1 Analysis of the dielectron continuum in 200 GeV with PHENIX Alberica Toia for the PHENIX Collaboration 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

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

3 e + e  Pair Continuum at RHIC Expected sources Light hadron decays –Dalitz decays    –Direct decays  and  Hard processes –Charm (beauty) production –Important at high mass & high p T –Much larger at RHIC than at the SPS Cocktail of known sources –Measure  ,  spectra & yields –Use known decay kinematics –Apply detector acceptance –Fold with expected resolution Possible modifications suppression (enhancement) Chiral symmetry restoration continuum enhancement modification of vector mesons thermal radiation charm modification exotic bound states R. Rapp nucl-th/ e- e+

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

5 Which belongs to which? Combinatorial background   e+ e-    e  e    e  e    e  e   0   e+ e-  0   e+ e- PHENIX 2 arm spectrometer acceptance: dN like /dm ≠ dN unlike /dm  different shape  need event mixing like/unlike differences preserved in event mixing  Same normalization for like and unlike sign pairs Combinatorial Background --- Foreground: same evt --- Background: mixed evt BG fits to FG 0.1% RATIO 

6 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% Combinatorial Background Systematic uncertainty:  0.25%  e + e -    e + e Foreground: same evt --- Background: mixed evt

7 Photon conversion rejection Mass [GeV/c 2 ] --- without conversion --- with conversion air Support structures Conversion removed with orientation angle of the pair in the magnetic field Photon conversion   e+e- at r≠0 have m≠0 (artifact of PHENIX tracking) effect low mass region have to be removed r ~ m ee beampipe

8 Subtracted spectrum All the pairs Combinatorix Signal BG normalized to Measured like sign yield Integral:180,000 above  0 :15,000 Green band: systematic uncertainty Acceptance Efficiency Run-by-run

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

10 A closer look at resonances Upsilon??? J/psi phi A. Kozlov, K. Ozawa H. Pereira, T.Gunji Agreement with other analyses

11 Cocktail comparison Data and cocktail absolutely normalized Cocktail from hadronic sources Charm from PYTHIA Predictions are filtered in PHENIX acceptance Good agreement in  0 Dalitz Continuum: hint for enhancement not significant within systematics What happens to charm? Single e  pt suppression angular correlation??? LARGE SYSTEMATICS!

12 Data/cocktail Measurement [10 -5 counts/event] Predictions [10 -5 counts/event] GeV/c ± 3.8 ± GeV/c ± 0.50 ±

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

14 A Hadron Blind Detector (HBD) for PHENIX S/B ~ 100x Irreducible charm background signal electron Cherenkov blobs partner positron needed for rejection e+e+ e-e-  pair opening angle ~ 1 m 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 S/B increased by factor 100 I.Ravinovich

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

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 Roughly parametrized from data charge/pT 00 00 z vertex 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

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 (0,2  ) Linearly falling mass (0-1GeV) Linearly falling pT (0-1GeV) Vertex(-30,30), rapidity (1unit), phi (0,2  ) 2D efficiency corrections: Mass vs pT

22 Single e distribution: Poisson 0-10% 20-30% 10-20% 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++( GeV) / IntegralBG++( GeV)) + (IntegralFG-- ( GeV) / IntegralBG-- ( 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