Quark Matter '08 Jaipur India

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Why does high-pt suppression persist at forward
rapidity?

• I.G. Bearden ( for the BRAHMS collaboration)
• Niels Bohr Institute, Københavns Universitet

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Outline

• Introduction
• Experiment
• RAA
• Results
• 200GeV AuAu
• 62GeV CuCu AuAu
• Model Comparisons
• Summary

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The BRAHMS Experiment
Two small solid angle spectrometers (FS and MRS)
provide excellent PID over broad range in y-pT
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Nuclear Modification
Quantified by
- yield relative to that from NN collisions,
scaled for the nuclear geometry (Nbin)

• Cronin Enhancement
• Shadowing/Saturation
• Jet-quenching

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Charged hadron pT spectra 200GeV AuAu
Nb filled are , open -, drawn lines are BRAHMS
pp data
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RAA for charged hadrons

• at high pT, the charged hadron yields produced in
  most central 0-10 AuAu collisions are
  suppressed compared to binary scaled pp yields
• Little change in RAA as function of eta

BRAHMS Preliminary
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RAA _at_ 62.4 GeV
?0
?3.1
AuAu
BRAHMS Preliminary
CuCu
BRAHMS Preliminary
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Caveat emptor!
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RAuCu0- 10 40-60
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RAuCu with same Ncoll at Midrapidity
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RAuCu mid vs. forward 62.4GeV (10)
Midrapidity Forward (3.1)
RAuCu
pT
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RAuCu with same radius
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RAA for identified particles (200GeV)

• the charged pion yields are suppressed by a
  factor of 2-3 as compared with binary scaled
  pp pion yields.
• RAA for pions is independent of rapidity
• the proton and antiproton yields in central AuAu
  at 200 GeV do not show suppression, baryon meson
  difference remains

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RAA at 62.4 GeV, y0
BRAHMS Preliminary
BRAHMS Preliminary
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RAA, ygt3, 62GeV, top 10 central
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HydroJet Interplay between Soft and Hard

• Calculation based on two models with CGC as
  initial conditions
• 3D hydrodynamic model (Hydro)?
• dynamics of high pT partons (Jet)?
• pT,cross is the point for YieldsoftYieldhard
• pT,cross moves toward high pT with mass of
  particles because of the effects of radial flow.

PT, cross 1.8 GeV/c for ? 2.7 GeV/c for K 3.7
GeV/c for p
Interesting region ?Intermediate pT (2ltpTlt3.5
GeV/c)? Pion ?hard, Proton ?soft
y0
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Model comparison I

• pT spectra for charged hadrons in 200 GeV AuAu
  reasonably well described by the hydrojet model
  calculation at ?3.2

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Model comparison II
Barnafoldi et al. hep-ph 0609023
opacity n L/l L effective length of
matter l mean free path less jet quenching
stronger initial effects at higher
rapidities maintain rapidity independent NMF
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Is RAA an appropriate measure?
Two rather different Models both reproduce RAA.
RAA only tells us that we only see particles
from near Hotdense surface?
Renk, Eskola, PRC 75, 054910 (2007)
Wicks,Horowitz,Djordjevic,Gyulassy NPA 784, 426
(2007)
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Summary

• At 200GeV, intermediate pT constant behavior vs.
  y
• For 62.4GeV, pronounced changes from mid to
  forward rapidity.
• ? suppressed for 62.4 y3.4, p suppressed but
  not partonically
• RAA flat for same mean radius (ltRgt). Why?
• Hydrojet looks good, but...
• RAA not a good model descriminator