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High p T g -h Correlations Using the PHENIX Detector

High p T g -h Correlations Using the PHENIX Detector. Matthew Nguyen, Stony Brook University. Why Direct Photon Correlations?. “Leading Order” Picture Exact Momentum Balance w/ Away-Side Jet Compton Dominance p+p : Measure Gluon Distribution Function A+A:

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High p T g -h Correlations Using the PHENIX Detector

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  1. High pTg-h Correlations Using the PHENIX Detector Matthew Nguyen, Stony Brook University

  2. Why Direct Photon Correlations? • “Leading Order” Picture • Exact Momentum Balance w/ Away-Side Jet • Compton Dominance • p+p: Measure Gluon Distribution Function • A+A: • Calibrated Probe of Energy Loss • More Sensitive Probe than single particle spectra, Di-Hadron Correlations • The Golden Channel for Jet Tomography? • the fine print • Fragmentation photons • Initial state effects (Shadowing , kT) • Still sensitive to geometry / space-time evolution • Quark vs. Gluon Energy Loss q γ γ q g q g q Compton Annihilation Direct Photon Processes at LO X.N. Wang, Z. Huang PRC55 (1997) 3047-3061 IAA Quark Matter 2008

  3. g-h Correlations in PHENIX Drift Chamber Electromagnetic Calorimeters Direct photon cross section in p+p h-h Correlations in Au+Au PbGl PbSc Quark Matter 2008

  4. PYTHIA 6.1 Statistical Subtraction Method Photon Sources in PYTHIA • Large background from meson decay • Direct g S/B increased by jet suppression in central Au+Au collisions • p0-h decayed via MC to get decay g-h • h-h measured in p+p • Rg = Ng (inclusive) / Ng (decay) • Can perform a statistical subtraction of per-trigger yields: • Method well-suited for high multiplicity environment (Au+Au) Rg measurement in Au+Au 200 GeV Rg Where Y = # hadrons per photon pT [GeV] Quark Matter 2008

  5. QM2006 Direct Photon-Hadron Results At QM2006 PHENIX showed “proof of principle” level g-h results Run 5 p+p @ 200 GeV Run 4/5 p+p/Au+Au @ 200 GeV • Subtraction method is robust • Near-side disappearance consistent with g+jet expectations • Away-side comparable to hadron triggered case • Away-side yields in Au+Au are suppressed compared to p+p • g-h measurement is statistics limited Quark Matter 2008

  6. New Results  Increased Precision 1/Ntrig dN/dDf (A.U.) • Systematic error dominated by uncertainty in Rg and h-h contribution *Note: Run 6, 7 Absolute Efficiencies not finalized – In Arbitrary Units Quark Matter 2008

  7. Direct Photon-HadronDf Correlations in p+p pT,hadron 1/Ntrig dN/dDf (A.U.) Df [rad] pT, photon Subtraction technique works over wide range of pT! Quark Matter 2008

  8. Photon Recoil Jets Run 6 p+p @ 200 GeV Per-Trigger Yield (A.U.) h-h measurement pT, photon • 1st approximation -- g recoil jets similar to hadron triggered jets in p+p • <Q2> of hard scattering should be lower due to z bias in hadron-triggered correlations • kT effect need not be the same – needs to be measured • g triggered yields sensitive to the FF g-h measurement Quark Matter 2008

  9. Quark Matter 2008 Vacuum Fragmentation Function p+p @ 200 GeV PRD 74 (2006) 072002 • Baseline for modified FF in Au+Au • p0-h xE distributions were shown to poorly discriminate between quark (– ) and gluon(--) fragmentation due to p0 trigger bias (z<1) Run 5 Direct g-h Run 3 p0-h 1/Ntrig dN/dxE( A.U.) – Quark FF --Gluon FF Preliminary • In “leading order” picture: • Direct photon xE distributions • should be sensitive to underlying • (quark) fragmentation function • Use Run 6 data to reduce errors – DELPHI, Eur. Phys. J. C13,543, (1996) -- OPAL Z.Phys. C 69, 543 (1996)

  10. Au+Au Run 7 Direct Photon-Hadron Correlations (7 < pT,g < 9) 0 Au+Au analysis is challenging: Additional sources of uncertainty from underlying event subtraction (ZYAM), elliptic flow subtraction and p0 combinatoric background Little or no near-side production associated with direct photon triggers Away-side yields indicate large jet suppression in g+jet channel Working to reduce systematics and more data to analyze (~2X) Quark Matter 2008

  11. The pT Landscape Direct Photon S/B (Rg) p0 Combinatoric S/B pQCD Accuracy pT,hadron Correlation Function(A.U.) Correlation Function(A.U.) Run7 Au+Au 200 GeV Run7 Au+Au 200 GeV 12-15 X 3-5 GeV 7-9 X 3-5 GeV Rate Jet S/B Near/Away-Side > 1 Near/Away-Side ≈ 1 pT, photon Quark Matter 2008

  12. Near-Side Fragmentation Run 6 p+p @ 200 GeV Per-Trigger Yield (A.U.) h-h measurement pT, photon 0.3 0.25 0.2 0.1 z • Near-Side yield small but non-zero • for z  0 • Expect near-side contribution from photon fragmentation • Systematic errors for g-h correlations • large on the near-side g-h (h-g) measurement Quark Matter 2008

  13. Fragmentation Photons q g γ • Fragmentation diagrams must be included at NLO to cancel divergences • |z| < 1 for fragmentation photons, but how much less? • Dparton/photon(z) not well constrained • May oppose either quark or gluon jets • Important for LHC • Suppressed by E-loss q γ q g g q Bremsstrahlung diagrams Induced Bremstrahlung I. Vitev, 2007 RHIC-AGS Meeting --RHIC – LHC Rate of fragmentation photons may be increased by presence of medium partons Quark Matter 2008

  14. h-g Correlations X10-3 • Idea: By triggering on a hadron and looking for near-side direct photon partners one can measure the fragmentation photon yield directly • Measure hadron - inclusive g and hadron - decay g correlations • Decay corr’s are made by tagging p0 and h by invariant mass • Must know tagging efficiency and false tagging rate precisely  dominant source of systematic uncertainty h-inclusive g X10-3 h-decay g X10-6 2.5 < pT,g < 3.5 10-6 h-direct g Quark Matter 2008

  15. h-g Correlations II • First measurement of it’s kind at RHIC! • Will measure jet shape distributions, e.g. xE, pout • Constrain photon FF? • Nfrag/Ninc ≈ 0.1 at intermediate pT • Measure in Au+Au Quark Matter 2008

  16. Outlook • Direct photon-hadron correlations are moving from the proof of principle stage towards the precision measurement stage • Finalizing Run 6 p+p for increased precision and broader kinematic range compared to previous results • Analyzing Run 7 Au+Au data set and working to reduce systematic uncertainties • Baseline measurements of g-h and h-g correlations are being made to ensure that theory is well under control • Fragmentation component measured – Nfrag ≈ 0.1 Ninc • Away-side jet suppression in the g+jet channel appear strong • PHENIX is currently working on Run 6 and Run 7 p+p and Au+Aug-h correlations at high pT Quark Matter 2008

  17. Backup Slides Quark Matter 2008

  18. Constructing the Decay Background • To start: Construct g(p0) - h from p0-h • Problem is that tagging the p0’s introduces a bias <pT> tagged = <pT> true • Developed a pair-by-pair correction procedure Correction Procedure: Per-Trigger Yield from g(p0): Sample MC weight function: • Decay kinematics reproduced by Monte Carlo include energy resolution effects and acceptance (P) • This is just for p0’s, What about contributions from other mesons?  This approach implicitly assumes they have the same associated yield – Analytic Form – w/ Detector Response Quark Matter 2008

  19. h-h Correlations Run 5 p+p @ 200 GeV • Parent-h • First measurement of h-h correlations at RHIC! • g(h)-h Yields generated by same procedure as g(p0)-h • Combinatorial background under h peak subtracted using a sideband procedure PRC 75, 024909 (2007) • h-h • g(h)-h Quark Matter 2008

  20. QGP Induced Photon Production Jet Conversion– Hard partons re-scatter off medium partons via Compton process Induced Bremsstrahlung – Rate of fragmentation photons increased by presence of medium partons Induced Bremstrahlung Jet Conversion • Both effects still speculative • Are there experimental signatures? • pT, v2 dependence (see other talks in session) • h-g correlations Zharkarov – JETP Lett. 80 (2004) Turbide et al., Phys. Rev. C72 (2005) Quark Matter 2008

  21. Prompt Photon Spectrum Quark Matter 2008

  22. p+p Integrated Yields Quark Matter 2008

  23. kT Effect kT Effect: Net momentum of di-jets (and g-jets) due to initial state radiation xE slopes for g-h from PYTHIA kT measurement for p0-h PRD 74 (2006) 072002 • kT = 2.5 GeV • kT = 0 PYTHIA shows that kT changes the xE distribution significantly kT measurement in g-h is needed kT measurements the di-jet channel show a large effect -- 2.5 – 3.0 GeV Quark Matter 2008

  24. Gluon Fragmentation Functions Inclusive production from AKK FF • Gluons contribute significantly to • single inclusive measurements at RHIC • However different parameterizations give divergent estimations of gluon contribution • e.g., AKK FF’s indicate much higher gluon contribution than previous fits Compilation by Hirai et al. Phys Rev D75 (2007) 94009 Quark Matter 2008

  25. Rg from Run 4 Au+Au 200 GeV Quark Matter 2008

  26. Using p+p data in FF‘s De Florian, Sassot, Stratmann STAR, Phys Lett B, 637 (2006) 161 hep-ph:0707.1506 Phys. Rev. D75, 114010 (2007). STAR identified hadron spectra constrain the gluon FF at large z Use twoparticle correlations to constrain g p at all z? New set of fragmentation functions include p+p (and DIS) data and obtain very different parameterizations of the gluon FF’s Quark Matter 2008

  27. γ q g q Identified Hadron Correlations • g recoil jets more likely to be a quark jet than p0 recoil jets • Quark and gluon jets may have different particle content •  p/p on the away-side of g vs. p0 recoil sensitive the FF’s For definiteness assume: In this simplified picture particle ratios can be written in terms of FF’s: & Kretzer Acta Phys. Polon. B36, 179 (2005) Quark Well constrained from e+e- Quark/Gluon Contribution Double ratio gives partial cancellation of systematics due to particle identification and direct photon subtraction Gluon pT(p) Quark Matter 2008

  28. g-Identified h Results (Under Construction) g- identified hadron correlations can be performed in the range 5 < pTg < 12 X 1 < pTh <2 This corresponds to about 0.1 < z < 0.2 5 < pTg < 7 X 1 < pTh < 2 PHENIX Arbitrary Normalization • Hadron efficiencies not finalized • Systematic errors are large but • will most cancel in ratio • So far run 5 only • Run 5 + 6 should enable a significant • measurement (integrated over away-side) Quark Matter 2008

  29. Rg from p+p Collisions In p+p there is no published Rg but deriveable from p0 and g spectra For pure power law: N(p0)/N(g) = (n-1)/2; n = p0 slope parameter Quoted from g x-sec analysis Quark Matter 2008

  30. Trigger Biases and the kT effect • Insensitivity of dN/dxE to true FF is the interplay of two trigger biases: • Trigger Bias #1: Trigger requirement • biases the near side FF to high z • Does not affect away-side jet • Absent in g-h kT Effect: Acoplanarity of di-jets (and g-jets) due to initial state radiation • Trigger Bias #2: Longitudinal • component of kT biases jet CM in • direction of the trigger for pT,t > pT,a • Does affect the away-side jet • Present in g-h kT effect affects: • away-side jet width • Inclusive high pT x-sections • Recoil spectra (dN/dxE) kT described by NLO, Why do we need to measure it? kT measurement for di-jets Quark Matter 2008

  31. Gluon Fragmentation The gluon fragmentation function has large uncertainties! Is there an independent test of gp? Quark Matter 2008

  32. Data Fit pQCD Limitations Auranche 2006 Closer: Agreement only 20% Not good enough to constrain kT Agreement w/ data is remarkable! Better than jet cross-sections Quark Matter 2008

  33. kT in the g-jet channe M. Begel, Nucl. Phys. B 79, 244 1999; Ph.d thesis (1998) Independent of process Measurement for g-p0 at lower Quark Matter 2008

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