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Direct Photon Measurements: initial conditions of heavy ion reactions at RHIC

Direct Photon Measurements: initial conditions of heavy ion reactions at RHIC. Alberica Toia for the PHENIX Collaboration Stony Brook University / CERN. IV Workshop on Particle Correlations and Femptoscopy Krakow, September 11-14 2008. Too hot for quarks to bind!!!

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Direct Photon Measurements: initial conditions of heavy ion reactions at RHIC

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  1. Direct Photon Measurements:initial conditions of heavy ion reactions at RHIC Alberica Toia for the PHENIX Collaboration Stony Brook University / CERN IV Workshop on Particle Correlations and Femptoscopy Krakow, September 11-14 2008

  2. Too hot for quarks to bind!!! Standard Model (N/P) Physics Too hot for nuclei to bind Nuclear/Particle (N/P) Physics HadronGas Nucleosynthesis builds nuclei up to He Nuclear Force…Nuclear Physics E/M Plasma Universe too hot for electrons to bind E-M…Atomic (Plasma) Physics SolidLiquidGas Today’s Cold Universe Gravity…Newtonian/General Relativity Evolutionof the Universe Quark-GluonPlasma??

  3. The “Little Bang” in the lab • High energy nucleus-nucleus collisions: • fixed target (SPS: √s=20GeV) • colliders • RHIC: √s=200GeV • LHC: √s=5.5TeV • QGP formed in a tiny region (10-14m) for very short time (10-23s) • Existence of a mixed phase? • Later freeze-out • Collision dynamics: different observables sensitive to different reaction stages • 2 counter-circulating rings, 3.8 km circumference • Top energies (each beam): • 100 GeV/nucleon Au-Au. • 250 GeV polarized p-p. • Mixed Species (e.g. d-Au)

  4. g g m e Jet p f p Time cc p L K time freeze-out expansion formation and thermalization of quark-gluon matter? hadronization hard parton scattering Space Au Au Probing Heavy Ion Collisions • Direct photon sources: • Compton scattering qg  gq • Annihilation qq  gg • Bremsstrahlung from inelastic scattering of incoming or thermalized partons Photons and dileptons: radiation from the media • direct probes of any collision stages (no final-state interactions) • large emission rates in hot and dense matter • according to the VMD their production is mediated in the hadronic phase by the light neutral vector mesons (ρ, ω, and φ) which have short life-time Changes in position and width: signals of the chiral transition?

  5. Energy density in heavy ion collisions • T.D.Lee: “In HEP we have concentrated on experiments in which we distribute a higher and higher amount of energy into a region with smaller and smaller dimensions.” [Rev. Mod. Phys. 47 (1975) 267] • Energy density: “Bjorken estimate” (for a longitudinally expanding plasma): Transverse Energy PHENIX 130 GeV central 2% int ~ 100x enucleus ~ 10x ecritical

  6. sQGP @ RHIC strongly interacting Quark-Gluon Plasma (sQGP) in HI collisions at RHIC The matter is so opaque that even a 20 GeV p0 is stopped The matter is so dense that even heavy quarks are stopped What does it emit? What is the temperature? The matter is so strongly coupled that even heavy quarks flow PHENIX preliminary The matter is so dense that it modifies the shape of jets The matter is so dense that it melts(?) J/y (and regenerates it ?)

  7. hard: Photon Emission • Quark Gluon Plasma • De-confined phase of quarks and gluonsshould emit thermal radiation • Direct photons are an important probe to investigate the characteristics of evolution of the matter created by heavy ion collisions. • Penetrate the strong interacting matter • Emitted from every stage of collisions • Hard photons (High pT) • Initial hard scattering, Pre-equilibrium • Thermal photons (Low pT) • Thermodynamic information from QGP and hadron gas measuretemperature of the matter • Dominant source for 1<pT<3 GeV/c • Measurement is difficut since the expected signal is only 1/10 of photons from hadron decays thermal: Decay photons(p0→g+g, h→g+g, …) S.Turbide et al PRC 69 014903

  8. Extended in RUN5 data Direct photons in p+p and d+Au • p+pTest of QCD • direct participant in partonic interaction • Less dependent on FF than hadron production • Reduce uncertainty on pQCD photons in A+A • good agreement with NLO pQCD • Important baseline for Au+Au • d+Au: • initial-state nuclear effects • no final-state effects (no medium produced) • Study initial-state effects 2

  9. Direct Photons in Au+Au Blue line: Ncoll scaled p+p cross-section THERMAL PHOTONS? Measurement at low pT (where an excess above the know sources may hint to thermal photon production) difficult because of detector resolution Au-Au data consistent with pQCD calculation scaled by Ncoll

  10. Alternative: Virtual Photons e+ q e- g* • Any source of real g can emit g* with very low mass. • If the Q2 (=m2) of virtual photon is sufficiently small, the source strength should be the same • The ratio of real photon and quasi-real photon can be calculated by QED  Real photon yield can be measured from virtual photon yield, which is observed as low mass e+e- pairs g q Kroll-Wada formula S : Process dependent factor • Case of Hadrons • Obviously S = 0 at Mee > Mhadron • Case of g* • If pT2>>Mee2 • Possible to separate hadron decay components from real signal in the proper mass window.

  11. Signal Extraction p+p Au+Au arXiv: 0802.0050 arXiv: 0706.3034 • Real signal • di-electron continuum • Background sources • Combinatorial background • Material conversion pairs • Additional correlated background • Visible in p+p collisions • Cross pairs from decays with 4 electrons in the final state • Pairs in same jet or back-to-back jet

  12. Hadronic Cocktail Calculation • Remaining pairs after background subtraction • Real signal + Hadron decay components • Estimate hadron components using hadronic cocktail • Mass distributions from hadron decays are simulated by Monte Carlo. • p0, h, h’, w, f, r, J/y, y’ • Effects on real data are implemented. • PHENIX acceptance, detector effect, efficiencies … • Parameterized PHENIX p0 data with assumption of p0 = (p++p-)/2 • Hadronic cocktail was well tuned to individually measured yield of mesons in PHENIX for both p+p and Au+Au collisions. arXiv: 0802.0050

  13. Cocktail Comparison p+p Au+Au arXiv: 0802.0050 arXiv: 0706.3034 • p+p • Excellent agreement with cocktail • Au+Au • Large enhancement in low mass region • Integrated yield in 150MeV < mee < 750MeV • data/cocktail = 3.4 ± 0.2(stat) ± 1.3(sys) ± 0.7(model)

  14. pT-Sliced Mass Spectra 0 < pT < 8 GeV/c 0 < pT < 0.7 GeV/c 0.7 < pT < 1.5 GeV/c 1.5 < pT < 8 GeV/c Normalized by the yield in mee < 100MeV • Au+Au • p+p PHENIX Preliminary • The low mass enhancement decreases with higher pT • No significant indication that this low mass enhancement contribute to m<300 MeV/c2 and pT>1 GeV/c • We assume that excess is entirely due to internal conversion of direct g

  15. Low mass & High pT region p+p Au+Au (MB) 1 < pT < 2 GeV/c 2 < pT < 3 GeV/c 3 < pT < 4 GeV/c 4 < pT < 5 GeV/c • p+p • Good agreement between real and cocktail • Small excess at higher pT • Au+Au • Good agreement in Mee < 50MeV/c2 • Enhancement is clearly seen above 100MeV/c2.

  16. Determination ofg* fraction, r Direct g*/inclusive g* is determined by fitting the following function for each pT bin. Reminder : fdirect is given by Kroll-Wada formula with S = 1. r : direct g*/inclusive g* • Fit in 80-300MeV/c2 gives • Assuming direct g* mass shape • c2/NDF=11.6/10 • Assuming h shape instead of direct g* shape • c2/NDF=21.1/10 • Twice as much as measured h yield • Assumption of direct g* is favorable. Mee (GeV/c2)

  17. directg*/inclusive g* p+p Au+Au μ = 0.5pT μ = 1.0pT μ = 2.0pT Base lineCurves : NLO pQCD calculations with different theoretical scales done by W. Vogelsang. • p+p • Consistent with NLO pQCD • better agreement with small µ • Au+Au • Clear enhancement above NLO pQCD

  18. Direct Photon Spectra exp + TAA scaled pp The virtual direct photon fraction is converted to the direct photon yield. • p+p • First measurement in 1-4GeV/c • Consistent with NLO pQCD and with EmCal method • Serves as a crucial reference • Au+Au • Above binary scaled NLO pQCD • Excess comes from thermal photons? Fit to pp NLO pQCD (W. Vogelsang) exponential scaled pp

  19. 1st measurement of Thermal Radiation • Au+Au = pQCD + exp.  T = 221  23 (stat)  18 (sys) • Initial temperatures and times from theoretical model fits to data: • 0.15 fm/c, 590 MeV (d’Enterria et al.) • 0.17 fm/c, 580 MeV (Rasanen et al.) • 0.2 fm/c, 450-660 MeV (Srivastava et al.) • 0.33 fm/c, 370 MeV (Turbide et al.) • 0.6 fm/c, 370 MeV (Liu et al.) • 0.5 fm/c, 300 MeV (Alam et al.) D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006) From data: Tini > 220 MeV > TC From models: Tini = 300 to 600 MeV t0 = 0.15 to 0.5 fm/c

  20. Dilepton Spectra SLOPE ANALYSIS • Single exponential fit: • Low-pT: 0<mT<1 GeV • High-pT: 1<mT<2 GeV • 2-components fits • 2exponentials • mT-scaling of p0 + exponential • Low pT: • inverse slope of ~ 120MeV • accounts for most of the yield p+p Au+Au • p+p • Agreement with cocktail • Au+Au • pT>1GeV/c: small excess  internal conversion of direct photons • pT<1GeV/c: large excess  q-q, p-p, …?

  21. Previous measurements NA60 CERES CERES measured an excess of dielectron pairs, confirmed by NA60, rising faster than linear with centrality attributed to in-medium modification of the r spectral function from pp annihilation. NA60 CERES The enhancement is concentrated at low pT

  22. Summary • We have measured e+e- pairs in p+p and Au+Au collisions at √sNN=200 GeV • Large excess above hadronic background is observed • For m<300MeV/c2 and 1<pT<5 GeV/c • Excess is much greater in Au+Au than in p+p • Treating the excess as internal conversion of direct photons, the yield of direct photon is deduced. • Direct photon yield in p+p is consistent with a NLO pQCD • Direct photon yield in Au+Au is much larger. • Spectrum shape above TAA scaled pp is exponential, with inverse slope T=221 ±23(stat)±18(sys) MeV • Hydrodynamical models with Tinit=300-600MeV at t0=0.6-0.15 fm/c are in qualitative agreement with the data. • Additional excess in Au+Au at pT<1GeV/c • Inverse slope T~120 MeV  Additional source of virtual g around Tcrit, responsible of most of the inclusive dilepton yield, so far not explained by theories…

  23. Backup

  24. Centrality Dependency LOW MASS p0 region: • Agreement with cocktail Low Mass: • yield increases faster than proportional to Npart enhancement from binary annihilation (ππ or qq) ? Intermediate Mass: • yield increase proportional to Ncoll charm follows binary scaling submitted to Phys. Rev. Lett arXiv:0706.3034

  25. Understanding the pT dependency • Comparison with cocktail • Single exponential fit: • Low-pT: 0<mT<1 GeV • High-pT: 1<mT<2 GeV • 2-components fits • 2exponentials • mT-scaling of p0 + exponential • Low pT: • inverse slope of ~ 120MeV • accounts for most of the yield

  26. e+ Gluon Compton g* e- q g q q q R.Rapp + H.vanHees K.Dusling + I.Zahed E.Bratkovskaja + W.Cassing Theory Comparison II • Freeze-out Cocktail + “random” charm + r spectral function Low mass • M>0.4GeV/c2: some calculations OK • M<0.4GeV/c2: not reproduced Intermediate mass • Random charm + thermal partonic may work Low-pT slope not reproduced PARTONIC HADRONIC p-p annihilation q-q annihilation

  27. Extract 2 components 2 EXPONENTIALS HAGEDORN + EXPONENTIAL • We fit the sum of 2 exponentials (a*exponential1 + b*exponential2) • We fit Hagedorn to Mee<100MeV (p0-dominated) • Then we fit (a*mT-scaling + exponential) to the other mass bins • Because of their different curvature, mT-scaling and the exponential account for more or less of the yield in the low-pT region.

  28. YIELDS Low-pT yield 2expo fit mT-scaling +expo fit Total yield (DATA) Yields and Slopes SLOPES • Intermediate pT: inverse slope increase with mass, consistent with radial flow • Low pT: • inverse slope of ~ 120MeV • accounts for most of the yield

  29. Theory Comparison II Calculations from R.Rapp & H.vanHees K.Dusling & I.Zahed E.Bratovskaja & W.Cassing (in 4p)

  30. Questions SPS RHIC 1. Enhancement at M<2Mp If pions are massless can pp annihilation produce ee with M<300MeV? 2. Enhancement at low pT, with T~120 MeV and now flow Is the same low-pT enhancement seen at SPS never reproduced by theory? Different initial temperature Different system evolution Do we miss something in the system evolution which may have different relevance at SPS and at RHIC?

  31. g p DC e+ e- PC1 magnetic field & tracking detectors PC3 PHENIX (Pioneering High Energy Nuclear Interaction eXperiment) designed to measure rare probes:+ high rate capability & granularity + good mass resolution and particle ID - limited acceptance Au-Au & p-p spin • 2 central arms: electrons, photons, hadrons • charmonium J/, ’ -> e+e- • vector mesonr, w,  -> e+e- • high pTpo, p+, p- • direct photons • open charm • hadron physics • 2 muon arms: muons • “onium” J/, ’,  -> m+m- • vector meson -> m+m- • open charm • combined central and muon arms: charm production DD -> em • global detectors forward energy and multiplicity • event characterization

  32. z Dalitz decay Conversion pair z e- B B y y x e+ e- x e+ Photon conversion rejection • ge+e- at r≠0 have m≠0(artifact of PHENIX tracking: • no tracking before the field) • effect low mass region • have to be removed Conversion removed with orientation angle of the pair in the magnetic field Photon conversion r ~ mee Inclusive Removed by phiV cut After phiV cut Beampipe MVD support structures

  33. Photon conversion cut No cut M<30 MeV & fV<0.25 & M<600 MeV & fV<0.04 M<600 MeV & fV<0.06 M<600 MeV & fV<0.08 M<600 MeV & fV<0.10 M<600 MeV & fV<0.12 M<600 MeV & fV<0.14 M<600 MeV & fV<0.20 M<600 MeV & fV<0.40

  34. γ e- e+ Conversion pair Dalitz decay e+ π0 z z e+ e- e- π0 B B π0 γ e- y y γ e- e+ e+ x x Physical background Semi-correlated Background Background is charge-independent Calculate the shape with MC Normalize to the like-sign spectra  Good description of the data • p0g g* e+e- e+e- • “jets” X arXiv: 0802.0050 Photon conversion ge+e- at r≠0 have m≠0(artifact of PHENIX tracking) Conversion removed with orientation angle of the pair in the magnetic field

  35. Combinatorial Background • PHENIX 2 arm spectrometer acceptance: • dNlike/dm ≠ dNunlike/dm different shape  need event mixing • (like/unlike differences preserved)Use Like sign as a cross check for the shape and to determine normalization • Small signal in like sign at low mass • N++ and N–- estimated from the mixed events like sign B++ and B-- normalized at high mass (> 700 MeV) Normalization: 2√N++ N-- • Uncertainty due to statistics of N++ and N--: 0.12% • Correction for asymmetry of pair cut • K=k+-/√k++ k-- = 1.004Systematic error (conservative): 0.2% LIKE SIGN SPECTRA TOTAL SYSTEMATIC ERROR = 0.25% Use same event topology (centrality, vertex, reaction plane) Remove every unphysical correlation

  36. Comparison of BG subtraction Methods Monte Carlo method Like sign method(with some variations) give consistent results over the full invariant mass range to determine syst. uncertainty: spread of two extreme cases (blue & orange): 5-10% 36

  37. Acceptance q0 • Define acceptance filter (from real data) • Correct only for efficiency IN the acceptance • “Correct” theory predictions IN the acceptance charge/pT z vertex pT f0 • Single electron pT > 200 MeV • Pair mT > 400 MeV Not an analysis cut, but a constrain from the magnetic field mass

  38. Ne Electron yield converter 0.8% 0.4% 1.7% With converter Photonic W/O converter Dalitz : 0.8% X0 equivalent radiation length Non-photonic 0 Material amounts: 0 Cross check Converter Method We know precise radiation length (X0) of each detector material The photonic electron yield can be measured by increase of additional material (photon converter was installed) The non-photonic electron yielddoes not increase Photonic single electron: x 2.3 Inclusive single electron :x 1.6 Combinatorial pairs :x 2.5 Photon Converter (Brass: 1.7% X0)

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