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PHENIX at RHIC: The Challenge of High Energies

PHENIX at RHIC: The Challenge of High Energies. RHIC Founding Fathers ’ View. before 1991 proposals for various experiments at RHIC STAR, TALES, SPARC, OASIS, DIMUON … except for STAR everything else is burned down from the ashes rises PHENIX

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PHENIX at RHIC: The Challenge of High Energies

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  1. PHENIX at RHIC: The Challenge of High Energies

  2. RHIC Founding Fathers’ View • before 1991 • proposals for various experiments at RHIC • STAR, TALES, SPARC, OASIS, DIMUON … • except for STAR everything else is burned down • from the ashes rises PHENIX • Pioneering High Energy Nuclear Interaction eXperiment • 1991: PHENIX “conceptual design report” • philosophy • measure simultaneously as many observables relevant for QCD phase transitions as you can imagine • all but one: low-mass dielectrons • why no dielectrons? • included in first TALES proposal • considered to be “too difficult” for PHENIX • 2005: A lot of work can make “impossible” things happen Axel Drees

  3. Low-Mass e+e- Pairs: The Problem • Electrons/event in PHENIX • Most electrons from Dalitz decays and photon conversions • PHENIX has now active rejection • m = Ne = (dN/dh)p0 * (BR+CONV) * acc * f(pT>0.2GeV) 350 (0.012+0.02) 0.5*0.7 0.32 = 1.3 • combinatorial background pairs/event (assume Poisson stat.) • B = ½ P(2) = ½½m2 e-m = 0.1 • expected signal pairs/event (m>0.2GeV, pT>0.2 GeV) • S = 4.2*10-4  Signal to Background: S/B = 1 / 250 • S/B signal/background as small as 1/ few hundred, • depends on mass Axel Drees

  4. Why can one not reject Conversions/Dalitz Decays? • Typically only one “leg” of the pair is in the acceptance • out of acceptance • “soft” tracks curl up in the magnetic field • Only option for rejection: • catch electrons before they are lost • need new detector and modification of magnetic field HDB (hadron blind detector) upgrade planned for 2009/2010 to solve issue Till then subtract background with accuracy of << 0.5% Axel Drees

  5. EMC PHENIX min. bias trigger BBC1 Au RICH Au BBC2 Coincidence BBC1+BBC2+|z|<30cm Au-Au ~92% of cross section p-p ~ PHENIX Measures Dielectrons • First attempt from 2002 Au-Au Run failed • S/B ~ 1/500 (!) for minimum bias events • not enough statistics • Success with Au-Au data taken in 2004 • minimum bias trigger • 8 108 events recorded (100x stat.) • Reduced material  reduced background • Reference p-p data taken in 2005 • Min. bias + single electron trigger (ERT) • xxx events sampled with ERT • Xxx minimum bias events • Low multiplicity  significantly smaller background PHENIX single electron trigger RHIC-EMC coincidence p>400 MeV Axel Drees

  6. p g e+ e- DC PC1 PC3 PHENIX: Tracking & Particle ID • Charged particle tracking • Precision tracking outside of B-field • Extrapolate to vertex to get momentum • Electron/Pion separation • Signal in RICH • EM shower • Match of E/p Axel Drees

  7. PHENIX Electron Acceptance fmax Single track acceptance charge/pT Df f=0 f fmin f • Acceptance not equal for + and – charged tracks! • Pairs will be recorded only if both tracks are within acceptance. • Different mass and pt distributions for like and unlike sign pairs! • Like sign pairs can not be used as estimate for combinatorial background! Axel Drees

  8. p+ e- Unphysical Background Rejection: “Pair Cut” • Pions identified as electrons in presents of electron • RICH measures angle only and not position!!! • Pion can be misidentified as electron • Leads to correlated but unphysical pairs • Not reproduced by mixed events • Different probability and kinematics for like and unlike sign pairs pT UNLIKE mass Remove by rejecting events with parallel tracks in RICH Axel Drees

  9. pool (j,j) e1 e2 e3 e4 . . en p1 p2 p3 p4 . . pn Combinatorial Background: Event Mixing Event with e+ (p)and/or e-(e) Centrality i Vertex j event cut pass no example 1 track e0 Axel Drees

  10. mix with pool Store like sign pairs pool (j,j) e1 e2 e3 e4 . . en p1 p2 p3 p4 . . pn Store unlike sign pairs Combinatorial Background: Event Mixing Event with e+ (p)and/or e-(e) Centrality i Vertex j event cut pass no example 1 track e0 Axel Drees

  11. mix with pool Store like sign pairs pool (j,j) e0 e1 e2 e3 . . en-1 p1 p2 p3 p4 . . pn Store unlike sign pairs Combinatorial Background: Event Mixing Event with e+ (p)and/or e-(e) Centrality i Vertex j event cut pass no example 1 track e0 update pool Axel Drees

  12. Mixing Without Pair Cut Large unphysical background! Axel Drees

  13. Combinatorial Background: Like Sign Pairs • Shape from mixed events • Excellent agreements for like sign pairs • 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 TOTAL SYSTEMATIC ERROR = 0.25% Axel Drees

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

  15. yield in4p yield in acceptance Subtraction of “Cross” Pairs • p0g g* unlike cross like cross unlike 4-body e+ e- X e+ e- Unlike: data - mixed Like: data - mixed Monte Carlo: Cross Like Cross Unlike Include also h decay Axel Drees

  16. submitted to Phys. Rev. Lett arXiv:0706.3034 Raw unlike-sign mass spectrum 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 0.25% large!!! Axel Drees

  17. Cross Check with Converter Method Increase background by increasing radiation length in experiment Add brass sheet around beam pipe (1.7% X0/X) Number of electrons increases by factor ~1.6 Combinatorial background increases by factor 2.5 ~ (1.6)2 If “signal” really not subtracted background signal must be larger with converter! Photon Converter (Brass: 1.7% X0) Axel Drees

  18. submitted to Phys. Rev. Lett arXiv:0706.3034 Raw unlike-sign mass spectrum Unlike sign pairs data Mixed unlike sign pairs normalized to: Independent check of background normalization ~ 0.1% Axel Drees

  19. Background Subtraction in pp What works in Au-Au does not work in p-p … some months later … Unlike sign pairs data Mixed unlike sign pairs normalized to: Axel Drees

  20. The Background in p+p • Near side located at small mass and high pT • Away side at low pT and large mass • In between exists a region that can be described by mixed events γ e- e+ e+ π0 e+ e- π0 π0 γ e- γ • Could jet correlations show up as signal? • Would produce like and unlike sign pairs • Generated p+p events with PYTHIA • compare same event spectra with mixed events Observe difference from mixed events at near- and away-side In like and unlike sign Background normalized to yield in Δφ = (π/2± π/10) rad Axel Drees

  21. Correlated Background Data & MC: pp • Cross pairs • Simulate cross pairs with decay generator • Normalize to like sign data for small mass • Jet pairs • Simulate with PYTHIA • Normalize to like sign data • Unlike sign pairs • Use same simulations • Use normalization from like sign pairs • Alternative methode • Correct like sign correlated background with mixed pairs Axel Drees

  22. Comparison of BG Subtraction Methods Monte Carlo method Like sign method(with some variations) give consistent results over the full invariant mass range 22 Axel Drees

  23. Correlated Background Data & MC: Au-Au • Cross pairs • Simulate cross pairs with decay generator • Normalize to like sign data for small mass • Jet pairs • Simulate with PYTHIA • Normalize to like sign data • No away side jet contribution! Complicated method to measure jet quenching! Background subtraction ok within systematic errors Axel Drees

  24. Efficiency Correction • Analysis requires that electron and positron are in the detector acceptance, but we correct for detector and analysis artifacts: • Correct for losses due to dead detector areas • Correct for losses due to analysis cuts, e.g. electron ID • Correct for losses due to pair cut hadron decay generator ~40% at higher pT pair efficiency: 10-18% Single track efficiency: including cuts and dead areas m (Gev/c2) pT (Gev/c) For pp collisions trigger efficiency is corrected in similar way Axel Drees

  25. Cocktail Tuning (p+p) • Start from the π0 , assumption: π0 = (π+ + π-)/2 • parameterize PHENIX pion data: arXiv: 0802.0050 • Other mesons well measured in electronic and hadronic channel • Other mesons are fit with: • mT scaling of π0 parameterization pT→√(pT2+mmeson2-mπ2) fit the normalization constant • All mesons mT scale!!! PHENIX Preliminary Axel Drees

  26. p+p Cocktail Comparison submitted to Phys. Lett.B arXiv: 0802.0050 Axel Drees

  27. Determine Charm and Bottom Cross Sections Charm: integration after cocktail subtraction • sc=544 ± 39 (stat) ± 142 (sys) ± 200 (model) mb Simultaneous fit of charm and bottom: • sc=518 ± 47 (stat) ± 135 (sys) ± 190 (model) mb • sb= 3.9 ± 2.4 (stat) +3/-2 (sys) mb Axel Drees

  28. Charm and bottom cross sections BOTTOM CHARM 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!!! Axel Drees

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

  30. Au+Au Cocktail Comparison submitted to Phys. Rev. Lett arXiv:0706.3034 Low-mass continuum: enhancement 150 <mee<750 MeV: 3.4±0.2(stat.) ±1.3(syst.)±0.7(model) Axel Drees

  31. Au+Au Cocktail Comparison Charm from PYTHIA filtered by acceptance sc= Ncoll x 567±57±193mb Charm “thermalized” filtered by acceptance sc= Ncoll x 567±57±193mb Intermediate-mass Continuum: consistent with PYTHIA if charm is modified room for thermal radiation Axel Drees

  32. Comparison to Theoretical Models • 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 Axel Drees

  33. Yield in Different Mass Ranges 0-100 MeV:p0dominated; scales approximately with Npart 150-750 MeV: continuum; scaling? 1.2-2.8 GeV: charm dominated; scales with Ncoll study centrality dependence of yields in these regions Axel Drees

  34. Centrality Dependence • p0 production scales approximately with Npart • expectation for low-mass continuum • if in-medium enhancement is related to pp or qq annihilation  yield should scale faster than Npart (and it does) • charm is a hard probe • total yield follows binary scaling (known from single e±) • intermediate mass yield shows the same scaling Axel Drees

  35. pT Dependence arXiv: 0706.3034 arXiv: 0802.0050 Au+Au p+p 0<pT<8.0 GeV/c 0<pT<0.7 GeV/c 0.7<pT<1.5 GeV/c 1.5<pT<8 GeV/c p+p: follows the cocktail Au+Au: enhancement concentrated at low pT Axel Drees

  36. 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

  37. Acceptance as function of pT and mass 0<m<100 200<m<300 100<m<200 300<m<400 400<m<500 500<m<600 600<m<700 700<m<800 20% 10% pT=5 GeV 800<m<900 1000<m<1050 900<m<1000 2900<m<3300 37 pT Axel Drees

  38. pT dependence II p+p Au+Au p+p: follows the cocktail for all the mass bins Au+Au: significantly deviate at low pT Axel Drees

  39. Understanding the pT Dependence • 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 Axel Drees

  40. 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 Axel Drees

  41. Theory Comparison II • Calculations from • R.Rapp & H.vanHees • K.Dusling & I.Zahed • E.Bratovskaja & W.Cassing (in 4p) Models fail to describe data in particular low pT raise! Axel Drees

  42. Summary p+p Low mass • Excellent agreement with cocktail Intermediate mass • Extract charm and bottom • sc = 544 ± 39 (stat) ± 142 (sys) ± 200 (model) mb • sb= 3.9 ± 2.4 (stat) +3/-2 (sys) mb 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 • pT dependency: enhancement concentrated at low pT Intermediate mass • Agreement with PYTHIA: coincidence? • First measurements of dielectron continuum at RHIC • Theory models fail to describes data • Huge enhancement • Very soft component Axel Drees

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