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Studying Jet Modifcation with Direct Photon-Hadron Correlations in RHIC Collisions

Studying Jet Modifcation with Direct Photon-Hadron Correlations in RHIC Collisions. Justin Frantz Ohio University DNP 10/13/09. g. Outline. Introduction Overview of RHIC Photon-Hadron Methods Direct Photon-Hadron Results Implications for Energy Loss esp. Geometric Biases in HI

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Studying Jet Modifcation with Direct Photon-Hadron Correlations in RHIC Collisions

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  1. Studying Jet Modifcation with Direct Photon-Hadron Correlations in RHIC Collisions Justin Frantz Ohio University DNP 10/13/09 g Justin Frantz DNP 2009

  2. Outline • Introduction • Overview of RHIC Photon-Hadron Methods • Direct Photon-Hadron Results • Implications for Energy Loss esp. Geometric Biases in HI • Fragmentation Photons • Future Directions/Implications for g-jet • talk posted at http://www.phy.ohiou.edu/~frantz/ Justin Frantz DNP 2009

  3. g Direct Photons @ RHIC Jet Suppression! • Direct g: excellent probe of strongly interacting medium • they escape medium without interacting • At RHIC we’ve established (as predicted!) that: • high pT hadrons are suppressed • Direct g aren’t • Status of latter conclusion: (slight supression @ highest pT?) • Effect not significant within errors & Cu+Cu data does not show • Progress: New Au+Au h measurement: highest pT’s? (T. Sakaguchi DNP09) Cu+Cu 200 GeV Great Cross Check (No “EMCal merging” effects as w/ p0) Justin Frantz DNP 2009 X.N. Wang, Z. Huang PRC55 (1997) 3047-3061

  4. Photon to Photon-Jet • Thus Established: Colorless photon natureexcellent complement to single jet/ di-jet • Perfect complement to any jet observable • replace jet / hadron w/ g • At Leading Order / Compton Scattering Dominance: • Exact Momentum Balance w/ Away-Side Jet • ignoring Photons from Jet Fragmentation (TBD) • Still good approximation including higher orders • p+p: Measure Gluon Distribution Function • A+A: Calibration of Energy Loss for awayside jet: D(z) w/o jet reco • More sensitive (?) probe than single particle spectra, Di-Hadron Correlations q γ γ q g q g q Compton Annihilation Direct Photon Processes at LO D(z) Jetg Fragmetation Direct Photon Also LO Process  Complication Justin Frantz DNP 2009

  5. A “golden” HI jet channel: g-jet Will discuss this in more detail after we take a look at the data • Purely hadronic observables clouded by the fact that hadrons are in the modified jets themselves • Geometric surface bias of the single jet, di-jet ? • Away jet should have longer path length  > suppression • Direct g production unmodified by medium: no surface bias for g vertices • Plus dominant partonic type becomes purely q-jets compared mainly g-jet/mix in jet-jet head-on view of Au+Au collision overlap region Compton Diagram Justin Frantz DNP 2009

  6. Overview of RHIC g-jet (g-h) Methods STAR and PHENIX Justin Frantz DNP 2009

  7. Reminder: 2-p Angular Correlations Pair Yields (Df) jet-jet g-jet • Jets by 2-particle correlations in particle pair opening azimuthal angle • Au+Au Flow Subtraction: Underlying Event • Find Yield, Y, per-trigger particle • Soft particles: large underlying pedastal w/ flow (v2) modulation ! • Y  J(Df) 0 p+p dijet Au+Au p p 0 0 Au+Au Justin Frantz DNP 09 Hawaii

  8. Main Subtraction Formulae Note: inclusivemeans - for PHENIX inclusive single photons -for STAR , “g-rich” EMC cluster sample, but not 100% devoid of po decay means meson/meson decay bkg to direct photon signal (below) Trigger Eff divides out…crucial point No absolute g eff needed! STAR PHENIX direct includes frag For PHENIX “Rg” directly measured in other analyses Rg=Nall/Ndecay For STAR, Solve for “R” by forcing nearside to be zero (Compton only--implicit isolation) direct includes NO frag Justin Frantz DNP 2009

  9. Main Subtraction Formulae g-h: double subtraction of 2-p measurements rich 0 • For PHENIX full subtraction w/ flow modulation included (low pTassoc) • New Norm Normalization method employed dramatically reduced systematics • For STAR working so far with pTassoc > 3  actual v2 modulation negligible • Underlying event level still relevant  ZYAM like subtraction Yxxx∝ Ydirect∝ Cincl- Cdecay- - [ ] Bkg(Flow) Bkgincl (Flow) Bkgdecay(Flow) Norm Justin Frantz DNP 2009

  10. PHENIX Systematic Improvement In going from prelim to final, we found our biggest reduction in systematic from Underlying event normalization I’d like to recommend this method to STAR as they push down in pT assoc • Biggest improvement for publication Hard Probes ’08 Result from last summer New Result (~Same Scale) Justin Frantz DNP 2009

  11. PHENIX MSMP Normalization • MSMP or “Absolute Normalization” • Alternative to ZYAM “b0“ • Used previously in PHENIX publications • arXiv:0801.4545v1 Phys. Rev. C 78, 014901 (2008) • Njet pairs “=“ <NtrigNassoc> - <Ntrig><Nassoc> • ξ correction factor for background centrality multiplicity correlation ξ ξ b0 Bkg(Flow) Justin Frantz DNP 2009

  12. Direct g Bkg-h Correlations rich h+- 0 h+- • Both Experiments: Derive from p0-h corr. • STAR: (p02g decay merges to single EMcluster) • Tightly contained shower cut leaves only small angle symmetric p decay in “grich” sample • Use easily-identifyiable, less symmetric p0 sample to form p bkg-h correlation Ybkg-h • Assume (after checking ;>) all other background has same Nearside/Awayside ratio • PHENIX: (2-g inv mass reco of all p0) • Bkg-h is simply meson decay g-h Ydecay • Also use p-h to calculate Ydecay, by mapping Np-h(pT)  Ng-h (pT) (& pg) • Build Ydecay (Df) up from p0 -h pair by pair with map weights • Low trigger pt bins, systematics from combinatoric under p0 peak p0 g g Justin Frantz DNP 2009

  13. Other contributions to Bkg-h • Bkg-h not only from p0, e.g. for PHENIX: • Raw Fraction of decay g from other mesons 20% : > 18% from h • δ = gother/gp = 1.24 • h, etc… triggered 2-p correlation not possible in Au+Au (Comb Bkg) • STAR cuts away some fraction of above w/ shower cut • Assumption : as w/ other contrib’s, same NS/AS ratio negligible effect • PHENIX Assumption: same suppression as p for other mesons • (Fragmentation largely outside medium) • Well justified by single h (other high pT meson) measurements • Measure h-h in p+p, scale to Au+Au as correction to p0-h • First h-h measurement at RHIC!!! Justin Frantz DNP 2009

  14. …point is so far is these are very complicated analyses!!! and now… Overview of g-h Results What we’ve measured Justin Frantz DNP 2009

  15. STAR and PHENIX g-h Results • Both RHIC experiments busily making measurements in this channel • STAR • Impressive array of prelimary results including 2007 long Au dataset • Au+Au , d+Au, p+p (baselines) Frag. Function D(z), Ratios (IAA) • Wide z (pTh/pTtrig) coverage and centrality dependence • PHENIX • Recent Publication of 2004 Au+Au dataset measurement • My other simultaneous baby, with SUNY grad student Matt Nguyen, (my wife and I picked a better name for the the first one) • Preliminary 2007 dataset w/ increased precision • More precise isolation cut measurements in p+p • Exploration of fragmentation photons themselves! • Both collaborations need more precision to make definitive statements about Eloss contraints Ava Justin Frantz DNP 2009

  16. Au+Au Awayside Yield Measurement PHENIX STAR • Both collab’s focus on the 1800 Back to Back Region only pTh(GeV/c) Phys. Rev. C 80, 024908 (2009) STAR always bins in zT = pTh/ pTg • 4 x statistics collected • new data fully consistent w/ 2004 data Justin Frantz DNP 2009

  17. Look at z > 1 ? • Because of Fragmentation g and NLO (2->3) and the way we define our z, it can be > 1 • in fact some (Wang, Arleo) suggest that is a sensitive region Highest precision! STAR Preliminary all pT combo’s ~scale (like frag fn should) Justin Frantz DNP 2009

  18. Slope comparison • Fit a ~universal curve for all jet energies • p+p slope of 6.89 ± 0.64more consistent with quark fragmentation of b=8 than b=11 for gluons (LEP) • Slope of Au+Au is 9.49 ±1.37 • Au+Au slope is ~1.3σ higher than p+p • Still just hint: need better stats Justin Frantz DNP 2009

  19. How do they compare? Actually first we should ask… Some Questions: Aren’t they measuring different quantities? For that a short digression… Justin Frantz DNP 2009

  20. Measuring Different Quantities? • STAR method like statistical isolation cut • PHENIX statistical method includes fragmentation photons • Does the difference in the STAR/PHENIX method affect the interpretations ? It turns out, not subtantially • Especially, can we interpret PHENIX results ignoring the fragmentation photons? • In principle PHENIX leaves this to theory • But in practice YES… • Because PHENIX has also measured yields in p+p using isolation cuts which addresses this question partially Justin Frantz DNP 2009

  21. Real (Ev. by Ev.) Isolation cut: p+p • Idea: most p0/decay g can be removed by looking at other event properties • Other detector activity around g Jet • ie cuts di-jet events • PHENIX: Isolation cuts • Sum of track pT and Emcal E must be < 0.1 * Eg • In cone around g of size DR = sqrt(Df2+Dh2) = 0.5 • Also cuts Fragmentation Direct g pure Compton Direct g • Still not 100% pure, still apply subtraction of iso p0 Justin Frantz DNP 2009

  22. Comparison of Stat/Iso Methods 2 <pT,a< 3 GeV 1 <pT,a< 2 GeV 3 <pT,a< 5 GeV 5 <pT,t< 7 GeV • Much more precise because subtraction so much smaller • Results consistent with statistical method on the away (jet) side 200 GeVp+p 1/Ntrig dN/dDf 7 <pT,t< 9 GeV 9 <pT,t< 12 GeV Preliminary 12 <pT,t< 15 GeV Df [rad] p Awayside Awayside Only 22 Justin Frantz DNP 2009

  23. Fragmentation g Contribution • Even though for PHENIX Stat. Method: • Direct g = Compton (Iso) + Fragmentation Prompt • … in practice results ≈ Compton Only • Why ? Two Reason • Frag. Prompt should be suppressed w/ associated jet • Au+Au stat method: Compton + ~1/5 Frag ≈ Compton • But then p+p wrong baseline for IAA (use Iso) • Au+Au/(p+p Iso) -- current p+p error bars encompass p+p • Actually this is not established. Zakarov, Gale, Qiu arguments: at some pt could be enhanced! • Near future publication of PHENIX p+p Isolation method: make IAA both ways (iso p+p in denominator) Justin Frantz DNP 2009

  24. Double Checking the Baseline • Both PHENIX and STAR compare Au+Au data to p+p baseline data. • In the low multiplicity p+p environment both statistical methods seem to yield fairly precise data • This can be used to double check the pQCD calculations the Eloss models use as a basis Justin Frantz DNP 2009

  25. Evaluating pQCD Performance Df > p/2 • Using Isolation Cut p+p Data PHENIX: non-universal slopes • can relate to phenomenological LO kT (non-back to back-ness) • NLO reproduction of this effect? • Although NLO description overall pretty good • Not large enough effect to be concerned? May need to keep in mind [Same group: Au+Au Eloss info sensitive to slope of D(z)] Zhang, Owens, Wang, Wang: arXiv:0902.4000 CTEQ + NLO + KKP I would say: not clear! (low z) Justin Frantz DNP 2009

  26. STAR Baseline Measurements • STAR comparision to same calculations • very good: imply consistency of STAR/PHENIX results? • (same pT scales, pdfs etc.. inputs?) • STAR one step further: checks baseline cold nuclear effects in d+Au (~null) Justin Frantz DNP 2009

  27. Suppression level IAA • We quantify this with the ratio IAA = DAA/Dpp • In preparation for comparisons of Au+Au data, also useful to consider di-hadron data • STAR analysis also produces high quality p-h for comparison STAR Preliminary Justin Frantz DNP 2009

  28. Quantifying g-jet A+A Suppression Constraints for Energy Loss (…Just taking the same ratio for the Au+Au data shown before) Justin Frantz DNP 2009

  29. Au+Au Suppression Level IAA STAR Preliminary STAR Preliminary Dd+Au Baseline -h • Strong Suppression Signal both collaborations • At about ~0.3 PHENIX splits into smaller trigger bins IAA Justin Frantz DNP 2009

  30. Au+Au h-h vs.g-h • Interesting to compare to h-h • Very similar • Compares well w/ theory (3 models) • Where is path length effect? Dd+Au Baseline Justin Frantz DNP 2009

  31. Centrality Dependence • PHENIX: IAA ~= RAA (~= IAA) • First results from STAR showed hint of z- (or pTassoc-) ordered centrality dependence • Latest data ~z independent with better precision than before PHENIX Phys. Rev. C 80, 024908 (2009) Justin Frantz DNP 2009

  32. Regarding model comparisons… Justin’s opinion about the current state of things…. • At least 4 Energy Loss Modeling Methods • Opacity Expansion, Multiple soft thermal pQCD interactions (AMY), etc… • RAA alone can’t discriminate models: other observables must be included (we’ve learned by now) • But not understood what role parameter adjustments, extensions should be included to unambiguously kill models • In fact, there is already lots of other Eloss data to confront • Until theorists resolve these issues (TEQHQM) model comparisons to g-jet can not yet be expected to used to distinguish models • e.g. including g-jet in global fit • Instead • Look for simple statements the experimental data might be telling us independent of specific models • Test specific behaviors the models may expect under certain assumptions in order to identify possible areas that may need addressed in order to make “the global fits” work for each model Justin Frantz DNP 2009

  33. Basic ideas for g-h expectations Picture jet vtx distribution njet vtx(x,y,z) within medium…. • Direct g–jet vtx’s ∝ ncoll(x,y,z) • therefore so is ndirectg (x,y,z) (unmodified) • g-jet IAA (# jet/g) = single jet RAA • high zfragh g-h ~= single-h RAA qualitatively confirms geometric picture • not necessarily surface bias!!!! • Big question becomes why is Di-hadron suppression IAA same? • In surface bias should have longer path length • So away jet yield (JAA = Away jet/Ncoll * pp Away) reduced  smaller IAA? • Possible resolution: tangential dominance of hadron (jet) trigger sample • Thus trigger yield reduced also: Per-trigger yield goes back up JAA ~= RAA2 • Reality (X-h NOT X-jet) g-h IAA ≠ g-jet etc.. • Need model for translation: one model with this property is ZOWW Justin Frantz DNP 2009

  34. ZOWW Model Realization of This Zhang, Owens, Wang, Wang: e.g. arXiv:0902.4000 • High z g-h: IAA = RAA expectation confirmed • Low z g-h IAA probes interior of medium • Tangent/Corona dominates in h-h IAA, b.c. Eloss averaged over pathlengths di-hadron g-hadron Single hadron ZOWW high z jet Origin vertex distributions Justin Frantz DNP 2009

  35. ZOWW Realization of This: Data? • Data: more precision at low z needed to confirm (hint of no change?) • Problem: centrality dep  pTassoc dep now not seen with STAR Justin Frantz DNP 2009

  36. PHENIX Model Comparisons • Run 7: Not enough precision here either to resolve the question ZOWW Justin Frantz DNP 2009 Renk

  37. What do we conclude? • Renk/ASW : same observables esp. h-h IAA can be described by allowing for fluctuations in Eloss for a given path through the medium • No surface bias (disagrees w/ jet reco results?) • But no explanation of why IAAh-h ~= RAAh-h • conspiracy of several effects? It’d be nice to hear why… • some effects (e.g. spectral shape) may be shared by g-h Justin Frantz DNP 2009

  38. PHENIX Results on Fragmentation g The other direct photon Justin Frantz DNP 2009

  39. Fragmentation Photons γ q g q g • Fragmentation diagrams must be included at NLO to cancel divergences • |z| < 1 for fragmentation photons, but how much less? • Dparton/g(z) not well constrained • May oppose either quark or gluon jets • Larger fraction of direct @ @ LHC • Suppressed by E-loss? • Enhanced by Medium? γ q g Medium Induced Bremsstrahlung diagrams I. Vitev, 2007 RHIC-AGS Meeting --RHIC – LHC Justin Frantz DNP 2009

  40. The idea: "Reverse Isolation Cut” • If we require a high pT hadron to be in the event • …and look at hard photons on the same side of the event • We brutally suppress the Compton direct photons • Which leaves only the fragmentation direct g after statistical subtraction Justin Frantz – DNP 09 Hawaii

  41. Latest Measurement: Frag g pout Z. Belghobsi et. al. arXiv:0903.4834v1 • Rates measured, starting to be addressed by theory • Latest: Low E frag g have much broader correlation inside (mini) jet pout = momentum tranverse to jet axis • If persists at higher Q2/Eg, implications for frag g suppression/ medium induced g arguments? Justin Frantz DNP 2009

  42. Future Directions for g-h correlations A cornerstone of the RHIC program in the next decade Besides the obvious extensions of the g-h (e.g. exploring reaction plane/path length dependence of g-h) I wanted to leave you with 2 thoughts about future directions… Justin Frantz DNP 2009

  43. g-jet in the jet reco era • Does jet reconstruction put us out of business? • No. Even if jet reco is completely unbiased is the difference, still useful b.c. of q uark jet dominance of g-jet • but pretty unlikely there won’t be some difference… • g-jet would probably be needed to determine that anyway • Next step combine jet reco on awayside with g subtraction/ID methods already developed • LHC plans to use it to constrain the reco energy scale • Complication: Likely will always be a thin region in pT of overlap Jet Reco pT reach Direct g trigger pT reach 15 5 Justin Frantz DNP 2009 pT, photon (GeV/c)

  44. Other things to do w/ g-hadron • Test for presence of exotic modifications! • Ridge and Shoulder • Do extended rapidity correlations exist between direct photon and hadrons? Excellent insight for jet origination models • Does shoulder/cone correlation appear in direct photon-hadron • Actually many wonder if it’s there for ANY high pT trigger: • STAR po-h looks like it may have quality to check if push slightly lower in pTh (maybe slightly lower pT trig) ? Justin Frantz DNP 2009

  45. Conclusions • STAR and PHENIX have been actively pursuing direct photon hadron correlations • First measurements already provide some insights, but much more precision will be needed in the next decade in order fully constrain energy loss descriptions Justin Frantz DNP 2009

  46. The End!!! • Back up slides follow Justin Frantz DNP 2009

  47. Future directions I: • Develop event by event methods to purify direct gamma in Au+Au • Limited Isolation Cuts • Anti-tagging p0 • High Multiplicity cause low efficiency • Need to find optimal Rejection/Efficiency Efficiency (Loss) of Accidental Tagging of Direct g pTg Justin Frantz DNP 2009

  48. Future Directions • Purification: Isolation other E.by E. • Check for Ridge • Reaction Plane • LHC? • Energy Loss Theory • How to combine with Jet Reco studies? Justin Frantz DNP 2009

  49. Future Directions II FOCAL • Explore rapidity dependence of g-jet • Need large rapidity coverage detector • FOCAL!!! • Test ridge models more directly by using gamma-jet w/ more rapidity coverage • Crisp test of Ridge models: Associated w/ gamma too? • Test other deviations of NLO seen at Fermilab • If present @ RHIC, is that physics modified in Au+Au straight-forward application of current method? Justin Frantz DNP 2009

  50. Fragmentation Photon Results Justin Frantz DNP 2009

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