Mihael Makek for the PHENIX Collaboration

1. Measurement of low mass dielectrons in Au+Au collisions with the HBD upgrade of the PHENIX detector Mihael Makek for the PHENIX Collaboration QM2011, Annecy

2. Outline: • Low mass dileptons • The HBD Performance in RHIC Run-9 and Run-10 • Status of the dielectron analysis in Au+Au • Conclusion and outlook M.Makek, QM2011

3. Low mass dileptons • Dileptons interact only electromagnetically, their path from the interaction region to the detectors almost undisturbed • Probes for chiral symmetry restoration and in-medium modification of the low mass vector mesons • Results* from RHIC Run-4: Yield in mee = 0.15 - 0.75 GeV/c2 larger by a factor 4.7 +/- 0.4(stat.) +/- 1.5(syst.) +/- 0.9(model) compared to the expected hadronic contribution • S/B in this mass region is 1/200 • combinatorial background should be reduced! *Phys.Rev.C81, 034911 (2010) M. Makek, QM2011

4. Low mass dileptons How to reduce the combinatorial background? • The main sources of the combinatorinal background in PHENIX originate from: π0     e+ e- π0  e+ e- • Distinguish e+e-from π0Dalitz decays and conversions by the opening angle Simulation f->e+e- p0->e+e-g comb. backg. pair qop • Preserve the opening angle by creating a magnetic field-free region • Install a detector which can distinguish single electron hits from open pairs and double electron hits from close pairs •  the goal of the Hadron Blind Detector! M. Makek, QM2011

5. HBD – the basic concept qpair opening angle Cherenkov blobs e- e+ B≈0 • The HBD is a Cherenkov detector • Proximity focus configuration no window, no mirror • CF4 radiator and active gas. Lrad=50 cm • Very large bandwidth 108 – 200 nm (6.2 - 11.5 eV) • triple GEMs for signal multiplication • CsI photocathode • hexagonal pad readout (pad side 1.55 cm) • total radiation length within acceptance: 2.4% 0.65 m 1.21 m NIMA (2011) doi:10.1016/j.nima.2011.04.015 M. Makek, QM2011

6. HBD – hadron blindness • Electron signals are relatively rare (compared to hadrons) mesh primary ionization from dE/dx photo electron 140 μm El. Field CsI (350 nm) 70 μm F. Sauli ,NIM A 386 (1997) 531 GEMs pads Detector operated in reverse bias mode to repel the ionization charge from dE/dx Cherenkov light is formed only by e+ or e- M. Makek, QM2011

7. HBD performance:single vs. double hit recognition HBD charge single hit Close pair Open pair • Single electron charge peaks at 20 pe • Double electron charge peaks at ~40 pe • Good single to double separation • Plot data from fully reconstructed 0Dalitz pairs (m < 150 MeV/c2) in the central arms. Matched to HBD into two separate clusters (open pairs) or one single cluster (close pairs). HBD charge double hit Run 9, p+p M. Makek, QM2011

8. HBD performance:figure of merit N0 and single electron detection efficiency • The average number of photo-electrons Npe in a Cherenkov counter: with: • bandwidth: 6.2 eV (CsI photo-cathode threshold) - 11.5 eV (CF4 cut-off) Quantum efficiency kept constant during the two years of operation! The highest ever measured N0! The high photoelectron yield excellent single electron detection efficiency:  Single electron efficiency using a sample of open Dalitz decays:  ~ 90 % Single electron efficiency derived from the J/Y region:  = 90.6  9.9 % M. Makek, QM2011

9. Background rejection in p+p Pairs in Central Arms Present status from Run-9 p+p: Background reduction in mee > 0.15 GeV/c2 (not fully optimized) Pairs matched to HBD Pairs after HBD reject. Run 9, p+p M. Makek, QM2011

10. HBD analysis in Au+Au • Challenge in Au+Au measurement: • Large multiplicity -> scintillation from CF4 -> Large detector occupancy -> Statistical fluctuations of the underlying event charge But there are solutions! • Analysis steps: • Subtract the underlying event • Reject the electrons created downstream of the HBD • Reject the p0 Dalitz and the conversions created upstream M. Makek, QM2011

11. HBD analysis in Au+Au:subtraction of the underlying event • Two approaches for underlying event subtraction (event-by-event): • local charge subtraction • average module charge subtraction -> consistent results • Occupancy reduced to ~15% in MinBias • After subtracting the underlying event, the remaining fluctuations can still mimic electron signals (fake electrons) • Two pattern recognition algorithms are developed for electron ID HBD module before subtraction: HBD module after subtraction: Posters: E. Atomssa and Y. Watanabe Run 10, Au+Au M. Makek, QM2011

12. The HBD analysis in Au+Au:matching of tracks to the HBD • Use the track projection point on the HBD to pre-determine the cluster size • The cluster is accepted if its charge is above a predetermined threhold Plots by Y. Watanabe • This algorithm slightly truncates the charge but ensures high rejection and good efficiency M. Makek, QM2011

13. The HBD analysis in Au+Au:matching of tracks to the HBD • Very high rejection achieved while keeping a high efficiency even in the most central events • Monitoring the efficiency and the rejection: • Efficiency studied using MC electrons from f-> e+e- embedded in Au+Au data • Rejection of mis-identified hadrons and random matching determined from the data Centrality: M. Makek, QM2011

14. The HBD analysis in Au+Au:rejection of p0Dalitz electrons and upstream conversions • The rejection of the upstream conversions and p0 Dalitz pairs is achieved with single/double charge cut • This requires good gain calibration throughout the entire run • Single electrons hits studied using MC electrons from f->e+e- emebedded in Au+Au data • Double electron hits studied using MC p0->gg emebedded in Au+Au data Single vs. double charge Single efficiency vs. double rejection Singles efficiency Doubles rejection Run 10, Au+Au M. Makek, QM2011

15. The HBD analysis in Au+Au:monitoring the analysis chain • Full event HIJING simulation of 0-10% central Au+Au events • Study the full analysis chain: electron detection efficiency, rejection of upstream and downstream conversions, electrons from p0 Dalitz decays, hadrons and the subtraction of the residual combinatorial background • Example - applying the matching to the HBD: • Pairs: S  S (0.83)2 B  B /(3.5)2 Only by matching to the HBD: S/B  8.4 S/B M. Makek, QM2011

16. Conclusion and Outlook • The HBD was successfully operated in the PHENIX set-up in Run-9 (p+p) and Run-10 (Au+Au) • Detector performed up to design specifications • excellent electron detection efficiency • good separation between electrons and hadrons • good separation of single and double electron hits • Two different algorithms for HBD electron ID in central Au+Au collisions were developed showing consistent results: • successful handling of scintillation background • High rejection of fakes keeping high electron efficiency First mass spectra from Run-10 Au+Au are expected soon. M. Makek, QM2011

17. BACKUP slides M. Makek, QM2011

18. Measured data sets Run-9: • p+p collisions at 500 GeV: • Recorded 0.01 nb-1 • p+p collisions at 200 GeV • Recorded 0.02 nb-1 Run-10: • Au+Au collisions at 200 GeV: • Recorded 1.3 (1.1) nb-1 or 8.2B (7.0B) events in +/-30 (+/-20) cm vertex (5x data than in Run-4) • Au+Au collisions at 62.4 GeV: • Recorded 0.1 nb-1 or 700M events in +/-30 cm vertex • Au+Au collisions at 39 GeV: • Recorded 0.04 nb-1 or 250M events PHENIX Integrated luminosity in Run-10 200 GeV M. Makek, QM2011

19. PHENIX detector Inner and outer magnet coils producing field-free region for r < 55 cm M. Makek, QM2011

20. Quantum efficiency Quantum efficiency of the photocathode was monitored with the “Scintillation Cube” Quantum efficiency kept constant during the two years of operation! M. Makek, QM2011