Measuring bremsstrahlung photons in sqrt(s)=200 GeV pp collisions

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Measuring bremsstrahlung photons in sqrt(s)200
GeV pp collisions

• Ali Hanks
• Hard/Photon analysis fest
• September 15, 2006

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Motivation
Zakharov hep-ph/0405101

• Understanding jet quenching
• a probe of the QGP

no quenching
with quenching

• Provides direct measurement of radiation spectrum
• Expect strong final state interaction effects on
  bremsstrahlung contribution
• significant enhancement for pT
  lt 10GeV/c

Just Bremsstrahlung photons

• pp collisions important for study of nuclear
  modifications

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In pp - pQCD predictions

• NLO pQCD describes data well
• At NLO distinction between direct photons and
  bremsstrahlung (fragmentation) photons blurs
• isolation cuts made to distinguish direct
  photons
• difficult to match to theory
• fragmentation component gt 20 for pT lt 10 GeV/c
• direct measurement of fragmentation contribution
  good test of theory
• direct photons will be excluded

Curves INCNLO 1.4
INCNLO(v1.4) J. Ph. Guillet, M. Werlen et al
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Methodology

• Pick out bremsstrahlung photons by selecting
  photons associated with a jet

• Trigger on high pT hadrons
• Calculate correlations with associate photons
  in various pT bins
• Look at yields for determining bremsstrahlung
  contribution

• Remove background sources
• Tag photons coming from p0 decays
• Correct for tagging efficiency
• Estimate contribution from other decay sources

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Inclusive h-? correlations
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Next step - removing p0 background

• Tag photons from pairs that fall within p0 mass
  peak 120-160 MeV/c2

p0 invariant mass
trigger photon has pT gt 1 GeV/c sister
photon has pT gt .5 GeV/c
To do

• Calculate combinatorial background
• Evaluate tagging efficiency

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Summary

• Have inclusive hadron-? correlations
• Working on calculating hadron-?tag correlations
  with
• combinatorial background correction
• tagging efficiency estimate
• Next begin subtraction to obtain hadron-?dir

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Backup Slides
?0 tagging efficiency for pT gt 3GeV/c
??
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Initial QA hot tower maps

• pT 1-2GeV/c

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hot tower maps

• pT 2-3 GeV/c

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hot tower maps

• pT 3-10 GeV/c

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Evaluating the p0 tagging efficiency

• Simulations of p0 decays provide efficiency
  correction
• p0s generated with pT dependent Gaussian
  distribution around trigger hadron
• ?? distribution of decay photons calculated
• Compare to photons from pairs where both photons
  accepted
• Ratio gives tagging efficiency
• pT dependence of input p0 distribution implies pT
  dependence in efficiency

( pT gt 3GeV/c )
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Evaluating the p0 tagging efficiency

• Separate efficiency into several bins in pT of
  decay photon

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Other sources of background

• ? decay also source of background
• distribution of decay photons may differ from
  that of p0s
• Try simulation with ? kinematics
• Same input pT spectrum weighted by ? /p0 ratio
• Fit decay distributions to check similarity
• Similarity means ? decays can be subtracted using
  p0 distribution

3.5 lt pT lt 4.0GeV/c