21st Winter Workshop on Nuclear Dynamics

1
Nuclear Modification of Heavy Flavor Production
in AuAu Collisions at vsNN 200 GeV

• 21st Winter Workshop on Nuclear Dynamics
• Breckenridge, CO
• Sergey Butsyk
• SUNY at Stony Brook
• For the PHENIX Collaboration

2
Outline

• Physics motivation
• Measurement techniques
• PHENIX at RHIC
• Non-photonic electrons from pp and AuAu
  collisions at ?sNN 200 GeV
• Binary scaling of the yield
• Nuclear modification factor RAA
• Electron elliptic flow
• Summary and Outlook

3
Heavy Flavor Physics motivation

• pp collisions
• Check pQCD predictions at ?s 200 GeV
• Sensitive to initial gluon density
• Baseline measurement for nuclear collisions
• dAu collisions
• Study gluon energy loss in cold nuclear matter
• Gluon shadowing
• AuAu collisions
• Measure medium modification effects (charm energy
  loss, collective flow)
• Study potential thermal production of charm from
  QGP
• Set the baseline for J/y production

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Heavy Quark Production in pQCD

• Perturbative QCD uses decomposition of the
  scattering amplitudes in powers of as(Q)
• k 0 Leading Order Diagrams (LO)
• k 1 Next to Leading Order (NLO)
• k 2 Next-to-Next Leading Order (NNLO)
• is partonic energy in center of
  mass,
• mR is an energy scale. mQ/2 lt mR lt 2mQ
  renormalization scale

• In LO the main contribution comes from gluon
  fusion

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How to measure Heavy Flavor ?

• STAR
• Direct D mesons hadronic decay channels in dAu
• D0?Kp
• D?Kpp
• D?D0p
• Single electron measurements in pp, dAu

• PHENIX
• Single electron measurements in pp, dAu, AuAu
  ?sNN 130,200,62.4 GeV

• Experimentally observe the decay products of
  Heavy Flavor particles (e.g. D-mesons)
• Hadronic decay channels D?Kp, D0?p p- p0
• Semi-leptonic decays D?e(m) K ne

Meson D,D0
Mass 1869(1865) GeV
BR D0 --gt Kp- (3.85 0.10)
BR D --gt e X 17.2(6.7)
BR D --gt m X 6.6
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PHENIX detector at RHIC

• Electron measurements
• hlt0.35
• Two separate arms 2xDf 900
• dp/p 1 p
• Electron ID
• RICH (gthr35)
• e/p separation up to pT 4.8 GeV/c
• Muon measurements
• 1.2 lt h lt 2.4
• Two separate arms in forward and backward rapidity

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Clear electron sample

• Clean sample of electrons is selected by
• Requiring well defined RICH ring
• Good track matching to Electromagnetic
  Calorimeter cluster
• Energy over Momentum cut

E/p
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Background Contributions

• Main source
• Random combinations of EMC cluster and RICH ring
• pT independent and P(he) 3?10-4
• Minor source
• d-electrons knocked by the hadron in RICH active
  volume
• P(hd) lt 10-6

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Electron ERT Trigger

• ERT Level-1 trigger matches RICH hit position
  with EMCal energy, deposited in 2x2 tile region
• Threshold on deposited energy tags electrons with
  energy higher then the threshold (800 MeV)
• Trigger efficiency in the stable acceptance
  region saturates around 95 - 100

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Cocktail Subtraction Analysis

• Calculate inclusive single electron spectrum from
  all known electron sources
• p0 Dalitz decay (dominant contributor at low pt)
• Photon conversions in PHENIX material
• Other light mesons (h, h, r, w, f) leptonic
  decays
• Direct radiation contribution
• Weak kaon decays electrons (Ke3)

• Input to the Cocktail
• p0 p invariant pT distributions as published
  by PHENIX
• Yield ratios of other light mesons to pions as
  measured at RHIC (where available)
• Use mT scaling of pion momentum distribution for
  light mesons
• Photon conversions from full simulation of PHENIX
  apparatus
• Direct g and Ke3 from PHENIX measurements

• Excess of the data over the Cocktail prediction
  can be interpreted as heavy flavor particle
  semi-leptonic decays contribution

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Cocktail vs. Data
pp
AuAu Min. Bias
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pp Non-photonic Electrons

• Cocktail subtracted electrons compared to PYTHIA
  expectations
• Spectrum is harder then PYTHIA prediction
• Bottom enhancement?
• Higher twist contributions to charm crossection?
• The reference for nuclear modification studies is
  obtained
• Statistical error is dominant uncertainty at pT
  gt2.5 GeV/c

PHENIX
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AuAu Non-photonic Electrons

• Converter subtraction method used
• Limiting factor - statistics of Converter run
  period
• Statistics is too low to make a definitive
  statement about spectral shape or centrality
  dependence

S.S. Adler, et al., nucl-ex/0409028
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Binary Scaling of Electron Yield

• dN/dy of Non-photonic electrons for pT gt 0.8
  GeV/c indicates scaling with Ncoll
• dN/dy Ncolla where 0.906 lt a lt 1.042 within
  90 C.L.

S.S. Adler, et al., nucl-ex/0409028
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Cocktail Subtraction analysis in AuAu

• Cocktail analysis has 4.5 higher statistics
• Systematic error is higher at low pT due to small
  S/B
• Clear suppression of electron yield at high pT is
  observed at most central bins

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Nuclear Modification Factor RAA

• Strong modification of the spectral shape in
  AuAu is observed at high pT
• Systematic error on TAA is large for 60-80
  centrality bin
• Statistics insufficient to quantify centrality
  dependence

17
Theory Comparison

• Observed suppression is in good agreement with
  theoretical predictions for the final state
  energy loss of heavy flavor
• Charm only component is used for comparison,
  Bottom electron contribution need to be taken
  into account at pT gt 4 GeV/c

Theory curves from N. Armesto, et al.,
hep-ph/0501225
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Open Charm Flow

• Final state energy loss implies interaction of
  the heavy quark with the medium
• Does charm participate in collective flow?
• With 90 C.L. the Charm flow!

PHENIX
S.S. Adler, et al., nucl-ex/0502009 Theory
curves fromGreco, Ko, Rapp Phys. Lett. B595
(2004) 202
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Summary

• Electrons from heavy flavor decays were measured
  at ?s 200 GeV in pp and AuAu collisions at
  RHIC
• Spectral shape of Non-photonic electron cross
  section in pp is harder then PYTHIA predicts
• Total Non-photonic electron yield in AuAu
  collisions exhibits binary scaling
• Cocktail subtraction results from AuAu improved
  the statistical significance of the previous
  Converter subtraction analysis
• Nuclear modification factor RAA indicates a
  strong modification of the electron spectra in
  AuAu collisions
• Observed modification of RAA is in agreement with
  theoretical expectations from final state energy
  loss
• There is an indication of measurable Charm
  electron elliptic flow
• Much higher statistics of Run03/04 will
  significantly improve the statistics ? study of
  centrality dependence

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Future outlook

• pp Run03, Run04
• 4 times acceptance
• 5 times ERT trigger statistics
• 20 times total electron statistics allows us to
  go up to 8-10 GeV/c in pT
• - RICH have 5 GeV threshold for pions require
  additional subtraction of p signal at pT gt
  5GeV/c
• AuAu Run04
• at least factor of 20 in MB statistics
• 1.5 times the acceptance
• gt 30 times total electron statistics

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Backup slides
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Photon Conversions Electrons

• Conversions electrons were introduced by full
  simulation of p0 decay in PHENIX aperture
• The ratio of reconstructed electrons from
  conversion to Dalitz was used to scale Dalitz
  component of the Cocktail
• The ratio is around 0.73 independent on pT