Measurement of the dielectron continuum in p+p & Au+Au collisions at √s NN = 200 GeV with PHENIX

1. Measurement of the dielectroncontinuum in p+p & Au+Au collisions at √sNN = 200 GeV with PHENIX - Torsten Dahms - Stony Brook University 2nd Berkeley School of Collective Dynamicsin High-Energy Collisions May 24, 2007

2. Outline • Motivation: probe the medium • Preliminary Au+Au result • Status of the p+p measurement • Summary Torsten Dahms - Stony Brook University

3. Motivation 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 Why dielectrons • Don’t interact via strong force • Signal integrated over full evolution of the system 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 Torsten Dahms - Stony Brook University

4. 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 • Remove π contamination with pair cut on parallel tracks in RICH • Remove conversions in detector material with cut on orientation in magnetic field p g e+ e- Torsten Dahms - Stony Brook University

5. Combinatorial Background • Normalization of unlike sign needs to be corrected for pair cuts bias between like and unlike sign pairs(i.e. pair cut on RICH ghosts) • Statistical uncertainty: 0.1% • + uncertainty on pair cut bias lead to total syst. uncertainty of ±0.25% like sign unlike sign --- Foreground: same evt --- Background: mixed evt BG fits to FG 0.1% --- Foreground: same evt --- Background: mixed evt Which belongs to which? 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  Same normalization for like and unlike sign pairs RATIO Torsten Dahms - Stony Brook University

6. Au+Au Result PHENIX Preliminary Systematic and Normalization Error 870 Million MinBias events Torsten Dahms - Stony Brook University

7. Au+Au: MB comparison to Cocktail PHENIX Preliminary • Data have been corrected for single electron ID efficiency • Data and Cocktail absolutely normalized • Cocktail from hadronic sources (i.e. light mesons via mT scaling) • open charm from PYTHIA • Cocktail filtered into PHENIX acceptance • Indication of enhancement in region:150<mee<740MeV Torsten Dahms - Stony Brook University

8. Au+Au Mass Ratios PHENIX Preliminary • Ratio of yield in various mass regions to the yield within the π0 mass range, 0-100 MeV • Yield in mass window 450-600 MeV increases faster with centrality than π0 mass range Torsten Dahms - Stony Brook University

9. Intermediate Mass Region • IMR dominated by open charm • c-cbar produced in pairs • mee = p1p2(1- cos(θ12)) • Suppression of IMR in addition to observed high pT suppression of charm in single electron measurement • If decrease true, it could indicate modified dynamical correlation of c-cbar pair • But within errors constant with Npart Torsten Dahms - Stony Brook University

10. p+p Status Au+Au • Important to provide reference beyond “cocktail” • Much smaller combinatorial background (S/B~1, compared to 1:100 in Au+Au collisions) • But also much smaller multiplicity triggered data p+p Torsten Dahms - Stony Brook University

11. p+p Analysis • Used ERT triggered dataset to increase statistics: • select events that were triggered by a track which fires RICH and showers in EMC (i.e. single electron trigger with pT threshold: 0.4GeV) • Trigger bias on combinatorial background • Remove random benefit (only accept pairs in which at least one electron has fired the trigger) • Generate mixed events from MinBias dataset, with same requirement on the pair • Need to correct for trigger efficiency: • Hadron cocktail • Depends on trigger dead area • Determine trigger efficiency from MinBias (triggered electron/all electrons) • Simulate trigger efficiency for every EMC sector • Project into mass vs. pT Torsten Dahms - Stony Brook University

12. First estimate of trigger effects • Inv. mass spectrum for e+e- pairs from hadronic cocktail: • for MinBias events • with trigger condition on at least one of the electrons • Small mass dependence (up to the φ) • Difference mainly due to dead area of trigger MinBias ERT Implementation Torsten Dahms - Stony Brook University

13. p+p Combinatorial Background Signal due to double conversion Like Sign Pairs: FG ERT BG MinBias • To get shape of combinatorial background correct: • Mixed events from MinBias sample • Apply trigger condition as pair cut, i.e. at least one of the electrons has fired the ERT trigger • BG normalized to like sign above 700MeV Torsten Dahms - Stony Brook University

14. p+p Raw Spectra Unlike Sign Pairs: FG ERT BG MinBias Subtracted • Resonances very well reproduced • Photon conversions in detector material removed after BG subtraction Torsten Dahms - Stony Brook University

15. The ω and φ Torsten Dahms - Stony Brook University

16. Summary • Background shape and normalization well understood • Au+Au: • Hint of low mass excess • Centrality dependent supression in charm region • Medium breaks opening angle correlation of c-cbar? • p+p: • Benefit of much smaller background: S/B~1 • Combinatorial BG from triggered data understood • Early stage of analysis • Could give answers to: • pT distribution of hadrons/input for cocktail • Baseline for charm • Identify thermal photons contribution to Au+Au • IMR: RCP  RAA Torsten Dahms - Stony Brook University

17. Backup

18. Background Normalization • Background shape well reproduced • Four independent normalization factors: • like sign yield (no like sign signal):FG+-/BG+- = (FG++/BG++ + FG--/BG--)/2(needs to exclude low mass region, due to signal from double conversions) • pair production (geometrical mean):N+- = 2√N++N-- • number of mixed events: Nevt/Nmix • number of tracks: <N+-> = <N+><N-> • Very good agreement:within 0.5%  syst. uncertainty of ±0.25% Torsten Dahms - Stony Brook University

19. Trigger Efficiency all tracks (after eID cuts) from MB events + ERT triggered tracks from MB events + ERT triggered tracks from ERT triggered events E3 W3 W0 E0 • pT spectra for each sector • Good agreement of ERT triggered tracks from MinBias and ERT triggered events • Ratio of ERT triggered/MinBias trigger efficiency Torsten Dahms - Stony Brook University

20. Trigger Efficiency Torsten Dahms - Stony Brook University

21. “Wrong” Combinatorial BG • Mixed events from ERT triggered events • What if we apply a trigger condition on the mixed event • require every MinBias event that is used for mixed events to have at least one trigger particle • this should reproduce a ERT trigger sampleBG should have same trigger bias like sign:FG ERT BG ERT BG MinBias Torsten Dahms - Stony Brook University