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Measuring fragmentation photons in p+p collisions. Ali Hanks for the PHENIX collaboration APS Conference April 15, 2007.  frag /  inc for direct photons. Phys. Rev. Lett. 98, 012002 (2007): hep-ex/0609031. Phys. Rev. Lett. 98, 012002 (2007): hep-ex/0609031.

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measuring fragmentation photons in p p collisions

Measuring fragmentation photons in p+p collisions

Ali Hanks

for the PHENIX collaboration

APS Conference

April 15, 2007

Ali Hanks - APS

pqcd predictions

frag/inc for direct photons

Phys. Rev. Lett. 98, 012002 (2007): hep-ex/0609031

Phys. Rev. Lett. 98, 012002 (2007): hep-ex/0609031

INCNLO(v1.4): J. Ph. Guillet, M. Werlen et al

pQCD predictions
  • NLO pQCD describes data well
  • fragmentation component significant (> 20%) at low pT
  • How can we test this prediction?
    • “isolation” cuts made to distinguish prompt photons
      • difficult to match to theory
  • direct measurement of fragmentation contribution necessary to really test theory
  • “direct” photons should be excluded

Ali Hanks - APS

medium modifications

Zakharov hep-ph/0405101

no quenching

with quenching

Medium modifications
  • Expect strong final state effects such as significant enhancement for pT < 10 GeV/c
  • Improve our understanding of jet quenching:
    • direct measurement of radiation spectrum

Just Bremsstrahlung photons

  • p+p cross section necessary for study of nuclear modifications

Ali Hanks - APS

photon sources

h (0,,)

Photon Sources
  • Decays are dominant source of photons detected
    • most (80%) from 0's
  • Major source of background for direct photon measurement

Decay photons

Direct photons

Medium induced modifications:

Leading order(LO) - prompt:


Next to leading order (NLO) - fragmentation:

  • Occurs at NLO - modifies the fragmentation component of direct photon spectrum only

Ali Hanks - APS

finding fragmentation photons


Finding Fragmentation Photons
  • Use jet correlations to single out fragmentation photons
    • Trigger on high p_T hadrons, 2-15 GeV/c, and calculate  distribution of associated photons

Use event mixing to correct for acceptance

Ali Hanks - APS

estimating 0 background

Assuming all decay photon correlations have the same shape as from 0s

Estimating 0 background
  • Tag 0 decay photons using invariant mass of photon pairs
    • Use simulations to calculate tagging efficiency
    • Use π0s to obtain full background

Ali Hanks - APS

tagging efficiency



 pair accepted

single  accepted

Tagging efficiency
  • Use fast MC to simulate 0 decays to obtain pT and  dependent efficiency correction
    • generate 0s in gaussian distribution around "trigger" hadron
    • Compare yield when both photons are accepted to yield for all single 0 photons accepted

Ali Hanks - APS

remaining decay background




Remaining decay background
  • Can we use π0 decays to estimate all decays?
    • Use PYTHIA to study difference in shape of distribution for other decays from that of π0 decays - first try 

Ali Hanks - APS

getting the shape right

Use to estimate remaining decay background (, ')

h - frag near side  distributions



  • Compare with what PYTHIA gives for actual frag distribution and yield
Getting the shape right
  • How well can we do measuring the eta directly?

Ali Hanks - APS

  • pQCD NLO calculations predict a frag spectrum > 20% of the inclusive spectrum in pp collisions
  • In Au-Au significant nuclear modification to the bremsstrahlung contribution may be seen
  • Looking for photons correlated with a hadron has the advantage of maximizing the fragmentation signal while removing uncorrelated prompt photons and random background sources
  • PYTHIA shows a difference in correlation shape between 0 and other sources that needs to be accounted for to do final background subtraction
    • may be possible to estimate this using simulation

Ali Hanks - APS