# Presentation: 484237

## Direct measurement of  fragmentation photons in p+p collisions at√s = 200GeV with the PHENIX experiment

**Ali Hanks**
***for the PHENIX collaboration***
**Hard Probes 2008**
**	****Illa da Toxa, Galacia-Spain**
**	****June 12****th****, 2008**

## Photon Sources

- _Direct photons_

- Leading order(LO) - prompt:

- Next to leading order (NLO) - fragmentation:

- _Decay photons_

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

**medium**

- _Medium induced modifications_:

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

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## Photons in heavy ion collisons

**Many sources of photons which contribute to total Au+Au direct photon cross-section**
- difficult to test predictions of modification of any single component from the inclusive spectrum
- Energy loss effects insignificant within current experimental limitations

**Turbide et al; hep-ph/0502248**

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## Predicting modification

**Measuring R****AA**** could highlight any modification to direct photon spectrum**
- Various predictions for affects of energy loss range from enhancement in to suppression
- Modification to production of fragmentation photons is dominant source of variation from unity
**Complication due to possible cold nuclear matter affects playing a role**

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## Why fragmentation photons?

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

**Improve our understanding of jet quenching:**
- direct measurement of radiation spectrum
**Modification very sensitive to energy loss of the jet**
- significant enhancement for pT<10 GeV/c

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## pQCD predictions

**NLO pQCD describes data well**
**fragmentation component significant (> 20%) at low p****T**** **
**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 **

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## Finding Fragmentation Photons

**Use jet correlations to single out fragmentation photons**
- Trigger on high pT hadrons, 2-5 GeV/c, and calculate  distribution of associated photons, focusing on the near side (-/2 to /2)

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## Tagging 0 and  decay photons

**Tagged correlations are needed for subtracting decay photons**
- tag photons in pairs that fall within the 0 or  peak

**To measure total decay yield, efficiency of tagging method is needed**

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## Tagging efficiency

**Use simulation of ********0****/******** decays to obtain p****T**** and ******** dependent efficiency correction**
- give 0’s and ’s  and pT dependent distributions around a "trigger" hadron
**Kinematics of simulated ********0****’s and ****’****s are estimated from data**
- fit 0 and  correlations to simulate their  distributions
- use input pT distribution from hadron trigger data to get conditional yield
**Compare yield when both photons are accepted to yield for all single ********0****/******** photons accepted**

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## Putting it all together

**The final decay yield is obtained by combining everything we've seen so far****:**

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## We see fragmentation photons!

- Final subtraction yields significant signal in most pT ranges
- Ratio of near-side fragmentation photon yield to inclusive ~0.1 for intermediate pT

-  systematic

- Scale systematic

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## Comparison with previous results

- Can compare with range of allowed values based on isolated photon measurement
- See agreement within systematic uncertainties
- Should be cautious: very different conditions (biases) went into these two measurements

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## Summary and Outlook

- pQCD NLO calculations predict a frag spectrum > 20% of the inclusive spectrum in pp collisions
- In Au-Au significant nuclear modification to the fragmentation contribution may be seen
- Preliminary p+p measurement shows fractional yield of fragmentation photons of ~0.1GeV/c at intermediate pT
- Success of method is promising for future measurements
- pout and jT measurements will provide information about the jet properties and may be more directly comparable with theory calculations
- fragmentation photons in d+Au and Au+Au
- Possibility for improvements in systematics at low and high pT
- more sophisticated tagging efficiency estimates
- Careful study of trigger pT dependence

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**Backup Slides**

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## Reconstructing 0's and 's 

**Reconstruct ********0****'s and ********'s from photon pairs, use event mixing to estimate combinatoric background**
**Fit remaining background to get S/B correction**

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## Subtraction method in Pythia

**Measure ********0**** and ******** photons directly to obtain decay background**
**test subtraction in PYTHIA**

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## Tagging efficiency - 2nd method

**Can also calculate tagging efficiency from full GEANT simulation using PYTHIA input**
**tag ********0**** photons as in the data, using simulated kinematics**
**construct true ********0**** photon distribution from PYTHIA information**
**extract tagging efficiency by calculating how often true ********0**** photons are successfully tagged**
**Compare this with what is calculated using the fastmc**
**Any differences are indications of systematic uncertainties in how well the efficiency can be determined**
**Full simulation can be used to understand them**

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## Tagging efficiency

**Comparison shows a small systematic difference that needs to be understood**
**several possibilities currently being explored**
**differences in single particle efficiencies that contribute to loss of pair photons would effect efficiencies**
**differences in the energy smearing**

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