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  1. Direct photons. Data review. Sergey Kiselev, ITEP Moscow Introduction Before QM’06 p+p: prompt γ A+B:experimental methods Subtraction method Momentum correlations method Internal conversion method QM’06 high-pt low-pt elliptic flow v2 γ-h azimuthal correlation Conclusions RELNP seminar, ITEP, S.Kiselev

  2. Introduction • Direct photons: not from hadron decays • Quark gluon level: • qq  gγ, qg  qγ, qq(g)  qq(g)γ. Initial hard NN collisions, pQCD  prompt γ. • Thermalised QGP stage  thermal γ from QGP. RELNP seminar, ITEP, S.Kiselev

  3. Introduction – cont. • Hadron level: • meson scatterings: ππ  ργ, πρ  πγ, πK  K* γ, Kρ  Kγ, KK*  πγ, πK*  Kγ, . . . • Thermalised hadron stage  thermal γ from hadron gas • Decay photons: • Long lived (c»cAB ~ 50-100 fm) π0 γγ,   γγ, ’ργ/ωγ/2γ • Shot lived (ccAB ~ 50-100 fm) ω  πγ, a1  πγ, Δ Nγ, K* Kγ,   γ, ρ ππγ, . . . RELNP seminar, ITEP, S.Kiselev

  4. Prompt g : pp data • A Compilation of Data, J. Phys. G,23 (1997) A1 • Data fit: D.Srivastava, Eur.Phys.J, C22 (2001) 129 RELNP seminar, ITEP, S.Kiselev

  5. Prompt g : data fit RELNP seminar, ITEP, S.Kiselev

  6. Prompt g : other data fit • PHENIX hep-ph/0609037 • d3σ/dyd2pT = (√s)5 · F(xT,y) RELNP seminar, ITEP, S.Kiselev

  7. Prompt g : data/theory P.Aurenche et.al, PR D73, 094007 (2006), with PHENIX, D0 data The data can be described (accuracy ~ 50%) within the NLO QCD (JETPHOX code), except E706 experiment. RELNP seminar, ITEP, S.Kiselev

  8. Experimental methods for direct photons • Last years: large progress in experimental techniques to extract direct photons • Direct photons have been revealed by methods: • Subtraction method, WA98/2000, PHENIX/2005 • Momentum correlation method, WA98/2004 • Internal conversion method, PHENIX/2005 • External conversion method, PHENIX/2006, QM’06 RELNP seminar, ITEP, S.Kiselev

  9. Subtraction method • Direct photon measurement by the subtraction method: WA98 PRL 85 (2000) 3595, PHENIX PRL 94 (2005) 232301 • WA98 subtracted from measured photons those from known hadronic source: from decays of reconstructedpo,h and other hadrons (with some assumption of its yield and spectrum) • At pt > 0,5 GeV/c • from po - 80% • from h - 13% • from w - 2% • from h’ - 1.3% RELNP seminar, ITEP, S.Kiselev

  10. Subtraction – WA98 RELNP seminar, ITEP, S.Kiselev

  11. Subtraction – PHENIX PRL 94, 232301 (2005) RELNP seminar, ITEP, S.Kiselev

  12. Momentum correlationsmethod • ggmomentum correlations, WA98 PRL 93 (2004) 022301, STAR nucl-ex/0511055 • gg correlations direct photons • direct photons correlation provide the system sizes at all stages of heavy-ion collisions • Needs larger statistics RELNP seminar, ITEP, S.Kiselev

  13. Momentum correlations – WA98 • Rinv (gg) ≈ 6 fm ≈ Rinv (pp) • emitted in the late, hadron gas, stage of the collision • Thermalcalculations substantialy underestimate the data. RELNP seminar, ITEP, S.Kiselev

  14. Internal conversion method • A novel technique (QM’05): internal conversion of direct photons into e+e-, PHENIX, nucl-ex/0511041 • any source of real g emits also virtual g with very low-mass • gdirect =( g*direct /g*incl.) gincl. Electrons in the central arms were identified by matching charged particle tracks to clusters in the ECAL and to rings in the RICH RELNP seminar, ITEP, S.Kiselev

  15. Internal conversion - details Main assumption: the Kroll-Wada (KW) formula, Phys.Rev.98(1955)1355 A given reaction produces, instead of real γ, γ*  e+e-. For decays: Nγ – number of real γ πo γγπo γγ* γe+e- I=KW = 0.0059 η γγη γγ*  γe+e- I=KW = 0.0081 γdirectγ*direct e+e- I=KW = 0.0162 phase space factor =1, if peeT » mee RELNP seminar, ITEP, S.Kiselev

  16. Internal conversion – cont. • Measurement for 1 < pT < 5 GeV/c consistent with calculations when thermal photon emission is taken into account (at high pT > 5 it is consistent with a NLO pQCD calculation). RELNP seminar, ITEP, S.Kiselev

  17. RAA vs pT for direct photons QM’05: High pT direct photons are not suppressed  strong suppression of mesons is not an initial effect, but a final state effect RELNP seminar, ITEP, S.Kiselev

  18. Run Year Species s1/2 [GeV ] Ldt 01 2000 Au+Au 130 1 b-1 02 2001/2 Au+Au 200 24 b-1 p+p 200 0.15 pb-1 03 2002/3 d+Au 200 2.74 nb-1 p+p 200 0.35 pb-1 04 2003/4 Au+Au 200 241 b-1 Au+Au 62 9 b-1 05 2004/5 Cu+Cu 200 3 nb-1 Cu+Cu 62 0.19 nb-1 Cu+Cu 22.5 2.7 b-1p+p 200 3.8 pb-1 06 2006 p+p 200 93.3 pb-1 ? p+p 62 1.1 pb-1 ? 07 2007 Au+Au 200 Quark Matter 2006 • PHENIX data • High-pT direct photon • Hard scattering • Talk by T.Isobe • Low-pT direct photon • Thermal emission from QGP. • Talk by D. Peressounko • Azimuthal anisotropy of direct photon • Talk by V.S. Panteuev • γ- h azimuthal correlations • Talk by J. Jin • Enter STAR into the game… RELNP seminar, ITEP, S.Kiselev

  19. Systematic Study of High-pT Direct Photon Production with the PHENIX Experiment at RHIC Tadaaki Isobe (CNS, Univ. of Tokyo) for the PHENIX Collaboration The 19th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions at Shanghai, China RELNP seminar, ITEP, S.Kiselev

  20. High-pT direct photons: where are we? • p+p: agreement with NLO pQCD. • Run2 high-pT direct photons in Au+Au: consistent with binary scaled NLO pQCD. • Binary scaling is not obvious. • 70% prompt photon + 30% jet fragmentation at high-pT. • Fragmentation photons are suppressed by jet-quenching effect at RHIC? • Compensated by other contribution? Phys. Rev. Lett. 94, (2005) 232301 30% contribution from jet frag. RELNP seminar, ITEP, S.Kiselev

  21. Jet g Fraction of fragmentation photons • Fraction of fragmentation photons is evaluated in terms of the fraction of isolated photons in 200GeV p+p collisions. • If energy of cone around a target photon is much less than the energy of photon, the photon is defined as isolated photon. • Isolation cut removes fragmentation (Bremsstrahlung) photons. • Data are consistent with theoretical expectation within the error. • hep-ex/0609031 Theoretical predictions where PHENIX acceptance is taken into account RELNP seminar, ITEP, S.Kiselev

  22. Data Fit Parameterization of the p+p data • For the calculation of nuclear modification factor, p+p reference is parameterized using a fitting function. • The fitting works well to parameterize the p+p photon data. • The parameterized photon data show enhancement comparing with NLO pQCD expectation by a factor of more than 20%. RELNP seminar, ITEP, S.Kiselev

  23. Hard scattered parton interacts with gluon in bulk matter All NLO-pQCD Jet-photon QGP-Thermal Jet-quenching effect for fragmentation photons are estimated with AMY formalism. Medium induced high-pT direct photons • Interaction of hard-scattered parton with dense matter. • Compton scattering (jet-photon conv.) • R.J. Fries, Phys.Rev. C72 (2005) 041902 • Over estimation at high-pT • S. Turbide, Phys.Rev. C72 (2005) 014906 • Reproduce data well • Bremsstrahlung • B.G. Zakharov, JETP Lett. 80 (2004) 1 • If it is observed, it can be used as hard probe of bulk matter. RELNP seminar, ITEP, S.Kiselev

  24. RAA with pQCD RAA with p+p data Direct photon RAA • Nuclear modification factor • Yield from hard processes can be scaled with Ncoll (# of collisions). • divided by expected (binary scaled) yield from p+p data. • RAA is consistent with Ncoll scaled p+p reference. • RAA of very high-pT direct photons is below one. • A hint of jet-quenching effect for fragmentation photons? • RAA using pQCD shows different tendency. First data using p+p data as reference! Thanks to the more accurate p+p/Au+Au data. RELNP seminar, ITEP, S.Kiselev

  25. Comparison of RAA with theoretical calculation • Turbide et al. • Phys. Rev. C72 (2005) 014906 + Private communication. • AMY formalism for jet-quenching effect for fragmentation photons. • Systematically data points are below theoretical prediction. • F. Arleo • JHEP 0609 (2006) 015 • High-pT suppression due to isospin effect, in addition to jet-quenching and shadowing. • BDMPS for jet-quenching. • Medium induced jet-photon is not taken into account. • The suppression of very high-pT photon is well reproduced. RELNP seminar, ITEP, S.Kiselev

  26. Summary/T.Isobe • Direct photons in 200GeV p+p collisions. • Published final year-3 p+p data. • Preliminary year-5 data. • Consistent with NLO pQCD within uncertainty. • Direct photons in 200GeV Au+Au collisions. • Clear signal of direct photon at high-pT thanks to the strong suppression of neutral hadrons. • Spectra up to 20GeV/c. • First direct photons RAA in 200GeV Au+Au collisions with p+p reference. • Almost consistent with binary scaled p+p data. • RAA seems to be below one at very high-pT region. RELNP seminar, ITEP, S.Kiselev

  27. Direct photons at low ptmeasured in PHENIX D.Peressounko RRC “Kurchatov institute” for the PHENIX collaboration RELNP seminar, ITEP, S.Kiselev

  28. Measure photon spectrum and remove those photons which make p0 mass with any other Correct direct photon candidate sample for contribution from p0 decay photons with missing partner and for direct photons with fake partners Subtract contribution from h, w etc. decays Measure yield of inclusive photons corrected for hadron contaminations, conversion, efficiency and acceptance. Measure spectrum of p0, h, w etc. Calculate yield of decay photons and subtract it from the inclusive yield. Subtraction and Tagging methods Tagging Subtraction Ngdir = Ngincl - Ngdecay RELNP seminar, ITEP, S.Kiselev

  29. d+Au collisions: Double ratio • Internal conversion method provides smaller systematic errors • But not as small as in the case of Au+Au collisions in Run-4 • (Large background of external conversion on MVD detector in Run-3) RELNP seminar, ITEP, S.Kiselev

  30. d+Au collisions: Spectrum Internal conversion extends range of significant points to pt > 2 GeV Data agrees with pQCD predictions in full pt range => No indication for nuclear effects RELNP seminar, ITEP, S.Kiselev

  31. Au+Au collisions: External conversion External conversion Tagging • Combining this photon with others measured in EMCAL with loose PID cut, estimate proportion of photons coming from p0 decays • Correct for missing p0 decay partners • Subtract h, w, h’ decay photons • Calculate ratio Ngall/Ngdecay • Use external photon conversion in beam pipe to produce very clean photon sample • Use very tight PID cut on EMCAL photons to produce very clean photon sample no pair cut with pair cut Dalitz Conversion avoid explicit calculation of p0 spectrum Uses very pure photon sample =>reduce systematic errors RELNP seminar, ITEP, S.Kiselev

  32. Comparison: Au+Au, central collisions • Systematic errors are still too large to extract thermal photons. • There is room for thermal photons This is consistent with excess above pQCD reportedpreviously Spectra agree within errors in entire pt range RELNP seminar, ITEP, S.Kiselev

  33. Two-photon correlations e+ g2 g2rec g1 g1 e- • External conversion: • No close cluster interference • No hadron contamination C2 calculated in EMCAL and converted+EMCAL agree => both effects are under control RELNP seminar, ITEP, S.Kiselev

  34. Conclusions/D.Peressounko • Measurement of direct photon yield at small pt is very interesting but extremely complicated task • PHENIX has developed a variety of methods for direct photon extraction in p+p, d+Au and Au+Au collisions producing consistent results • Because of the large systematic errors comparison of binary scaled d+Au spectrum with Au+Au does not allow to make a statement on the origin of the excess above pQCD observed in Au+Au. • More precise data are coming… RELNP seminar, ITEP, S.Kiselev

  35. PHENIX measurements of reaction plane dependence of high pT photons and pions in Au+Au collisions Vladislav Pantuev, University at Stony Brook for PHENIX collaboration RELNP seminar, ITEP, S.Kiselev

  36. In p+p Hard photons:direct component Annihilation Compton In A+A picture is much more complicated: Photons in A+A Direct Photons Decay Photons Preequilibriumphotons hard thermal hard+thermal direct fragmentation QGP Hadron gas jet-g-conv. Medium inducedg bremsstr. V2=0 V2>0 V2>0 V2>0 RELNP seminar, ITEP, S.Kiselev V2<0 V2<0

  37. There are theoretical predictions with a sizable azimuthal parameter v2 : Jets lose more energy where the medium is thicker - more jet-photon conversions (v2<0), + photons from fragmentation of quenched jets (v2>0) Thermal photons are produced throughout the expansion history and reflect quark anisotropy S.Turbide, C.Gale, R.J.Fries, PRL 96 032303 (2006) R. Chatterjee et al., PRL 96, 202302 (2006) RELNP seminar, ITEP, S.Kiselev 4

  38. How does PHENIX measure direct photons and v2? Measure inclusive photon yield, Ninc Measure hadron contribution components p0 and h By Monte Carlo calculate p0 and h decay background in inclusive sample, Nbg Calculate direct photon excess over hadron decays, R Measure inclusive and hadron v2 by reaction plane method Calculate direct photon v2 as R * v2inc – v2BG v2dir = R – 1 Use large statistics Run 4 Au+Au data set R = N inc / NBG See also poster 86 by Kentaro Miki RELNP seminar, ITEP, S.Kiselev 5

  39. Step by step calculations: PHENIX preliminary PHENIX preliminary PHENIX preliminary PHENIX preliminary RELNP seminar, ITEP, S.Kiselev 6

  40. PHENIX direct photon v2 result Systematic errors dominantly from R: Enormous background of decay photons at low pT Within statistical and systematic errors v2 is consistent with zero. v2=0 or cancellation of different contributions? RELNP seminar, ITEP, S.Kiselev 7

  41. PHENIX Measurement on High pT h-h and g-h Azimuthal Correlations Jiamin Jin For the PHENIX collaboration Quark Matter 2006, Shanghai Nov. 18, 2006 RELNP seminar, ITEP, S.Kiselev

  42. Method  • Construct inclusive g-h yield • Construct decay g-h yield via: • Pair by pair weighted summation • convolutes all p0-h pair contributions from higher pT • Weight reflects probability from kinematics for p0 at given pT to decay into photon in given pT range • Subtraction via:  p+p  # inclusive g #decay g R= RELNP seminar, ITEP, S.Kiselev

  43. PHENIX Preliminary PHENIX Preliminary • data • PYTHIA First direct g-h yield in p+p, cmp with PYTHIA! p+p: important baseline measurement! PYTHIA 6.1 kT = 2.5 GeV • A nice confirmation of our method RELNP seminar, ITEP, S.Kiselev

  44. First direct g-h yield in Au+Au! Au+Au: study medium induced modification! 1/NtrigdN/dDf 1/NtrigdN/dDf Df(rad) Df(rad) • v2 modulated bkgd for inclusive/decay g subtracted from each corr. fn under ZYAM assumption (v2 from single g/p measurements) • Near side yield consistent with 0 • small fragmentation photon yield • Away side yield also small, suppressed!? RELNP seminar, ITEP, S.Kiselev

  45. Comparison btw Au+Au and p+p Black: Au+Au Red: p+p 1/NtrigdN/dDf 1/NtrigdN/dDf Df(rad) Df(rad) • Hint of away side modification in AuAu! RELNP seminar, ITEP, S.Kiselev

  46. Away side integrated yield • trigger g pT dependence • assoc. h pT dependence • centrality dependence + pp • p+p shows a systematic trend of having higher yields in the • away side than Au+Au! RELNP seminar, ITEP, S.Kiselev

  47. Conclusions/J.Jin • First measurements of high pT direct g-h yields in Au+Au/p+p collisions • Near side yield ~ 0 • Hint of suppression of away side jets in Au+Au • Need more statistics to make conclusive statement • Upcoming Au+Au run RELNP seminar, ITEP, S.Kiselev

  48. Conclusions • High-pt: RAA in 200GeV Au+Au collisionsseems to be below one at very high-pT region. • Low-pt: more methods, but errors are still too large. More precise data are coming… • Elliptic flow v2: within statistical and systematic errors v2 is consistent with zero. • γ-h azimuthal correlations:near side yield ~ 0, hint of suppression of away side jets in Au+Au, need more statistics. • Multi-purpose Direct Photons RELNP seminar, ITEP, S.Kiselev