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Ali Hanks for the PHENIX collaboration Quark Matter 2009

Measurements of Fragmentation Photons and the Flavor Dependence of Jet Fragmentation with the PHENIX Detector. Ali Hanks for the PHENIX collaboration Quark Matter 2009 The 21 st International Conference on Ultrarelativistic Nucleus-Nucleus Collisions Knoxville, Tennessee

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Ali Hanks for the PHENIX collaboration Quark Matter 2009

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  1. Measurements of Fragmentation Photons and the Flavor Dependence of Jet Fragmentation with the PHENIX Detector Ali Hanks for the PHENIX collaboration Quark Matter 2009 The 21st International Conference on Ultrarelativistic Nucleus-Nucleus Collisions Knoxville, Tennessee 26th March, 2009

  2. Understanding energy loss • Strong suppression of charged hadrons • 0 and  show similar suppression • energy loss occurs at parton level? • Direct photons not strongly interacting unmodified? • initial state effects • new/modified sources in Au+Au collisions • do not interact with medium after production

  3. Compilation by Hirai et al.; Phys Rev D75 (2007) 94009 de Florian et al (DSS) PRD 76 074033 (2007) quark vs gluon dependence of energy loss • Naively expect more energy loss for gluon jets (color factor - 9/4) • theory claim: protons come from gluons • RAA inconsistent with this picture • large uncertainty in gluon FF Would like to have: • Better tag of quark/gluon jets • Better constraints on fragmentation functions

  4. PRD 74 (2006) 072002 p0-h while -jet to constrain parton fragmentation •  recoil jets more likely to be a quark jet than 0 recoil jets • p0-h xE can't distinguish between quark* and gluon+ fragmentation because of trigger bias (z<1) * DELPHI, Eur. Phys. J. C13,543, (1996) + OPAL Z.Phys. C 69, 543 (1996) • -h should do better • Quark and gluon jets may have different particle content • p/ on the away-side of  vs. 0 recoil sensitive the FF’s

  5. 200 GeV p+p PHENIX preliminary 7 < pTg < 9 GeV, 3 < pTh < 5 GeV Inclusive g-h Decay g-h Direct g-h dir-h correlations • high pT photon correlated with associated hadrons • decay background from reconstructed 0 and  trigger samples • Can compare associated h distributions to probe fragmentation functions

  6. Zhang, Owens, Wang, Wang: arXiv:0902.4000 Yield of hadrons per isolated, direct  CTEQ + NLO + KKP -jet  quark fragmentation • Well described by NLO pQCD • quark fragmentation function well constrained • additional power to test kT effect • Next step - identify associated hadrons • comparison with 0-hid will better constrain gluon FF J. Franz; Hard Probes 08

  7. Ivan Vitev and BWZ, arXiv:0804.3805 Inclusive direct photons Fragmentation/Bremsstrahlung photons Competition between suppression due to energy loss and bremsstrahlung enhancement • fragmentation/bremsstrahlung photon component highly sensitive to E-loss • direct measurement of radiation spectrum • important to have well understood p+p baseline Medium-induced enhancement predicted to be < 10% 7

  8. Ivan Vitev and BWZ, arXiv:0804.3805 Opal Eur.Phys.J.C2:39-48,1998 ~20% variation at high pT 40%-50% at low pT Phys. Rev. Lett. 98, 012002 (2007) INCNLO(v1.4): J. Ph. Guillet, M. Werlen et al Fragmentation photons from e+e- to p+p • Inclusive specturm well described by NLO pQCD • however, still room for significant variation in theory • Poorly constrained photon fragmentation function from e+e- •  large uncertainties in frag/inc in p+p

  9. Phys. Rev. Lett. 98, 012002 (2007): hep-ex/0609031 Constraining these predictions in p+p • “isolation” cuts made to distinguish prompt photons • large experimental uncertainties • difficult to match to theory • not practical in A+A • direct measurement of fragmentation contribution would do better • will be correlated with jets • “prompt” photons should be excluded 9

  10. h-inc    h -1 0  1 h-dec -1 0  1 h-frag  Measuring fragmentation photons directly • Select photons correlated with hadron within range of pT's • naturally eliminates uncorrelated background • no contribution from LO direct photons • random backgrounds removed • decay background estimated from tagged 0 and  photons near side only (-/3 to /3)

  11. Yield of fragmentation photons • Integrated near-side yields: compare fragmentation to inclusive • can this be compared to predictions of frag/inc?

  12. Phys. Rev. Lett. 98, 012002 (2007): hep-ex/0609031 Getting back to predictions • Theory predicts frag /all direct ~ 0.25 - 0.35 at pT, ~ 6 GeV/c • measured frag /inc ~ 0.09+/-0.04  frag /inc*Ndir/Ninc = frag /all direct ~ 0.56+/-0.25 or ~ 0.3 - 0.8 • Compare with previous results applying isolation cut • In both cases not a direct comparison • ratio of conditional yields in finite integration window NOT of cross sections • To get from frag /inc to frag /dir : • R = Ninc/Ndecay • ≈ 1.19 at pT ~ 6 GeV/c • Ndir/Ninc = 1 - 1/R ≈ 0.16 not including systematic error

  13. Newer results - vary hadron trigger • Systematics at low pT much smaller with new data and improved method • statistical fluctuations in background dominate systematics at high pT

  14. jTy,trig pT,trig tj  jet pout aj pT,assoc Going further - jet properties • Qualitatively and quantitatively different shape for pout of fragmentation photons • Look at variation as a function of trigger pT to quantify further pout = pT,assocsin()) on near-side pout ~ jTy,trig

  15. pout in trigger bins

  16. pout for frag vs hadrons • comparison of 0-h results with h-inc results • restricted angular separation in this analysis (||<1.0) will lower rms values • decay kinematics due to measurement of decay photons rather than 0 • direct measurement of vacuum radiation - should be similar to gluon • frag distribution broad relative to inclusive photon or 0-h from 0-h correlations Phys. Rev. D 74, 072002 (2006) s(Q2) difference?

  17. Summary/Conclusions • Using photon triggers provides additional constraints for parton fragmentation functions • correlated with identified hadrons can constrain the gluon FF • possibilities for studying energy loss of quarks vs gluons • First measurement of fragmentation photons shows significant yield on near-side of conditional trigger hadron • more direct comparison/constraint of theory will come from cross section • Fragmentation photon pout distributions provide direct measure of parton radiation • broad relative to inclusive photon and hadron • promising for similar measurements in A+A • implications for gamma-jet measurements

  18. Backup Slides

  19. h (0,,) Medium induced modifications: medium • Occurs at NLO - modifies the fragmentation component of direct photon spectrum only 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 Leading order(LO) - prompt: Next to leading order (NLO) - fragmentation: 3

  20. 2.0 < pT,h <5.0 and pT, from 1.5 - 15 inc frag PYTHIA Comparison • Results agree qualitatively with what PYTHIA predicts • Detailed comparisons of inclusive and fragmentation can give insight into details of parton radiation

  21. 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

  22. Cold Nuclear Matter effects • Complication due to possible cold nuclear matter affects playing a role • initial state energy loss effects may be non-negligible • not significant within current experimental limitation Ivan Vitev and BWZ, arXiv:0804.3805 6

  23. kT effect in -jet • kT bias effect in non-flat (non-universal) slopes still present up to ~ 10 GeV/c

  24. √s = 200GeV 5 < pTg < 7 X 1 < pTh < 2 Arb. Normalization -indentified h  gluon fragmentation • Correlations in the range • 5 < pTg < 12 X 1 < pTh <2 • corresponds to ~ 0.1<z<0.2 • Systematic errors will most cancel in ratio

  25. tag    h  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 this tagging is needed

  26. 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

  27. π0 trigger  pair accepted single  accepted Tagging efficiency • Use simple simulation of 0 decays to obtain pT and  dependent efficiency correction • give 0's gaussian  around "trigger" hadron • Kinematics of simulated 0's and  are estimated from data • use 0 and  correlations to simulate their  distributions around a "trigger" hadron • use input pT distribution from hadron trigger data to get conditional yield correct • Compare yield when both photons are accepted to yield for all single 0 photons accepted

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