Angular Correlations at High pt:

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Angular Correlations at High pt Jan Rak for the
Phenix Collaboration
hard-scattered parton during AuAu
hadron distribution softened, broadened?
increased gluon-radiation within plasma?
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Correlations

• 2-particle angular correlation functions
• X.N. Wang Phys. Rev. D47, 2754 (93)
• gluons radiated during passage through QGP gt
  correlations at small Df may be broadened
• study correlations from peripheral gt central
  reactions
• complementary to single-particle pt spectra and
  ghadron back-to-back correlations.

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Data

• 1.5M events, summer 2000 Phenix data
• -20 lt collision vertex lt 20 cm
• Central arm tracks
• momenta from drift chamber tracks
• 1 lt pt lt 2.5 GeV/c
• Centrality cuts in 2-dimensional space
• (zero degree energy) vs (charge in beam counters)
• Correlation functions
• mixed events from similar beam-vertex, centrality
• 2-track acceptance cuts on both real, mixed pairs

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Possible Causes of Angular Correlation

• Both flow and jet fragmentation can lead to
  azimuthal correlations
• jet fragmentation extends only to narrow angles
• 0-30 deg
• flow extends over full range with a harmonic
  oscillation
• analysis challenge to extract both

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Both hadrons in west arm
40-92 Centrality, 1.0 lt pt lt 2.5 GeV/c
Near-angle correlation falls more steeply than
back-to-back correlation Add correlations by
ensuring symmetry near 90o
One hadron in west arm, other in east arm
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Correlations

• Both hadrons between 1lt pt lt 2.5 GeV/c
• Two correlations formed
• both hadrons in west arm of PHENIX
• one hadron in east, west arm of PHENIX
• Studied as a function of centrality

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phenix preliminary
1ltptlt2.5 GeV/c (40-92)
symmetric (elliptic flow) fit but data
stronger near-angle correlation than back-to-back
phenix preliminary
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phenix preliminary
Centrality Dependence
40 to 92
0 to 5
phenix preliminary

• near-angle correlation in central reactions
  broader, smaller amplitude
• elliptic flow v.small in central reactions

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Centrality Dependence
width of correlation broadens for more central
reactions systematic errors how fit changes
for different normalization criteria
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Possible Causes of Near-Angle Correlation

• Fragmentation of high-pt parton
• note that in limited rapidity acceptance,
  back-to-back parton less likely to be observed
• Correlated decay of L, etc.
• cross-checks in progress , -- correlations
• this source of correlation should not increase
  its width for more central reactions.
• Other. ?

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Correlation function in Pythiapp root(s)
200 GeV/c

• Phenix accept. cuts
• 1ltptlt2.5 GeV/c

• Near-angle peak is the same for both
  like/unlike-sign correlations
• Far-angle peak is significantly wider in case of
  like-sign correlations

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Example of ?(1115) ?p? decay

• The majority of ?-decays does not pass through
  the symmetrical pt cut.
• The rest give rise to the narrow peak around ??
  0

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Width, Amplitude vs Centrality
phenix preliminary
phenix preliminary
l 1.5 central reactions
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Summary

• High-pt near-angle correlations slightly stronger
  than back-angle correlations
• well reproduced by gaussian superimposed on
  oscillation
• width of correlation broadens for more central
  reactions
• Possible causes of near-angle correlation
• fragmenting hard-physics
• in this scenario, increasing width consistent
  with increased QCD radiation before fragmentation
• decay of L etc.
• working to distinguish two