18th Winter Workshop on Nuclear Dynamics

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Suppression of high pt hadrons from Au-Au
collision at 130GeV/c

• Introduction
• Experimental Setup
• Suppression Results
• Centrality Dependence
• Summary and Outlook

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Motivation

• At RHIC we reach a new energy regime
• (Mini-) Jet production contributes substantially
  to the particle production
• Initial production rate can be calculated in pQCD
• Early phase of the collisions can be investigated

Use Jets as a calibrated source to study
the property of the medium
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Spectra Predictions
Several predictions have been done on what to
expect in the ultra-high-density medium formed in
RHIC collisions. -- Partonic energy loss
M.Gyulassy, I.Vitev, X.N.Wang PRL 86 (2001) 2557
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Ratio Central/Proton-Proton

• ratio of charged hadron spectra in central Au-Au
  _at_ 200GeV/c over p-p collisions normalized by the
  average number of binary collisions

X.N.Wang PRC 61(2000) 064910
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Centrality dependence
X.N.Wang PRC 63 (050821)01
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Phenix

• Our Observables
• Charged particles
• Neutral pions

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Trigger

• BBC and ZDC trigger up to 92 of the total cross
  section
• Precise centrality selections using correlation
  between ZDC energy and BBC charge
• Glauber model
• to extract the Number of Participants and Number
  of Collisions

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Charged Particles

• For this analysis
• 1.5M minimum bias events
• EAST arm DC, PC1 and PC3
• ?? 90 degree and ?? 0.7
• Momentum resolution 3.6 p (GeV/c)
• Pro
• single track, no combinatorial background
• Contra
• at high momentum difficult PID analysis
• contribution from different species
• (RiCh detectors)

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Neutral Pions

• For this analysis
• 1.17 M minimum bias events
• 2 sectors EMCal in the WEST arm
• (2 sector PbGl in the EAST arm)
• ?? 45 degree and ??0.7
• Pro
• invariant mass reconstruction up to very high pt
• Contra
• large combinatorial background
• need a lot of events
• hard at low pt, much easier at high pt

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Peak extraction

• Example of extraction of peak
• central collisions
• pt 1.5 GeV/c
• At higher pt the analysis gets easier
• pt gt2 GeV/c
• asym lt0.8
• M 136.7 MeV

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What to compare to ?

• Reference N-N spectra at 130GeV/c
• (h h-)/2 and neutral pions ( ?/h 1/1.6 ratio
  from ISR)
• Fit

Power Law
?pp d2N/dpt2 A (p0pt)-n
Scaling by number of binary collisions in Au-Au
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Pt spectra

• Charged particles and neutral pions spectra at
  mid-rapidity for two different centrality bins
• 0-10 and 60-80
• Pions two data sets (PbGl and PbSc)
• Comparison to Nbinary scaled reference of NN

Phys. Rev. Lett. (88) 2002
Central collisions show deficit
Peripheral collision are described at high pt
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Nuclear Effects (I)

• Modification due to nuclear effects have a long
  history started in 1975
• - p-A measurement
• - a (pt)
• -shadowing below 1.5 GeV/c
• -anomalous Cronin effect (initial state
  multiple scattering) above.

Cronin Effect
Shadowing
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Nuclear Effects (II)
At lower energies (SPS) , Cronin effect
ordinary anomalous nuclear effect
Compilation of X.N. Wang
Plateau at 2.
Crossing at 1.5 GeV/c
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Nuclear Modification Factor

• RAA shows
• Charged particles
• Increase up to 2 GeV/c
• Saturation at 0.6
• Neutral pions
• Roughly constant at 0.4

• RAA shows
• Charged particles
• Increase up to 2 GeV/c
• Saturation at 0.6
• Neutral pions
• Roughly constant at 0.4

• RAA shows
• Charged particles
• Increase up to 2 GeV/c
• Saturation at 0.6
• Neutral pions
• Roughly constant at 0.4

Brackets and bands represents sums of all
systematic errors Reference NN, corrections ,
binary Total 30-50
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Central/Peripheral

• Ratio of most central spectra (0-10) to the
  peripheral sample (60-80) appropriately scaled
  by the number of binary collisions

Particle composition Changing with centrality
Binary Scaling
Charged particles
Neutral pions
pT (GeV/c)
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Centrality evolution of spectra (I)

• Centrality dependence of the pt distributions in
  the range of 0.3 and 5 GeV/c
• Pronounced power law in peripheral collisions
• More exponential trend in central collisions

PHENIX preliminary
Central
Periph.
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Centrality evolution of specta (II)

• Ratio to minimum bias
• Pronounced curvature in peripheral collision
• Curvature decreasing with increasing centrality
• Change of curvature in most central collisions

PHENIX preliminary
Central (0-5)
5-15
15-30
30-60
Peripheral (60-92)
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Change in shape

• To quantify the change in shape vs centrality
  (Npart)
• Inverse slope or
• Mean pt (above a pt cut ) ?pT? - pTmin

Directly connect with the inverse slope
At high pt ltptgt decreases with Npart
PHENIX preliminary
NB at smaller pt, the trend is reversed
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Nuclear Modification Factor Evolution

• Gradual modification of the spectra with
  increasing centrality
• - Monotonic increase vs pt for peripheral data
  Cronin or proton contribution ?
• - Strongest Suppression for central data

Relative systematic error (mostly Nbinary)
Common systematic scale error
Peripheral (60-92)
Central (0-5)
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Which scaling ?

• Testing two scaling behaviours
• Soft production scales as Npart
• Hard processes scale as Nbinary

Nx Nbin
13 10 7 5 2 b(fm)
Nx Npart/2
0 1 2 3 4
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Summary

• We measured neutral pions and charged particles
  up to a pt 4-5GeV/c
• Suppression relative to the Binary scaling of
  proton-proton collisions
• Suppression relative to expected ordinary
  anomalous Cronin effect
• And to peripheral interactions.
• Suppression evolves gradually as function of
  centrality.
• Soon on your screens
• 170 million Au-Au at 200 GeV
• A continuously increasing number of proton-proton
  collisions at the same energy.
• A much better detector
• More detailed centrality dependence of
  suppression
• Separate study for the mesonic and barionic
  components of the spectra.
• Particle ratios at high pt

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Centrality Selections