1. Ali Hanks - APS 2008 1 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

2. Ali Hanks - HP 2008 2 12/06/08 Photon Sources h (  0, ,  ) 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

3. Ali Hanks - HP 2008 3 12/06/08 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

4. Ali Hanks - HP 2008 4 12/06/08 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 I. Vitev; Winter Workshop ‘08

5. Ali Hanks - HP 2008 5 12/06/08 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 p T <10 GeV/c

6. Ali Hanks - HP 2008 6 12/06/08 Phys. Rev. Lett. 98, 012002 (2007): hep-ex/0609031  frag /  inc for direct photons INCNLO(v1.4): J. Ph. Guillet, M. Werlen et al Phys. Rev. Lett. 98, 012002 (2007): hep-ex/0609031 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

7. Ali Hanks - HP 2008 7 12/06/08 Finding Fragmentation Photons Use jet correlations to single out fragmentation photons –Trigger on high p T hadrons, 2-5 GeV/c, and calculate  distribution of associated photons, focusing on the near side (-  /2 to  /2)  h Use event mixing to correct for acceptance  

8. Ali Hanks - HP 2008 8 12/06/08 Tagging  0 and  decay photons Tagged correlations are needed for subtracting decay photons –tag photons in pairs that fall within the  0 or  peak h     tag To measure total decay yield, efficiency of tagging method is needed

9. Ali Hanks - HP 2008 9 12/06/08 Tagging efficiency Use simulation of  0 /  decays to obtain p T and  dependent efficiency correction –give  0 ’s and  ’s  and p T 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 p T distribution from hadron trigger data to get conditional yield Compare yield when both photons are accepted to yield for all single  0 /  photons accepted π0π0 trigger   pair accepted single  accepted

10. Ali Hanks - HP 2008 10 12/06/08 Putting it all together The final decay yield is obtained by combining everything we've seen so far : Assuming heavier decay photon correlations can be estimated using  's

11. Ali Hanks - HP 2008 11 12/06/08 We see fragmentation photons! Final subtraction yields significant signal in most p T ranges Ratio of near-side fragmentation photon yield to inclusive ~0.1 for intermediate p T  systematicScale systematic

12. Ali Hanks - HP 2008 12 12/06/08 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

13. Ali Hanks - HP 2008 13 12/06/08 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 p T Success of method is promising for future measurements –p out and j T 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 p T –more sophisticated tagging efficiency estimates –Careful study of trigger p T dependence

14. Ali Hanks - HP 2008 14 12/06/08 Backup Slides

15. Ali Hanks - HP 2008 15 12/06/08 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

16. Ali Hanks - HP 2008 16 12/06/08 Subtraction method in Pythia Measure  0 and  photons directly to obtain decay background test subtraction in PYTHIA Use  to estimate remaining decay background ( ,  ') Compare with what PYTHIA gives for actual  frag yield and  distribution PYTHIA

17. Ali Hanks - HP 2008 17 12/06/08 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

18. Ali Hanks - HP 2008 18 12/06/08 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