Measurements of low pT direct photons in PHENIX

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Measurements of low pT direct photons in 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
• Experimental approach at RHIC
• vsNN 200GeV AuAu collisions
• Direct photons are an important probe to
  investigate the characteristics of evolution of
  the matter created by heavy ion collisions.
• Penetrate the strong interacting matter
• Emitted from every stage of collisions
• Hard photons (High pT)
• Initial hard scattering, Pre-equilibrium
• Thermal photons (Low pT)
• Carry the thermodynamic information from QGP and
  hadron gas

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Photon Measurement in PHENIX

• Hard photon
• Strong suppression of high pT hadrons helps to
  improve the S/N ratio
• Successfully measured for both pp and AuAu
  collisions
• Measured pT range
• up to 14GeV/c
• Good agreement with pQCD
• Thermal photon
• Thermal radiation from QGP
• Primary contributor in low pT range up to
  3-6GeV/c
• Direct evidence of thermal equilibration
• Thermal photon measurement is very challenging
  because it is very hard due to a large background
  from hadron decays.

AuAu
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Low pT Photons

• Long-awaited results for both pp and AuAu
• Experimental determination is very important
  since applicability of pQCD is doubtable in low
  pT region.
• In real photon measurement
• Measured yield with a large systematic error
• Difficulty on measuring low pT real direct
  photons
• Finite energy resolution of the EMCal
• Large hadron background

• Alternative method to measure low pT direct
  photons
• Measure ee- pairs from virtual direct photons

• Advantages on measuring virtual photons
• High momentum resolution of the Drift Chamber
• Reliable estimation of the hadron decay
  components using Kroll-Wada formula

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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

• Case of Hadrons
• Obviously S 0 at Mee Mhadron
• Case of g
• If pT2Mee2
• Possible to separate hadron decay components
  from real signal in the proper mass window.

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Signal Extraction
AuAu
pp
arXiv 0706.3034
arXiv 0802.0050

• 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 jets

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Hadronic Cocktail Calculation

• Remaining pairs after background subtraction
• Real signal Hadron decay components
• Estimate hadron components using hadronic
  cocktail

arXiv 0802.0050

• 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
  
• Hadronic cocktail was well tuned to individually
  measured yields of mesons in PHENIX for both pp
  and AuAu collisions.

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Cocktail Comparison
AuAu
pp
arXiv 0802.0050
arXiv 0706.3034

• pp
• Excellent agreement with cocktail
• AuAu
• Large enhancement in low mass region
• Integrated yield in150MeV
• Real/cocktail 3.4 0.2(stat) 1.3(sys)
  0.7(model)

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pT Sliced Mass Spectra
Normalized by the yield in mee

AuAu

pp

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

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pT Dependence
pp
AuAu

• AuAu
• pT
• Large enhancement ? thermal qqbar and pp
  annihilations?

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Cocktail Comparison
1 pT

pp

Good agreement between real and cocktail

Small excess at higher pT

• AuAu
• Good agreement in Mee
• 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 shape
• c2/NDF11.6/10
• Assuming h shape instead of direct g shape
• c2/NDF21.1/10
• Twice as much as measured h yield
• Assumption of direct g is favorable.

Mee (GeV/c2)
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direct g/inclusive g
pp
AuAu
Base line
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 1-4GeV/c
• Consistent with NLO pQCD
• Serves as a crucial reference

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

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Theory Comparison
D.dEnterria, D.Peressounko, Eur.Phys.J.C 46
(2006)
S.Turbide, R.Rap, C.Gale, Phys.Rev.C 69 (2004)
T0ave360 MeV (T0max590 MeV) t00.15 fm/c
T0max370 MeV t00.33 fm/c
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Summary Outlook

• Direct photon measurements with virtual photon
  method in pp and AuAu collisions have been done
  at RHIC-PHENIX.
• 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.
• This is the first time that direct photon
  production in pp collisions has been measured in
  1
• Direct photon yield in pp collisions is
  consistent with NLO pQCD.
• 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.
• pp analysis with more statistics is now
  ongoing.
• Result with higher quality will be provided.
• pT region will be extended upward.
• 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)
  Electron triggered data sample (pp)
• BBC
• z-vertex
• DC, PC1
• Tracking
• RICH EMCal
• Electron ID
• 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 M M M M M 20
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 ( 700
  MeV)
• Normalization 2vN N--
• Uncertainty due to statistics of N and N--
  0.12

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Centrality Dependence
arXiv 0706.3034
? AuAu ? pp Cocktail

• Integrated yield divided by Npart/2
• 150MeV
• Strong centrality dependence
• Increases faster than Npart
• mee
• Agreement with cocktail

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Theory Comparison 2

• Freeze-out Cocktail random charm r
  spectral function
• Low mass
• M0.4GeV/c2 some calculations OK
• M