Analysis of the dielectron continuum in Au+Au @ 200 GeV with PHENIX

1. Analysis of the dielectron continuum in Au+Au @ 200 GeVwith PHENIX • Physics Motivation • Analysis Strategy (765M events) • Cuts • Single electron cuts • Electron pair cuts: remove hit sharing • Spectra: Foreground, Background (mix events), Subtracted • Efficiency / Acceptance • Cocktail & theory comparison Alberica Toiafor the PHENIX Collaboration

2. Freeze-out Hadronization QGP Thermaliztion Hard Scattering Au Au Physics Motivation: em probes time e- e+ g  Expansion  space • electro-magnetic radiation: g, e+e-, m+m- • rare, emitted “any time”; reach detector unperturbed by strong final state interaction

3. e- e+ R. Rapp nucl-th/0204003 e+e- Pair Continuum at RHIC Expected sources • Light hadron decays • Dalitz decays p0, h • Direct decays r/w and f • Hard processes • Charm (beauty) production • Important at high mass & high pT • Much larger at RHIC than at the SPS • Cocktail of known sources • Measure p0,h 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)

4. All charged tracks RICH cut Real Net signal DC PC1 Background magnetic field & tracking detectors PC3 Energy-Momentum Electron Identification • PHENIX optimized for Electron ID • track + • Cherenkov light RICH+ • shower EMCAL Charged particle tracking: DC, PC1, PC2, PC3andTEC Excellent mass resolution (1%) p e+ e- Pair cuts (to remove hit sharing)

5. Combinatorial Background Which belongs to which? Combinatorial background g e+ e- g e+ e- g e+ e- g e+ e- p0  g e+ e- p0  g e+ e- p0  g e+ e- p0  g e+ e- PHENIX 2 arm spectrometer acceptance: dNlike/dm ≠ dNunlike/dm  different shape  need event mixing like/unlike differences preserved in event mixing  Same normalization for like and unlike sign pairs RATIO -- --- Foreground: same evt --- Background: mixed evt BG fits to FG 0.1%

6.  e+ e - po   e+ e - Combinatorial Background • Different independent normalizations used to estimate sys error • Measured like sign yield • Event counting: Nevent / Nmixed events • Poisson assumption: N± = 2√N++N— • Track counting: ‹N±› = ‹N+›‹N-› • All the normalizations agree within 0.5% Systematic uncertainty: 0.25% --- Foreground: same evt --- Background: mixed evt

7. Photon conversion rejection • ge+e- at r≠0 have m≠0(artifact of PHENIX tracking) • effect low mass region • have to be removed Conversion removed with orientation angle of the pair in the magnetic field r ~ mee Photon conversion --- without conversion --- with conversion beampipe air Mass [GeV/c2] Support structures

8. Subtracted spectrum Integral:180,000 above p0:15,000 BG normalized to Measured like sign yield All the pairs Combinatorix Signal • Green band: systematic uncertainty • Acceptance • Efficiency • Run-by-run

9. Signal to Background • Very low signal to background ratio in the interesting region main systematic uncertainty ssignal/signal = sBG/BG * BG/signal 0.25% large!!! Yellow band: error on combinatorial background normalization Green band: other systematics

10. A closer look at resonances phi Agreement with other analyses J/psi Upsilon???

11. Cocktail comparison • Data and cocktail absolutely normalized • Cocktail from hadronic sources • Charm from PYTHIA • Predictions are filtered in PHENIX acceptance • Good agreement in p0 Dalitz • Continuum:hint for enhancement not significant within systematics • What happens to charm? • Single e  pt suppression • angular correlation??? • LARGE SYSTEMATICS!

12. Data/cocktail

13. Comparison with theory • calculations for min bias • QGP thermal radiation included • Systematic error too large to distinguish predictions • Mainly due to S/B • Need to improve •  HBD R.Rapp, Phys.Lett. B 473 (2000) R.Rapp, Phys.Rev.C 63 (2001) R.Rapp, nucl/th/0204003

14. A Hadron Blind Detector (HBD) for PHENIX signal electron Cherenkov blobs e- partner positron needed for rejection e+ qpair opening angle ~ 1 m S/B ~ 100x • Dalitz rejection via opening angle • Identify electrons in field free region • Veto signal electrons with partner • HBD concept: • windowless CF4 Cherenkov detector • 50 cm radiator length • CsI reflective photocathode • Triple GEM with pad readout • Construction/installation 2005/2006 Irreducible charm background S/B increased by factor 100

15. Summary & Outlook • First dielectron continuum measurement at RHIC • Despite of low signal/BG • Thanks to high statistics • Good understanding of background normalization • Measurement consistent with cocktail predictions within the errors • Improvement of the systematic uncertainty • HBD upgrade will reduce background great improvement of systematic and statistical uncertainty “The most beautiful sea hasn't been crossed yet. And the most beautiful words I wanted to tell you I haven't said yet ...“ (Nazim Hikmet)

16. Backup

17. Single electron cuts • Event cut: • zvertex <= 25 • Single electron cuts: • Pt: = 150 MeV – 20 GeV • Ecore >= 150 MeV • Match PC3 & EMC • PC3 (Phi+z) < 3 sigma • EMC (Phi+z) < 3 sigma • Dispmax < 5 (ring displacement) • N0min >= 3 tubes • dep >= -2 sigma (overlapping showers NOT removed) • chi2/npe0 < 10 • Quality = 63, 51, 31

18. Pair cuts • DC ghosts (like sign): • fabs(dphi) < 0.1 rad • fabs(dz) < 1.0 cm --- Foreground: same evt --- Background: mixed evt RICH ghosts (like and unlike sign)Post Field Opening Angle < 0.988 like Cos(PFOA)

19. Systematic error • Systematic error of simulation • Acceptance difference between real/simulation is less than: 3%. • Single e eID efficiency difference between real/simulation is less than 8.8%. • Dep < 1%, emcsdphie < 1%, emcsdze < 1%, n0 < 7%, chi2/npe0 < 1%, disp < 5%. • Systematic error of real data • Stability of acceptance: 5% • Stability of eID efficiency: 5% • Other correction factor • Embedding efficiency < 10% (Run2 7%). • Background Normalization

20. Acceptance filter • Decoupling acceptance – efficiency corrections • Define acceptance filter (from real data) • Correct only for efficiency IN the acceptance • “Correct” theory predictions IN the acceptance • Compare ACCEPTANCE FILTER q0 charge/pT f0 z vertex Roughly parametrized from data

21. Efficiency • 2 sets of simulations of dielectron pairs • White in mass (0-4GeV) • White in pT (0-4GeV) • Vertex(-30,30), rapidity (1unit), phi (0,2p) • Linearly falling mass (0-1GeV) • Linearly falling pT (0-1GeV) • Vertex(-30,30), rapidity (1unit), phi (0,2p) 2D efficiency corrections: Mass vs pT

22. Single e distribution: Poisson 0-10% 10-20% 20-30% 30-40%

23. Normalization of combinatorial background • Same normalization used for like and unlike sign pairs • 4 different (independent) normalizations: • Akiba : Nevent / Nmixed events • Hemmick: N± = 2√N++N— • Zajc: ‹N±› = ‹N+›‹N-› • Drees: 0.5*( (IntegralFG++(0.15-4 GeV) / IntegralBG++(0.15-4 GeV))  + (IntegralFG-- (0.15-4 GeV) / IntegralBG-- (0.15-4 GeV)) ) • Normalization factors [10-2] • akiba: 8.74 • hemmick: 8.69 • zajc: 8.71 • drees: 8.70 • upper limit: Integral in charm region=0  Normalization factor 8.75 All normalizations agree within 0.5%

24. The unfiltered calculations • black: our standard cocktail • red : hadronic spectrum using the VACUUM rho spectral function • green: hadronic spectrum using the IN-MEDIUM rho spectral function • blue : hadronic spectrum using a rho spectral function with DROPPING MASS • magenta : QGP spectrum using the HTL-improved pQCD rate