Di-electron Continuum at PHENIX

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Di-electron Continuum at PHENIX

• Yorito Yamaguchi
• for the PHENIX collaboration
• CNS, University of Tokyo

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Introduction

• Quark Gluon Plasma
• De-confined phase of quarks and gluons
• Predicted by lattice QCD
• Temperature
• higher than 150-200MeV
• Energy density
• higher than 1GeV/fm3
• Relevance to the history of the early universe
• Properties of QGP are of keen interest
• Strongly interacting system
• Perfect fluid
• Experimental approach
• Heavy ion collisions at RHIC
• vsNN 200GeV AuAu collisions

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Photons Di-leptons
R. Rapp nucl-th/0204003

• Photons and di-leptons are emitted from
  thermally equilibrated medium.
• Penetrating the strongly interacting matter
• Carrying the thermodynamic information directly
• Topics on di-electron continuum in low mass
  region (Below 1GeV)
• Measurement of electron-pair yield in low-mass
  low-pT region in pp and AuAu
• Enhancement possibly due to thermal q-qbar and
  p-p annihilations
• Measurement of direct photon yield in pp and
  AuAu with virtual photon method
• Thermal photons are primary contributor in
  1-5GeV/c.

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Signal Extraction
AuAu
arXiv 0706.3034

• Real signal
• di-electron continuum
• Background sources
• Combinatorial background
• Material conversion pairs
• Additional correlated background
• Visible in pp collisions
• Cross pairs from decays with 4 electrons in the
  final state
• Pairs in same jet or back-to-back jet

pp
After subtraction
arXiv 0802.0050

• Remaining pairs
• Real signal Hadron decay components
• Estimate hadron components using hadronic
  cocktail
• Tuned to individually measured yields of mesons
  at PHENIX

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Cocktail Comparison
AuAu
pp
arXiv 0802.0050
arXiv 0706.3034

• pp
• Excellent agreement with cocktail
• AuAu
• Large enhancement in 150MeV lt mee lt 750MeV
• Integrated yield
• Real/cocktail 3.4 0.2(stat) 1.3(sys)
  0.7(model)

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Centrality Dependence
arXiv 0706.3034
? AuAu ? pp Cocktail

• Integrated yield divided by Npart/2
• 150MeVltmeelt750MeV
• Strong centrality dependence
• Increases faster than Npart
• meelt100MeV (p0 region)
• Agreement with cocktail

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Shape Difference

• AuAu
• pp

• Shape differences between pp and AuAu are
  larger at lower pT.

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pT Dependence
pp
AuAu

• pp
• Consistent with cocktail for all mass bins
• AuAu
• Large enhancement at low pT as presented

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Virtual Photon Measurement

• Any source of real g can emit g with very low
  mass.
• Convert direct g fraction to real direct photon
  yield

Kroll-Wada formula
S Process dependent factor
Invariant mass of g

• Hadron mass distribution
• Obviously S 0 at Mee gt Mhadron
• g mass distribution
• If pT2gtgtMee2
• Possible to separate hadron decay components
  from real signal in the proper mass window.

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Cocktail Comparison
1 lt pT lt 2 GeV 2 lt pT lt 3 GeV 3 lt pT lt 4 GeV 4 lt
pT lt 5 GeV

• pp
• Good agreement between real and cocktail
• Small excess at higher pT

• AuAu
• Good agreement in Mee lt 50MeV
• Enhancement is clearly seen above 100MeV.

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Determination of g fraction, r
Direct g/inclusive g is determined by fitting
the following function for each pT bin.
Reminder fdirect is given by Kroll-Wada formula
with S 1.
r direct g/inclusive g

• Fit in 80-300MeV gives
• Assuming direct g mass shape
• c2/NDF11.6/10
• Assuming h shape instead of direct g shape
• c2/NDF21.1/10
• Assumption of direct g is favorable.

Mee (GeV/c2)
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direct g/inclusive g
pp
AuAu
Curves NLO pQCD calculations with different
theoretical scales done by W. Vogelsang.

• pp
• Consistent with NLO pQCD
• better agreement with small µ

• AuAu
• Clear enhancement above NLO pQCD

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Direct Photon Spectra
The virtual direct photon fraction is converted
to the direct photon yield.

• pp
• First measurement in 1-3GeV/c
• Consistent with NLO pQCD
• Serves as a crucial reference

• AuAu
• Above binary scaled NLO pQCD
• Excess from thermal photons?

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Summary Outlook

• Di-electron analyses in pp and AuAu collisions
  have been done at RHIC-PHENIX.
• Large enhancement at low mass is observed in
  AuAu collisions while the result in pp
  collisions is consistent with a hadronic
  cocktail.
• Strong centrality dependence
• Large enhancement at low pT
• The fractions of direct g to inclusive g above
  pT of 1GeV/c are obtained by making a shape
  comparison between real pairs and a hadronic
  cocktail.
• Direct photon yield in pp collisions is
  consistent with NLO pQCD.
• First measurement of direct photons for
  1ltpTlt3GeV/c in pp collisions.
• The result in pp serves as a crucial reference
  to AuAu result.
• Excess of direct photon yield above binary
  scaled NLO pQCD in AuAu collisions is observed.
• The paper on direct photon measurement with
  virtual photon method will be submitted soon.
• pT region of direct photon yield in pp
  collisions will be extended upward with more
  statistics.
• Same analysis will be done in dAu collisions.

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backup
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PHENIX Detector

• Minimum Bias data sample (triggered by BBC) ERT
  triggered data sample
• Analyzed events 1.6B events
• BBC
• z-vertex
• DC, PC1
• momentum
• RICH EMCal
• Electron ID
• Energy
• Track Matching
• Electron Trigger

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Material Conversion Pair Cut
The pairs from material conversion should be
removed.
These pairs can be recognized by its orientation
relative to the magnetic field.
No cut Mlt30 MeV fVlt0.25 Mlt600 MeV fVlt0.04
Mlt600 MeV fVlt0.06 Mlt600 MeV fVlt0.08
Mlt600 MeV fVlt0.10 Mlt600 MeV fVlt0.12
Mlt600 MeV fVlt0.14 Mlt600 MeV fVlt0.20
Mlt600 MeV fVlt0.40
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Additional Correlated Background
Jet cross pair
Dalitz conversion cross pair
Correlated combinatorial background is very
good agreement with the real like sign mass
spectrum.
Systematic error due to background subtraction
2
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Combinatorial Background (AuAu)

• Normalization factor is determined by like sign
  pairs.
• N and N-- estimated from the mixed events like
  sign B and B-- normalized at high mass (gt 700
  MeV)
• Normalization 2vN N--
• Uncertainty due to statistics of N and N--
  0.12

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Hadronic Cocktail Calculation (pp)

• Mass distributions from hadron decays are
  simulated by Monte Carlo.
• p0, h, h, w, f, r, J/y, y
• Effects on real data are implemented.
• PHENIX acceptance, detector effect, efficiencies
  

arXiv 0802.0050

• Parameterized PHENIX p0 data with assumption of
  p0 (pp-)/2

• Other mesons are measured individually.
• Fit with mT scaling of p0 parameterization

• Hadronic cocktail is well tuned to PHENIX data.

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Theory Comparison 1

• Freeze-out Cocktail random charm r
  spectral function
• Low mass
• Mgt0.4GeV/c2 some calculations OK
• Mlt0.4GeV/c2 not reproduced

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Theory Comparison 2
T0ave360 MeV (T0max590 MeV) t00.15 fm/c