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Photons at RHIC: Intermediate and High pT - The Road from the Cleanest Probe to the Universal Probe

This workshop discusses the role of photons as a probe in high-energy collisions, exploring their different sources and their potential for jet energy scale determination and gluon structure function measurements.

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Photons at RHIC: Intermediate and High pT - The Road from the Cleanest Probe to the Universal Probe

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  1. Photons at RHIC: intermediate and high pT The road from the cleanest probe to the universal probe (and the blessings and curses that come with it) G. David, BNL Data mostly from PHENIX, interpretations not necessarily endorsed by PHENIX CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  2. In the beginning… In ancient times (maybe even two decades ago!) 2+1 major roles were anticipated for photons:  thermal radiation (low pT)  establish Ti (or Tc)  unambiguosly fix the jet energy scale (high pT)  bonus: due to dominance of Compton in LO measure polarized gluon structure functions How times have changed!  the best (so far only) Ti estimate comes from virtual photons  a plethora of recently conjectured photon sources make jet E-scale somewhat murkier  but that also means that photons are much richer in information than anticipated, we just have to learn to decode it… 0904.2184 CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  3. g g High pT photons in p+p and A+A: what is common and what is different? Hadron-hadron collisions At high pT and lowest order Compton dominates. Heavy ion collisions: add… plus Compton/annihilation with one quark from the thermal bath (abundant source of “low x”!)  “jet-photon conversion” CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  4. Outline Direct (prompt) photons, p+p, A+A spectra, RAA Baseline decomposition Correlations – photon-hadron (jet), the “golden channel” Some more exotic ideas CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  5. The baseline: direct photons in p+p collisions Run-5 (2005) data Domain of particle physics: low multiplicities, decay photons can be “tagged” (rejected) with high efficiency above 4-5 GeV/c  direct photons are identified Good agreement with NLO pQCD (favors QF=1/2pT fragmentation scale) Since Compton dominates, polarized gluon structure functions can be measured Much needed reference to establish what is different in nucleus-nucleus collisions Reference from the same experiment: reduced systematic errors Published Run-3 results: PRL 98 (2007) 012002 CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  6. The baseline: direct photons, world data Aurenche et al, PRD 73 (2006) 094207 Good description over - 9 orders of magn. in s - 2 orders of magn. in E_cms - both p+p and p+pbar (quark annihilation turned on!) One single outlyer CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  7. Best guess-wrong!! PRD79 012003 Aside: a cautionary tale on the baseline: “same experiment, same systematic errors” Same Au+Au p0 data, different p+p reference for RAA:  the “message” is quite different Even if individual measurements are not better, the ratio (or excitation function) is more precise if everything is done in the same experiment Implications for RHIC low energy beam scan! CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  8. The big picture in Au+Au: we got extremely lucky Hadron suppression is a huge discovery plus it makes photon measurements somewhat easier Photon RAA is essentially unity  TAB scaling for hadrons is meaningful CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  9. “Essentially unity” CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  10. Small deviations: isospin effect (uud, udd) 200GeV At 62GeV there is no experimental difficulty The isospin effect should be (almost) independent of centrality Little overlap with Cu+Cu, but the two are consistent within errors Could d+d collisions help? (One could tag pp, pn, nn collisions!) CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  11. Direct photon The picture so far We have a a probe that – to first order – barely changes in the pQCD regime from p+p to Au+Au Caveats:  so far we didn’t try to decompose p+p contributors (there’s life past LO!)  we are testing relatively large x (at midrapidity), but gluons pile up (and might saturate already in cold nuclear matter) at low x!  you can get a close-to-null result either because there is really no change (new physics) or because there is some rich physics: interplay of “suppressing” and “enhancing” mechanisms Are the same graphs relevant? Are these really the same? Does the medium influence this? CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  12. Baseline decomposition: fragmentation photons (two techniques) Traditional way: isolated photons tag direct photons (no p0, h partner) require no significant energy nearby Fragmentation photon: the non-isolated fraction Novel technique: trigger on hadrons look for near-side yield of photons both inclusive and tagged (p0, h partner) The difference: fragmentation photons CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  13. Baseline decomposition: jet shape pout: component perpendicular to trigger hadron RMS for fragmentation photons considerably larger CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  14. A+A decomposition (theory) 0807.4771 (Liu, Hirano, Werner, Zhu) 0904.2184 (Gale) (LO not plotted) CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  15. Direct photon flow and A+A decomposition (practice) jet fragment photon v2 > 0 annihilation Compton scattering jet v2 > 0 Medium induced (inc.energy loss) v2 < 0 Fragmentation: non-isolated Bremsstrahlung: non-isolated Jet-photon conversion: isolated “Primordial”: isolated In principle you can disentangle, but in practice… Hydro region 0904.2184 Needs *3 smaller errors to become decisive pQCD region Can be improved: in the works CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  16. To be more specific… Thermal photon flow, centrality dependence Flow from all processes, 20-40% 0902.1303 (Liu, Hirano, Werner, Zhu) 0904.2184 (Gale) CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  17. g The “golden channel” – photon-hadron (jet) correlations 0907.4816 The idea is simple: fix the jet energy scale …and its estimated magnitude: The potential problem: 0904.2184 CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  18. Bipartisanship …is starting with a brand-new STAR result (0912.1871) IAA ~= RAA Differences in p0-h and g-h, but similar IAA Where is the lost energy? CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  19. g-h correlations in PHENIX, high pT Slopes: • p+p: b = 6.89 ± 0.64 • Au+Au: b = 9.49 ±1.37 Significant change in slope (due to medium). How literally can this FF be taken? CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  20. g-h correlations in PHENIX, high pT (cont) pT dep. cent. dep. PRC 80, 024908 (2009) Compared to h-h, where available zT dep. Calculations reject fragmentation photons (isolation cut) data do not Is this what we are seeing? Is it compatible with this? CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  21. Inclusive g-h correlations, low pT If all these correlations make sense… But 2*2=4! All right: - leading hadrons are suppressed - jets are suppressed STILL: WHERE IS MY CHANGE??? Will I ever find it (and get it from the same model?) Assoc. yields in bins of h CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  22. Some exotica (I don’t want to pick a fight with theorists, so these are called exotica, because they are experimentally very challenging) nucl-th/0404050 High pT photon HBT (2-3+ GeV/c pair pT)  pre-equilibrium size and time  rate and resolution challenged with real photons  rate^2 limited with gg* (or needs converter jeopardizing other measurements)  needs extremely strong, solid theory support to consider Low pT photon HBT  g cross-section at low pT a la WA98  size (and shape!) of thermal system very different from pions! (time-average vs freeze-out)  RP-dependent measurement, hard, but feasible as gg* or g*g* 0907.1292 CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  23. -p 0 p ASSO TRIG Some more exotica leads to (Esumi, QM’09) Remember: Trigger on a (direct) photon, study the (asymmetric) yields away side. What fraction of energy is accounted for? Not exotic, just not done yet: photons at low x CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  24. Quo Vadis, Photons? (Summary) No big surprises in p+p so far (good) decomposition of higher order effect underway (even better) A+A so far not inconsistent with expectations but the jury is still out  trivial effects only?  rich physics, several mechanisms counteracting?  can flow components be disentangled? How to make full use of the “golden channel”?  Is there a way to tag “primordial” photon (the real measure of jet energy)?  Is there a way to “find the change”? Photons are overloaded with information; decoding it is probably impossible without very active collaboration with theorists. Well, this is why we are here, I guess. CATHIE workshop, Dec. 16, 2009 – G. David, BNL

  25. CATHIE workshop, Dec. 16, 2009 – G. David, BNL

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