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Measurements of thermal photons and the dielectron continuum with PHENIX

Measurements of thermal photons and the dielectron continuum with PHENIX. - Torsten Dahms - Stony Brook University Workshop on Hot & Dense Matter in the RHIC-LHC Era February 13 th , 2008. charm & bottom cross section. centrality dependence. p T spectra. chiral symmetry. mass spectra.

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Measurements of thermal photons and the dielectron continuum with PHENIX

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  1. Measurements of thermal photons and the dielectron continuum with PHENIX - Torsten Dahms - Stony Brook University Workshop on Hot & Dense Matter in the RHIC-LHC Era February 13th, 2008 charm & bottom cross section centrality dependence pT spectra chiral symmetry mass spectra thermal photons Au+Au collisions medium modification p+p collisions low mass enhancement

  2. Dileptons at RHIC 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 Possible modifications Space Chiral symmetry restoration continuum enhancement modification of vector mesons Au Au thermal radiation charm modification exotic bound states suppression (enhancement) Expected sources • Light hadron decays • Dalitz decays p0, h • Direct decays r/w and f • Hard processes • Charm (beauty) production • Much larger at RHIC than at SPS • 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?

  3. The Data submitted to Phys. Lett.B arXiv: 0802.0050 p+p at √s = 200GeV γ e- • 800M MinBias Au+Au events • 2.25pb-1 of triggered p+p data • Combinatorial background removed by mixed events • additional correlated background: • cross pairs from decays with four electrons in the final state • particles in same jet (low mass) • or back-to-back jet (high mass) • well understood from MC e+ e+ π0 e+ e- π0 π0 γ e- γ

  4. The Raw Subtracted Spectrum submitted to Phys. Rev. Lett arXiv:0706.3034 • Same analysis on data sample with additional conversion material • Combinatorial background increased by 2.5 Good agreement within statistical error ssignal/signal = sBG/BG * BG/signal large!!! 0.25% 300,000 pairs 50,000 above p0 From the agreement converter/non-converter and the decreased S/B ratio scale error < 0.1%(well within the 0.25% error we assigned) 2007-12-14 Torsten Dahms - Stony Brook University 4

  5. Cocktail Tuning (p+p) PHENIX Preliminary • Start from the π0 , assumption: π0 = (π+ + π-)/2 • parameterize PHENIX pion data: • Other mesons • well measured in electronic and hadronic channels • Other mesons are fit with:mT scaling of π0 parameterization pT→√(pT2+mmeson2-mπ2) fit the normalization constant All mesons mT scale!

  6. p+p Cocktail Comparison Data absolutely normalized Excellent agreement with Cocktail Filtered in PHENIX acceptance Cross Sections: Charm: integration after cocktail subtraction σcc = 544 ± 39 (stat) ± 142 (syst) ± 200 (model) μb Simultaneous fit of charm and bottom: σcc = 518 ± 47 (stat) ± 135 (syst) ± 190 (model) μb σbb = 3.9 ± 2.4 (stat) +3/-2 (syst) μb Charm cross section from single electron measurement: σcc = 567 ± 57 ± 193 μb submitted to Phys. Lett.B arXiv: 0802.0050

  7. Cocktail Comparison Data and cocktail absolutely normalized Cocktail from hadronic sources Charm from PYTHIA Single electron non photonic spectrum w/o angular correlations σcc= Ncoll x 567±57±193mb Predictions are filtered in PHENIX acceptance & resolution Low-MassContinuum:enhancement 150 < mee < 750 MeV3.4 ± 0.2 (stat) ± 1.3 (syst) ± 0.7 (model) Intermediate-Mass Continuum: Single e  pT suppression PYTHIA softer than p+p but coincide with Au+Au Angular correlations unknown Room for thermal contribution? submitted to Phys. Rev. Lett arXiv:0706.3034

  8. Au+Au & p+p Comparison p+p and Au+Au normalized to π0 region Agreement in intermediate mass and J/ψ just for ‘coincidence’(J/ψ happens to scale as π0 due to scaling with Ncoll + suppression) p+p normalized to mee<100 MeV

  9. Comparison with Theory • Ingredients: • Freeze-out Cocktail • “random” charm • ρ spectral function • LMR • m>0.4 GeV:some models describe data • m<0.4 GeV:enhancement not reproduced • IMR • Randomized charm + thermal may work

  10. Yield in Different Mass Ranges 0-100 MeV:π0 dominated; approximately scales with Npart 150-750 MeV: continuum 1.2-2.8 GeV: charm dominated;scales with Ncoll Study yield in these mass regions as a function of centrality

  11. Centrality Dependence π0 production scales with Npart Low Mass: If in-medium enhancement from ππ or qq annihilation yield should increasefaster than proportional to Npart Intermediate Mass: charm follows binary scaling yield should increaseproportional to Ncoll LOW MASS INTERMEDIATE MASS submitted to Phys. Rev. Lett arXiv:0706.3034

  12. Mass Spectra: pT dependency 0 < pT < 8 GeV/c 0 < pT < 0.7 GeV/c 0.7 < pT < 1.5 GeV/c 1.5 < pT < 8 GeV/c • p+p in agreement with cocktail • Au+Au low mass enhancement concentrated at low pT Study pT dependency of the low mass enhancement in Au+Au

  13. pT Spectra • p+p: follows the cocktail • Au+Au: significantly deviates at low pT

  14. Understanding the pT dependency • Comparison with cocktail • Single exponential fit: • Low-pT: 0<mT<1 GeV • High-pT: 1<mT<2 GeV • 2-component fits • 2 exponentials • mT-scaling of p0 + exponential

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

  16. Mass Spectra: pT dependency 0 < pT < 8 GeV/c 0 < pT < 0.7 GeV/c 0.7 < pT < 1.5 GeV/c 1.5 < pT < 8 GeV/c • high pT region provides window for thermal photon measurement Study pT dependency of the low mass enhancement in Au+Au

  17. Virtual Photons e+ Compton e- γ* q γ γ g q π0 π0 e+ γ* γ e- Compton γ q g q • Start from Dalitz decay • Calculate inv. mass distribution of Dalitz pairs invariant mass of Dalitz pair invariant mass of Dalitz pair invariant mass of virtual photon invariant mass of virtual photon form factor form factor phase space factor phase space factor • Now direct photons • Any source of real γproduces virtual γwith very low mass • Rate and mass distribution given by same formula • No phase space factor formee<< pT photon • Improved S/B by measuring direct photon signal in mass region in which π0 are suppressed

  18. PHENIX Preliminary p+p Au+Au (MB) Cocktail comparison • QM2005 • Results from Au+Au • QM2008 • long awaited result from p+p • important confirmation of method • p+p • Agreement of p+p data and hadronic decay cocktail • Small excess in p+p at large mee and high pT • Au+Au • data agree for mee <50MeV • Clear enhancement visible above for all pT 1 < pT < 2 GeV 2 < pT < 3 GeV 3 < pT < 4 GeV 4 < pT < 5 GeV

  19. Shape Comparison At m=0 Dalitz and internal conversion pairs have indistinguishable shape Shape differs as soon as π0 is suppressed due to phase space limitation Assume internal conversions of direct photons Fix absolute normalization of cocktail and direct photons by normalizing to data in mee<30MeV Fit paramater r is fraction of direct photons Two component fit in80 < mee < 300MeV gives: χ2/DOF=11.6/10 It’s not the η: Independent measurement of η in Au+Au fixes π0/η ratio to: 0.48 ± 0.08 Fit with eta shape gives:χ2/DOF = 21.1/10

  20. Fraction of direct photons Fraction of direct photons Compared to direct photons from pQCD p+p Consistent with NLO pQCD favors small μ Au+Au Clear excess above pQCD p+p Au+Au (MB) μ= 0.5pT μ= 1.0pT μ= 2.0pT

  21. Comparison • Agreement of all three methods within their errors • Internal conversion method observes clear excess above decay photons • Extract direct photon spectrum by multiplying with measured inclusive photon spectrum: Nγdirect = r · Nγinclusive

  22. The spectrum • Compare spectra to NLO pQCD p+p • consistent with pQCD Au+Au • above binary scaled pQCD • If excess of thermal origin:inverse slope is related to initial temperature

  23. Conclusions p+p LOW MASS: Excellent agreement with hadronic decay cocktail INTERMEDIATE MASS: Extract charm and bottom cross sections σcc = 544 ± 39 (stat) ± 142 (syst) ± 200 (model) μb σbb= 3.9 ± 2.4 (stat) +3/-2 (syst) μb THERMAL PHOTONS p+p in agreement with pQCD Au+Au LOW MASS: Enhancement above the cocktail expectations:3.4±0.2(stat.) ±1.3(syst.)±0.7(model) Centrality dependency: increase faster than Npart Enhancement concentrated at low pT INTERMEDIATE MASS: Coincidence agreement with PYTHIA Room for thermal radiation? THERMAL PHOTONS: Dielectron mass shape for pT > 1 GeV and mee < 300MeV consistent with internal conversions of virtual photons Au+Au above pQCD • First dielectron continuum measurement at RHIC • Despite of low signal/BG • Thanks to high statistics • Very good understanding of background normalization • HBD upgrade will reduce background great improvement of systematic and statistical uncertainty (LMR) • Silicon Vertex detector will distinguish charm from prompt contribution (IMR)

  24. Backup

  25. Charm and bottom cross sections CHARM BOTTOM Dilepton measurement in agreement with single electron, single muon, and with FONLL (upper end) Dilepton measurement in agreement with measurement from e-h correlation and with FONLL (upper end) First measurements of bottom cross section at RHIC energies!

  26. Theory Comparison II • Cocktail not subtracted from data • (necessary for comparison) • Calculations from • R. Rapp & H. van Hees • K. Dusling & I. Zahed • E. Bratovskaja & W. Cassing (in 4π)

  27. Cu+Cu dN/dm – Minimum Bias π η η’ ω ρ φ cc J/ψ ψ’

  28. Dielectrons at RHIC – Intermediate Mass • Transverse momentum spectra of dielectrons at constrained transverse masses: • RHIC with PHENIX acceptance, pT > 1 GeV and 2 GeV < Mee < 3 GeV • Mee hard chance to find thermal dileptons with Mee > 2 GeV. • The double differential rate dNe+e−/dMT2 dQT2 with MT in a narrow interval and with a suitable pTmin cut on the individual leptons seems to allow for a window at large values of the pair pT where the thermal yield shines out K. Gallmeister, B. Kämpfer and O. P. Pavlenko Phys. Rev. C 57, 3276 (1998)Phys. Lett. B 419, 412 (1998) see also e.g.: E. V. Shuryak, Phys. Rev. C 55, 961 (1997)

  29. Dielectrons at RHIC Expected Sources: Light hadron decays Dalitz decays π0, η Direct decays ρ, ω and φ Hard processes Charm (beauty) production Important at high mass & high pT Much larger at RHIC than at the SPS Cocktail of known sources Measure π0, η spectra & yields Use known decay kinematics Apply detector acceptance Fold with expected resolution Possible modifications Chiral symmetry restoration continuum enhancement modification of vector mesons thermal radiation charm modification exotic bound states suppression (enhancement) R. Rapp nucl-th/0204003 • Strong enhancement of low-mass pairs persists at RHIC • Open charm contribution becomes significant

  30. Relativistic Heavy Ion Collider

  31. The PHENIX Experiment Charged particle tracking: DC, PC1, PC2, PC3 Electron ID: Cherenkov light RICH shower EMCal Photon ID: shower EMCal Lead scintillator calorimeter (PbSc) Lead glass calorimeter (PbGl) charged particle veto Central arm physics(|y|<0.35, p ≥ 0.2 GeV/c): charmonium J/ψ, ψ’→ e+e- vector mesonρ, ω, φ → e+e- high pTπ0, π+, π- direct photons open charm hadron physics Two muon arms at forward rapidity (1.2 < |η| < 2.4, p  2 GeV/c) • Measure rare probes in heavy ion collisions (e.g. Au+Au) as well as in p+p (+spin program) p g e+ e-

  32. Electron Identification Charged particle tracking (δm: 1%) DC, PC1, PC3 PHENIX optimized for Electron ID Cherenkov light RICH + shower EMCAL Emission and measurement of Cherenkov light in the Ring Imaging Cherenkov detector→ measure of min. velocity Production and of em. shower in the Electro-Magnetic Calorimeter  measure of energy E Electrons: E ≈ p Hadrons: E < p RICH All charged tracks RICH cut Real Net signal Background Energy-Momentum

  33. The Double Challenge Need to detect a very weak source of e+e- pairshadron decays (m>200 MeV, pT>200 MeV) ~ 4x10-6 / π0 In the presence of hundreds of charged particlescentral Au+Au collision dNch / dy ≈ 700 And several pairs per event from trivial originπ0Dalitz decays ~ 10-2 / π0+ γ conversions (assume 0.5% radiation length) ~ 10-2 / π0 huge combinatorial background  (dNch / dy)2 pairing of tracks originating from unrecognized π0 Dalitz decays and γ conversions no means to reduce combinatorial backgroundbeyond reducing conversion length to 0.4% andpT cut at 200 MeV  Signal to background depending on mass up to1 : few hundred Electron pairs are emitted through the wholehistory of the collision: need to disentangle the different sources. need excellent reference p+p and d+Au data. Experimental Challenge Analysis Challenge

  34. Combinatorial Background Which belongs to which? Combinatorial background γ→ e+ e-γ→ e+ e-γ→ e+ e-γ→ e+ e- π0 → γ e+ e-π0 → γ e+ e-π0 → γ e+ e-π0 → γ e+ e- PHENIX 2 arm spectrometer acceptance: dNlike/dm ≠ dNunlike/dm  different shape  need event mixing like/unlike differences preserved in event mixing Produce like and unlike sign in the mixed events at the proper rate (B+- = 2√B++B--) Like sign used as a cross check for the shape  provide absolute normalization for unlike sign background Use same event topology (centrality, vertex, reaction plane) Remove every unphysical correlation

  35. Background Normalization: N+-=2√N++N-- • Small signal in like sign at low mass • N++ and N-- estimated from the mixed events like signB++ and B-- normalized at high mass:B++/N++ = 1 B--/N-- =1for mass > 700 MeV  Uncertainty due to statistics of N++ and N--: 0.12% --- Foreground: same event --- Background: mixed event TOTAL SYSTEMATIC ERROR = 0.25% • Correction for asymmetry of pair cut • Pair cut works differently in like and unlike sign pairs κ =κ+-/√ κ++κ-- = 1.004 • estimated with mixed events • Systematic error (conservative): 0.2%

  36. Des einen Freud – des anderen Leid: Conversions Conversion pair Dalitz decay z z e- B B y y MVD support structure r ~ mass e- e+ e+ x x f PHENIX Beam Pipe • γ→e+e- at r ≠ 0 have m ≠ 0 (artifact of PHENIX tracking: • i.e. 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 conversions Inclusive Removed by phiV cut After phiV cut

  37. Cocktail Ingredients p+p at √s=200 GeV π0→ γγ(Phys. Rev. D 76, 051106 (2007)) π±(Phys. Rev. C 74, 024904) • Start from the π0 , assumption: π0 = (π+ + π-)/2 • parameterize PHENIX pion data:

  38. p+p Cocktail Tuning (ω & φ) • ω and φ are fit with: • modified Hagedorn (as on previous slide: all parameter free) • π0 parameterization with modified Hagedorn + mT scaling (as on previous slide: A is only free parameter, pT→√(pT2+mω2-mπ2)) • exponential in mT • Fits of ω cross section • mod. Hagedorn: χ2/NDF = 21.6/18 • mT scaled π0: χ2/NDF = 34.1/22 • expo in mT: χ2/NDF = 77.0/8 • Fits of φcross section • mod. Hagedorn: χ2/NDF = 30.5/13 • mT scaled π0: χ2/NDF = 32.4/17 • expo in mT: χ2/NDF = 73.3/15 38

  39. p+p Cocktail Tuning (J/ψ) • Fits of J/ψcross section • mod. Hagedorn: χ2/NDF = 9.86/12 • mT scaled π0: χ2/NDF = 12.5/16 • expo in mT: χ2/NDF = 15.0/14 Published J/ψ is fit with: • modified Hagedorn (all parameter free) • π0 parameterization with modified Hagedorn + mT scaling (one free parameter) • exponential in mT • also shown is the published fit with a power law 2007-12-14 Torsten Dahms - Stony Brook University 39

  40. 0-30 90-140 140-200 200-300 MeV Rdata ÷ ÷ ÷ In practice • Material conversion pairs removed by analysis cut • Combinatorial background removed by mixed events • Calculate ratios of various mee bins to lowest one: Rdata • If no direct photons: ratios correspond to Dalitz decays • If excess:direct photons • Fit of virtual photon shape to data in principle also possible(done for d+Au) From conventional measurement

  41. Low pT mass spectra

  42. Direct Photons Decay photons(p0→g+g, h→g+g, …) hard: thermal: • Direct photon sources: • QCD Compton scattering • Annihilation • QCD Bremsstrahlung • Hard photons from inelastic scattering of incoming partons • Thermal photons are emitted via same processes but from thermalized medium carry information about the temperature of the medium

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