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Dileptons with PHENIX

Dileptons with PHENIX. D ilepton analysis PHENIX setup with and w/o HBD Key: background subtraction Results Well understood baseline pp w & w/o HBD, dAu A new handle on charm/bottom separation in dAu Dilepton puzzle in AuAu Comment on STAR data HBD analysis

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Dileptons with PHENIX

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  1. Dileptons with PHENIX • Dilepton analysis • PHENIX setup with and w/o HBD • Key: background subtraction • Results • Well understood baseline pp w & w/o HBD, dAu • A new handle on charm/bottom separation in dAu • Dilepton puzzle in AuAu • Comment on STAR data • HBD analysis • HBD performance in central AuAu collisions • Status at QM2012 • Recent progress • Summary Axel Drees, May 22nd 2013, Trento

  2. DC PC1 magnetic field & tracking detectors PHENIX Setup without HBD background p0e-e+ g g e-e+ e+e- pairs E/p and RICH e- and e+ acceptance are different No background rejection! dilepton S/B < 1:150 200 GeV Run-4 AuAu Run-5 pp, CuCu Run8 dAu e- e- e+ e+ Axel Drees

  3. DC PC1 magnetic field & tracking detectors PHENIX Setup with HBD PHENIX Setup with HBD background p0e-e+ g g e-e+ e+e- pairs E/p and RICH field free region e- e- background rejection in field free region! S/B improves by 5-10 200 GeV Run-9 pp Run-10 AuAu 62.4 GeV Run-10 AuAu e+ e+ HBD Axel Drees

  4. γ e- e+ π0 e+ π0 e+ X π0 e- e- γ γ Key: Understanding the Background Subtraction • Combinatorial background: e+and e-from different uncorrelated source • Need event mixing because of acceptance differences for e+ and e- • Use like sign pairs to check event mixing • Unphysical correlated background • Track overlaps in detectors • Not reproducible by mixed events: removed from event sample (pair cut) • Correlated background: e+ and e- from same source but not “signal” • “Cross” pairs  “jet” pairs • Use Monte Carlo simulation and like sign data to estimate and subtract background Subtractions dominate systematic uncertainty Axel Drees

  5. Component-by-Component Bkg Subtraction • Cross pairs • Simulate cross pairs • with decay generator • Normalize to like sign • data for small mass pp raw data Like Sign Data Correlated Signal = Data-Mix • Jet pairs • Simulate with PYTHIA • Normalize to like sign data Mixed events • Unlike sign pairs • same simulations • normalization from • like sign pairs Unlike Sign Data • Alternative methode • Correct like sign correlated background with mixed pairs Signal: S/B  1 Axel Drees

  6. Key to dilepton analysis is to understand the background 2 10-7/MeV 1 10-9/MeV p0: NpAA/Nppp*eAA/epp jet: Ncoll*RAA*eAA/epp 1 10-10/MeV 2 10-8/MeV Correlated background roughly consistent with expectation

  7. Alternative Method: Relative Acceptance Correction S = FG12 – a FG1122 FG1122 = PHENIX d+Au run-8 A large number of simulations show that relative acceptance correction can not be controlled to better than a few % Method not usable for AuAu

  8. Sys. Uncertainties on Relative Acceptance • Relative acceptance correction depends on • Physics source, thus mixed events not perfect description • Difference in number of electron and positrons • Differences in pt spectra of electrons & positrons • z-vertex position • Variations in active area (within similar run groups) • Case study: variation of active area in dAu setup • In MC simulation remove randomly drift chamber readout cards (1/80 of acceptance) Axel Drees

  9. Dilepton Continuum in p+p Collisions PHENIX Phys. Lett. B 670, 313 (2009) Data and Cocktail of known sources represent pairs with e+ and e- PHENIX acceptance Data are efficiency corrected Excellent agreement of data and hadron decay contributions with 30% systematic uncertainties Consistent with PHENIX single electron measurement sc= 567±57±193mb * * PYTHIA, 1st order, D/B semileptonic decay Axel Drees

  10. p+p Data with the HBD • Cocktail updated • BR, trans. form factors, r-shape, bremsstrahlung, c/b • New Charm/Bottom simulation • MC@NLO using cross sections determined in d+Au • Like sign contribution subtracted in data and simulation Consistent with known sources! Axel Drees

  11. Dilepton Continuum in d+Au Collisions PHENIX preliminary PYTHIA 3 data sets consistent with cocktail within 20-30% pp,dAu w/o HBD; and pp w HBD Baseline at RHIC energies well understood! Axel Drees

  12. d+Au Data: pT Dependence Detailed double differential data Subtract hadron decay contribution Remaining yield dominated by heavy flavor production Common wisdom: charm bottom (but not quite true!) Axel Drees

  13. What can we learn about Charm/Bottom • Use MC@NLO 2nd order pQCD Monte-Carlo • Significant improvement over 1st PYTHIA (gg-fusion) • Subtle differences compared to “tuned” PYTHIA • New features of bottom production not relevant for charm • Feed-down: BR (B→e) ~ BR (B→D→e) ~ 10% • B0B0 oscillations • 60% of pairs unlikesign ee-pairs from bottom production ee-pairs from charm & bottom production Any like sign subtraction in data must be accounted for if compared to simulation! in PHENIX acceptance Axel Drees

  14. Charm/Bottom Cross Section from d+Au Data • Hadron decay cocktail and like-sign pairs subtracted • MC@NLO normalized to double differential data (m vspT) • Extrapolated heavy flavor cross sections: • cc = 710  62 (stat)  183 (syst)  80 (model) mb • bb= 4.5  0.7 (stat)  1.1 (syst)  0.2 (model) mb • DY not considered, bottom might be overestimated Axel Drees

  15. Double Differential View d+Au at 200 GeV • Relative importance of c/b changes with mass and pT • c/b discrimination power • Low pT low mass → charm • High pT low mass → bottom • Possible new opportunity for thermal radiation (Au+Au) • Mass – pT independent • Inverse slope in mass reflects temperature of ~ few 100 MeV • Should be distinguishable from c/b • Will be effected by heavy flavor modification c > b c < b c ~ b Axel Drees

  16. Au+Au Dilepton Continuum Excess 150 <mee<750 MeV: 4.7 ± 0.4(stat.) ± 1.5(syst.) ± 0.9(model) hadron decay cocktail tuned to AuAu PHENIX Phys. Rev. C 81 (2010) 034911 Charm from PYTHIA filtered by acceptance scc= Ncoll× 567±57±193mb Charm “thermalized” filtered by acceptance scc= Ncoll× 567±57±193mb Need better handle on heavy flavor Intermediate-mass continuum: consistent with PYTHIA since charm is modified room for thermal radiation Axel Drees

  17. Soft Low Mass Dilepton Puzzle acceptance corrected • mTspectrum of excess dileptons • Subtract cocktail • Correct for pair acceptance • Fit two exponentials in mT –m0 • 1st component T ~ 260 MeV • Consistent with thermal photon yield • 2nd “soft” component T ~ 100 MeV • Independent of mass • More than 50% of yield • Soft component also seen at SPS in NA60 and CERES data 300 < m < 750 MeV 258  37  10 MeV 92  11  9 MeV PHENIX Phys. Rev. C 81 (2010) 034911 Soft component eludes theoretical explanation Axel Drees

  18. Soft Low Mass Dilepton Puzzle acceptance corrected • mTspectrum of excess dileptons • Subtract cocktail • Correct for pair acceptance • Fit two exponentials in mT –m0 • 1st component T ~ 260 MeV • Consistent with thermal photon yield • 2nd “soft” component T ~ 100 MeV • Independent of mass • More than 50% of yield • Soft component also seen at SPS in NA60 and CERES data 300 < m < 750 MeV 258  37  10 MeV CERES 92  11  9 MeV PHENIX Phys. Rev. C 81 (2010) 034911 Soft component eludes theoretical explanation Axel Drees

  19. A look at STAR p+p Dilepton Data STAR arXiv:1204.1890 charm cross section: STAR s = 920 mb PHENIX(MC@NLO) s= 710 mb* acceptance: STAR Df=2p |Dh|=2 PHENIX cocktail in STAR acceptance PHENIX cocktail and STAR cocktail consistent Axel Drees *consistent with PYTHIA 570mb

  20. A look at STAR pp and AuAu Data Ratio STAR data/ PHENIX cocktail in STAR acceptance • On a different note: comparing PHENIX data to cocktail • Not acceptance corrected • Soft component emphasized • 20%-30% increase of enhancement Compared to PHENIX cocktail no appreciable enhancement Axel Drees

  21. Data from PHENIX HBD Upgrade Window less CF4 Cherenkov detector GEM/CSI photo cathode readout Operated in B-field free region Improve S/B by rejecting combinatorial background • HBD fully operational: • Single electron ~ 20 P.E. • Conversion rejection ~ 90% • Dalitz rejection ~ 80% • Improvement of S/B factor 5-10 to published results p+p data in 2008/9 Au+Au data in 2009/10 Axel Drees

  22. Effect of CF4Scintillation on HBD Performance 10% central, 62 GeV: HBD module in AuAu before subtraction: • Material of HBD 2-3% conv. probability • 3-4x larger conversion background • Most material behind radiator • Reduced p-rejection in RICH • Effect centrality dependent • Scintillation ~ 10pe/pad in central AuAu • Subtract statistically; multiple algorithms; 2 presented at QM11 • Fluctuation in scintillation results in centrality dependent rejection • In central collisions estimate 90% rejection at 65% efficiency e- efficiency 60% Rejection 90% Rely on veto by HBD e- HBD module after subtraction: Scintillation light limits HBD veto Axel Drees

  23. Status at Quark Matter 2012 • Background subtraction with rel. acceptance corrected like-sign pairs • Systematic uncertainty to large to obtain result in central collisions • Observe over subtraction in central collisions • Ongoing analysis improvements • Component-by-component background subtraction (run-4 AuAu) • Improved pion rejection • Increased statistics Semi central (20-40%) Au+Au Peripheral (60-92%) Au+Au Axel Drees

  24. Analysis Improvements Since QM2012 • Improved data analysis of RICH information • Issue: parallel track point to same ring in RHIC • New algorithm resolves ring • MC simulation: Purity 70% 90% at 80% pair efficiency • Include TOF information for hadron rejection • EMCal 450 ps; TOF ~120 ps • Small improvement of S/B • Increased statistics by 1.25 PbSc timing central AuAu • New run of data production completed Axel Drees

  25. X Quantitative Understanding of Background Central (10-20%) AuAu no HBD rejection data • Simulation of background from p0 production • p0 e-e+g & g  e-e+ • Full PHENIX Geant simulation • Unlike sign pairs • Like sign cross pairs • Work in progress • Absolute normalization • Quantitative understanding of rejection • Simulation of jet background • Once completed  look at unlike sign data early conversion late conversion conversions Dalitz Data and MC normalized to each other dito with HBD rejection Late conversions rejection factor 6 early conversion rejection factor 4 • Blind analysis Axel Drees

  26. Summary and Outlook • PHENIX measured ee-pairs at √s=200 GeV • Well understood baseline in pp and dAu collisions • Within 20-30% consistent with hadron decays & heavy flavor production • Constrains heavy flavor production • Puzzles in AuAucollisons • larger excess beyond contribution from hadronic phase with medium modified r-meson properties • soft momentum distribution • Thermal photon puzzle (see talk by Gabor David) • Large thermal yield with T > 220 MeV (10-20% of decay photons) • Large elliptic flow (v2) • HBD analysis moving towards completion • Expect results this year Axel Drees

  27. Backup slides Axel Drees

  28. CERES 95/96 Centrality Dependence of Enhancement PHENIX Warning this is for illustration only!!! 0-30% cross section Npart Nch CERES pT > 200 MeV/c In+In Axel Drees

  29. Acceptance Function Polarized unpolarized: difference smaller than 10% Axel Drees

  30. Acceptance for Virtual Photons Data presented as e+ and e- in acceptance, this is not the same as virtual photon in acceptance! Physical distribution requires that virtual photon is in acceptance! detector Case A e+ Virtual photon and electron and positron in the acceptance g* e- B-field detector e+ Virtual photon in acceptance electron and/or positron NOT in the acceptance Case B g* e- B-field Acceptance depends on pair dynamics! Axel Drees

  31. Combinatorial Background: Like Sign Pairs • Shape from mixed events • Excellent agreements for like sign pairs • also with centrality and pT • Normalization of mixed pairs • Small correlated background at low masses from double conversion or Dalitz+conversion • normalize B++ and B- - to N++ and N- - for m > 0.7 GeV • Normalize mixed + - pairs to • Subtract correlated BG • Systematic uncertainties • statistics of N++ and N--: 0.12 % • different pair cuts in like and unlike sign: 0.2 % --- Foreground: same evt N++ --- Background: mixed evt B++ Au-Au Normalization of mixed events: systematic uncertainty = 0.25% Axel Drees

  32. Au-Au Raw Unlike-Sign Mass Spectrum arXiv: 0706.3034 Run with added Photon converter 2.5 x background Excellent agreement within errors! Unlike sign pairs data Mixed unlike sign pairs normalized to: Systematic errors from background subtraction: ssignal/signal = sBG/BG *BG/signal  up to 50% near 500 MeV as large as 200!! 0.25% Axel Drees

  33. Centrality Dependence of Background Subtraction Compare like sign data and mixed background Evaluation in 0.2 to 1 GeV range For all centrality bins mixed event background and like sign data agree within quoted systematic errors!! Similar results for background evaluation as function pT Axel Drees

  34. Background Description of Function of pT Good agreement Axel Drees

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