200 have the same shape
• Substantial amp. change with energy

f?
f?
??
??
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Minijet Same-Side Peak
Peak Amplitude
Peak ? Width
Peak f Width
Statistical and fitting errors shown Systematic
error is 9 of correlation amplitude
STAR Preliminary
STAR Preliminary
STAR Preliminary
200 GeV 62 GeV
X-axis shows mean participant path-length

• Observations
• Amplitude and ? widths start small and
  experience a sharp transition
• Transition occurs at 55 centrality at 200 GeV,
  is more central (40) for 62
• f width has a very different centrality
  dependence

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Binary Scaling
Peak Amplitude
Peak ? Width
Peak f Width
STAR Preliminary
STAR Preliminary
STAR Preliminary
200 GeV 62 GeV
constant widths
small increase before transition
Large increase in amplitude and ? width are
unexpected
minijet amplitude assuming binary scaling in
Kharzeev and Nardi model
Deviations from binary scaling represent new
physics unique to heavy ion collisions
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HIJING Minijets
Peak Amplitude
Peak ? Width
Peak f Width
STAR Preliminary
STAR Preliminary
STAR Preliminary
200 GeV 62 GeV
HIJING 1.382 default parameters, 200 GeV, quench
off Quench on causes slight amplitude decrease
The observed minijets correlation is actually far
greater than predicted by HIJING (factor of 4)
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Consistency Check
Does interaction between same-side peak and
cos(f?) terms cause the transition?
Two-stage fit
STAR Preliminary
The results are consistent Cancellation in fit
terms does not cause the amplitude increases.
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Transition
Does the transition from narrow to broad ?? occur
quickly or slowly?
data - fit (except same-side peak)
83-94
55-65
STAR Preliminary
STAR Preliminary
?? width
Shape changes little from peripheral to the
transition
The transition occurs quickly
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Scaling
What is the best way to compare different
energies? Does the transition scale?
Peripheral bins are compressed.
Minijet parameters seem to scale with system
density
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Yield Estimates
See also T. Trainor, arXiv0710.4504, accepted to
J Mod Phys E
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Summary

• We measure 2D angular autocorrelations on (?,f)
  to study minijets in AuAu collisions and make
  two primary observations
• Transition
• Minijet correlations follow binary scaling in
  peripheral AuAu collisions and deviate at a
  sharp transition point
• The transition points for 62 and 200 GeV occur
  at about the same value of transverse particle
  density
• Yield
• Beyond the transition point the peak amplitude
  and ? width increase dramatically
• The same-side correlations include a large
  number of hadrons, estimated at up to 1/3 of the
  particle yield in central AuAu collisions

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Backup Slides
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Yt-Yt Correlations
proton-proton
By looking in yt-yt space we can find where these
new correlations are in momentum
Corresponding to excess minijet amplitude and
width, we see a sudden onset of localized growth
on yt lt 2.5 ? pt lt 0.8
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Away-side
STAR Preliminary
??
??

• Momentum conservation for an (other-wise)
  independent-particle many-body system produces a
  cos(f?) component whose amplitude is constant for
  our measure.
• Instead, the amplitude exactly follows the
  minijet peak amplitude

We have found the away-side jet! Instead of
suppression we can now study momentum transport
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Away-side
Since f is periodic, any AS Gaussian at f?p will
also need to copied to f? -p and f? 3p. I
tried replacing cosine term with Gaussians at odd
multiples of p, and letting ROOT freely fit the
amplitude and width. The result was
indistinguishable from a cosine.

• The data demand a cos(f?) shape.
• A model with away-side Gaussians is
  statistically indistinguishable, though it has
  one extra parameter.

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Longitudinal Fragmentation
We follow the 1D Gaussian along ?? which is seen
in p-p collisions and described by the Lund
String Model
amplitude
width
62 200

• But from studying the charge-dependence, we know
  that we are seeing two distinct processes.
• longitudinal fragmentation soft hadronization
  process, charge-dependent, energy-independent,
  monotonically decreasing with centrality.
• A second process that is charge-independent,
  energy-dependent, non-monotonic with centrality.
  Seems to have constant mean at nu 3.5, width
  decreases with energy
• The other charge-dependent results are
  interesting, but thats another story.

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Finer Centrality Bins
Fine bins test, 200 GeV, all parameters
As a test, I tried offsetting and doubling the
number of centrality bins around the step for 200
GeV. Here the red points are the fine bins
test
Lets zoom in on the minijet parameters
Results are consistent and support a sudden
turn-on
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Why minijets?

• We hypothesize that the transition is due to
  particles associated with minijets rather than a
  new physical process
• The F? width doesnt change at the transition
• The pT correlation amplitude trend doesnt
  change, the widths change slightly

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What about the sharp peak?

• Many different processes compete near f? ??
  0
• ee- pairs from ? conversion (big)
• HBT / Coulomb correlations (big)
• resonance decay (small, lt10 of peak)
• tracking inefficiencies splitting, merging,
  crossing, etc. (small)
• high pt jets (very small for minbias)
• plus other structures that we want

• Four general approaches
• kill these structures with cuts
• use simulations to model the big contributors
• ignore the bins near the origin
• fit a sharp exponential

I tried them all and got the sensible result No
matter how you handle the bins near the origin
the large-scale structure doesnt change Or,
what happens at the origin, stays at the origin