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Measurements of low pT direct photons in PHENIX

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Title: Measurements of low pT direct photons in PHENIX


1
Measurements of low pT direct photons in PHENIX
  • Yorito Yamaguchi
  • for the PHENIX collaboration
  • CNS, University of Tokyo

2
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

3
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
4
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

5
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.

6
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

7
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.

8
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)

9
pT Sliced Mass Spectra
Normalized by the yield in mee
  • AuAu
  • pp
    • Shape differences between pp and AuAu are
      larger at lower pT.

    10
    pT Dependence
    pp
    AuAu
    • AuAu
    • pT
    • Large enhancement ? thermal qqbar and pp
      annihilations?

    11
    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.

    12
    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)
    13
    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

    14
    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?

    15
    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
    16
    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.

    17
    backup
    18
    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

    19
    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
    21
    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

    22
    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

    23
    Theory Comparison 2
    • Freeze-out Cocktail random charm r
      spectral function
    • Low mass
    • M0.4GeV/c2 some calculations OK
    • M
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