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PHENIX measurement on direct photon production. Jiamin Jin Journal Club 07/12/2006. Outline. motivation direct photon in pp & AuAu (RUN2) -- hep-ex/0502006, nucl-ex/0503003 direct photon in AuAu (RUN4) extend to higher pT two analyses to improve on mid pT range

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phenix measurement on direct photon production

PHENIX measurement on direct photon production

Jiamin Jin

Journal Club

07/12/2006

outline
Outline
  • motivation
  • direct photon in pp & AuAu (RUN2)

-- hep-ex/0502006, nucl-ex/0503003

  • direct photon in AuAu (RUN4)
    • extend to higher pT
    • two analyses to improve on mid pT range
      • using stachastic cuts (David et al)
      • photon internal conversion (Y. Akiba)

-- various ANs

J. Jin - JClub

what are direct photons
What are direct photons?
  • @LO level, 3 channels:
    • Compton scatting g+qg+q
    • Annihilation q+qbarg+g
      • The latter is suppressed for p+p, because qbar is less probable than g in the proton
    • The parton-parton scattering with the scattered quark or gluon fragmenting to a g
  • @NLO level:
    • Bremstrahlung emission of g from the quarks undergoing hard scattering

J. Jin - JClub

slide4
photons not from hadronic decays
    • Experimental challenge to differentiate gdirect & gdecay
      • cocktail subtraction method
      • p0 tagging method
      • Isolation cuts
  • Direct photons != isolated photons
    • Fragmentation and bremstralung processes will produce photons with jets

J. Jin - JClub

why direct photons
Why direct photons?
  • p+p
    • Test on QCD
      • explore PDF of the gluon inside the proton
      • Reduce uncertainty on pQCD photons in AA
  • A+A
    • Photons don’t strong interact with medium
    • Hard photons
      • Allow test of Ncoll scaling for hard processes
      • Interpret high-pt hadron suppression at RHIC
    • Thermal photons
      • Carry information of early stage of collisions
      • Detect QGP via thermal photon radiation

J. Jin - JClub

direct photons in p p
Direct photons in p+p
  • 1st step: determine direct-photon-candidate hits
    • reject a EMCal cluster if:
      • form an invariant mass in the p0 or h range with other clusters
      • asymmetry cut
    • combinatorial background studied by embedding (small effect in p+p)
    • a factor of 2 rejection achieved for cluster pT>5GeV/c

J. Jin - JClub

formula of differential cross section
Formula of differential cross section
  • N: # of direct-photon-candidates in a DpT&Dy bin
  • Creco: acceptance and efficiency correction (MC sim.)
  • Cconv: correction for photon conversions (<7.4%)
  • Closs: correction for the loss of true direct photons due to combinatorial bkgd (<2%)
  • f = 0.75(+/-0.02): MinBias trigger efficiency
  • L: integrated luminosity (~41nb-1 for RUN2 p+p)

J. Jin - JClub

slide8
2nd step: deal with the remaining hadronic decay photons
    • mostly from missing the partner photon
    • also from other decay sources (w,h’,…)
    • plot from data
    • feed p0 spectrum into MC, simulate
    • get double gamma ratio Rg
    • mT scaling for other decay sources

J. Jin - JClub

results
Results
  • Ratio of direct-photon-candidates to the p0 spectrum (data & MC)
  • Direct g cross section
    • small but significant signal observed
    • agree with NLO pQCD

J. Jin - JClub

other methods i
Other methods I
  • p0 tagging method:

J. Jin - JClub

other methods ii
Other methods II
  • Isolation cuts
    • photon energy < 10% of cone energy
    • cone size: 0.5 around the candidate photon
    • cone energy: sum of track momentum in DC & EMCal cluster energy
    • correct for limited PHENIX acceptance, which causes fake isolated photons
  • Emphasize the Compton process
    • direct access to gluon distribution

J. Jin - JClub

results run3 p p
Results (RUN3 p+p)
  • direct photon production
    • spectrum up to 16GeV/c
    • well described by NLO pQCD for pT>5GeV/c
  • ratio of isolated photons to all direct photons
    • large systematics in low pT
    • agree with pQCD(lines) for pT>7GeV/c

J. Jin - JClub

direct photons in au au
Direct photons in Au+Au
  • What’s new in Au+Au?
    • centrality determination by BBC&ZDC
    • high pT hadron suppression in central Au+Au, measure Rg directly:
      • Rg > 1, indicates direct photon signal
  • Direct photon spectra extracted as:

J. Jin - JClub

results1
Results
  • double gamma ratio plot
    • advantage: many systematics cancel
    • an excess over unity is observed at high pT
    • magnitude increases with centrality due to hadron suppression
    • expectation for Ncoll scaling of direct photons holds for all centrality bins

J. Jin - JClub

slide15
spectra shown
    • 9 centrality bins and MB
    • comparison to NLO
    • at low to mid-pT where additional production of thermal photons appears, we have large error bars

J. Jin - JClub

slide17
goal: improve upon mid-pT region systematic errors
  • using most stable subset of RUN4 data
  • improving various corrections
      • p0 peak extraction
      • energy scale/linearity
      • energy smearing
      • fitting function to p0 spectrum
  • using stochastic cuts to extract photons

J. Jin - JClub

slide19

Excess above the scaled pQCD?

  • new points agree with, but consistently higher than the old ones ~4GeV/c

J. Jin - JClub

the idea dalitz decay

g

g

p0

p0

e+

g*

g

e-

The idea: Dalitz decay
  • mass distribution of e+e- pairs (Knoll-Wada formula):

1) pure QED part, universal to all Dalitz decays

2) phase factor part, depends on M of parent hadron

3) hadronic form factor

at low mass, both the second and third part become 1

at high mass, suppressed by phase factor

J. Jin - JClub

now direct photons

e+

0-30

90-300 MeV

Rdata

÷

Compton scattering

e-

Now direct photons:

g*

q

g

q

  • Any source of real g produces virtual g with very low mass
  • Rate and mass distribution given by same formula
    • No phase space factor for mee<< pT photon

We measure Rdata:

N(90<Mee<300)/ N(0<Mee<30)

We can calculate:

Rp0, Rh, Rdirect

J. Jin - JClub

slide23

True in very

low mass window!

Rdata excess over calculated expected ratio from hadronic decays:

virtual & real direct photon signal

J. Jin - JClub

slide24

Results: Rdata

0-20 %

J. Jin - JClub

slide25

g*direct/g*inclusive

0-20 %

Significant 10% excess of very-low-mass virtual direct photons

J. Jin - JClub

slide26

more peripheral

Centrality Dependence

Indication for centrality dependence

J. Jin - JClub

slide27

( + 1 )

Comparison to conventional results

J. Jin - JClub

slide28

gdirect

J. Jin - JClub

slide29

The Spectrum

Compare to the published Run2 results (black arrow)

J. Jin - JClub

slide30

Compare to NLO pQCD

  • excess above pQCD

The Spectrum

J. Jin - JClub

slide31

Compare to NLO pQCD

  • excess above pQCD

Compare to thermal model

  • data above thermal at high pT

The Spectrum

J. Jin - JClub

slide32

Compare to NLO pQCD

  • excess above pQCD

Compare to thermal model

  • data above thermal at high pT

Compare to thermal + pQCD

  • data consistent with thermal + pQCD

The Spectrum

J. Jin - JClub

backup
Backup

J. Jin - JClub

schematic photon spectrum in au au

thermal:

Decay photons

hard:

Schematic Photon Spectrum in Au+Au

J. Jin - JClub

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