Jet Structure From Dihadron Correlations in d Au Collisions

1
Jet Structure From Di-hadron Correlations in dAu
Collisions

• Nathan Grau
• Iowa State University
• For the PHENIX Collaboration

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Outline

• The Cronin effect and theoretical models of dAu
  collisions with hard-scattering events
• Di-hadron correlations in dAu
• Results on kTztrig
• Results on dN/dxE

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Cronin Effect

• Cronin enhancement of the single particle
  spectrum has been measured at RHIC.
• Common interpretation multiple scattering of
  parton prior to hard scattering.
• Increase pT of parton kT
• Increase in di-jet acoplanarity

PRL91, 072303(2003)
Enhancement in dAu collisions
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Broadening at Lower Energies
Previous Fermilab fixed-target experiments have
measured a sizable increase in the di-jet
broadening in hA collisions. RHIC data will
extend the energy dependence of the effect.
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Theoretical Models

• In multiple scattering models the increase in kT
  is related to TA(b)
• Compare dAu to pp and measure centrality
  dependence compare directly to models
• Compare also to Hwa and Yang recombination in
  cold nuclear matter. Predict little increase in
  kT and increase in near-side yield.

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dN/dxE Distributions
Definition of xE. Pioneered at ISR to
approximate the fragmentation variable z
At high pT, i.e. when di-jets are nearly
back-to-back.
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Data overview

• Trigger hadrons used h/-, p0, p/-
• All correlated with h/-
• p0 identified by gg decay channel
• High-pT p/- identified in RICH (cerenkov
  threshold of 4.9 GeV/c)
• Data spans trigger pT range from 3-16 GeV/c
• Corresponds roughly to a Q2 of 10-1500 (GeV/c)2
• Examined both pp and dAu data from Run-3 RHIC

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Correlation Functions in pp and dAu
Pion trigger 5-10 GeV/c
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Jet Structures from Azimuthal Correlations

• Peak at Df 0 ? di-hadron fragmentation
• Yield related to D(ztrig,zassoc)
• z pT/pTjet
• Peak at Df p ? Two parton fragmentation
• Width related to kTztrig
• Yield related to the product of fragmentation
  functions D(ztrig)D(zassoc)

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Angular Widths

• Widths from gaussian fits
• Far side width gt Near side width

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Parton Transverse Momentum

• kT quoted is per parton
• kTy perpendicular to trigger particle.

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kTyztrig
dAu Min Bias

• kTyztrig for different trigger pT ranges and
  different trigger particles
• kTyztrig flat with pT,trig

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Comparison to pp

• Weighted averages of separate data
• Weighted average of combined data

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Model Comparison

• Limit set on increase in kT
• ztrig 0.5-0.7 implies kT2 increase lt 0.6 -
  1.2 (GeV/c)2 in dAu
• Qiu and Vitev, PLB 570, 161 (2003)
• Semi-hard multiple scattering model
• Predicted kT2 increase of 1 (GeV/c)2
• Hwa and Yang, PRL 93, 082302 (2004)
• Recombination of soft and hard partons
• Reproduce RdAu and predict little increase in kT2
• Although prediction of increase in near-side
  yield is not seen

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Centrality Dependence
p0
p/-
TA(b) dependence of kTyztrig for different
trigger ranges Simultaneous fit with common
slope, slope consistent with 0.
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Summary of kT results

• kTztrig is flat vs. pT,trig with a value of
• No significant increase over pp collisions is
  seen
• No significant trend in centrality seen
• Data sets an upper bound on nuclear effects on
  di-jet acoplanarity in dAu collisions

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dN/dxE Results

• xE distributions for trigger p/- for several
  different trigger ranges
• Overlap of data consistent with fragmentation
  being independent of parton momentum.

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Scaling of dN/dxE
dAu Min Bias
pp
Scaling with pTtrig found at lower root-s at ISR
Scaling holds for pp and dAu at RHIC.
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Scaling Violation of dN/dxE

• Fit to find the fractional change with pT
• Scaling in both pp and dAu and no systematic
  difference between them.

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Summary and Conclusions

• PHENIX has measured jet structures in dAu
  collisions with a number of different hadrons
  over a large pT range.
• No significant increase in kT compared to pp
  collisions is observed and no significant
  centrality dependence is seen.
• dN/dxE distributions scale in both pp and dAu
  collisions and no systematic difference between
  the two systems is observed.
• Data places a limit on the cold nuclear medium
  modification to di-jet structure.
• Data also provides a baseline for measurements in
  AuAu collisions.

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Backup Slides
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p0 Identification

• Instead of Jet reconstruction use a high-pT
  trigger particle
• Lvl-1 EmCal-RICH trigger
• Large increase in photon statistics
• Excellent p0 detection
• S/B 10-20 for p0 gt 5 GeV

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p/- Identification

• RICH fires on charged pions gt 4.9 GeV/c
• Use EmCal to reject other background
• Total background lt 5 above 5 GeV/c

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Extracting Quantities
Jet properties extracted from azimuthal
correlation shape
background
jet
di-jet
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Detailed jT, kT formulae
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jTy
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Centrality Dependence of Yields
pT-dependence of the associated hadron yields.
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Yield Comparison to pp
Ratio of yields to pp collisions. No systematic
deviation. Contrast to the prediction of Hwa and
Yang for an increase in the near-side yield for
recombination by a factor of 2.