SKY ROLNICK University California Riverside APS April Meeting

1. Baseline study for Chiral Symmetry Restoration usingthe Hadron Blind Detector in the PHENIX Experiment SKY ROLNICK University California Riverside APS April Meeting

2. Low Mass Dilepton Pairs in Heavy Ion Collisions Electron pairs landscape • Chiral symmetry is a symmetry of QCD which is expected to be restored at high temperatures achievable at RHIC • Dileptons are our best probe to study chiral symmetry restoration and medium modification of vector mesons. • Ideal EM probes, no strong interaction, long mean free path, carry information about the medium. • Produced throughout the history of the collision.

3. PHENIX Experiment Dielectron Measurement Currently, electrons are tracked by drift chamber and pad chamber The Ring Imaging Cherenkov Counter is primary electron ID device Electromagnetic calorimeters measure electron energy Typically, in normal magnetic field configuration, only 1 electron from a pair falls within the PHENIX acceptance. Both members of the pair are needed to reconstruct a Dalitz decay or a  conversion. p g e+ e- Experimental challenge: huge combinatorial background arising from e+e- pairs from copiously produced from 0 Dalitz decay and  conversions.

4. Dilepton Pair Analysis in p+p arxiv: 0912.0244v1 [nucl-ex] • Low mass: • Dalitz decays: • p0ge+e-, hge+e-, wp0e+e-, fhe+e- • Direct decays: • re+e-, we+e-, fe+e-, J/ye+e-, y’e+e- • Heavy flavor: • cce+e- +X, bbe+e- +X • Drell-Yan: • qqe+e- • Intermediate mass: • Extract charm and bottom: • σcc = 518 ± 47 (stat) ± 135 (syst) ± 190 (model) μb • σbb = 3.9 ± 2.4 (stat) +3/-2 (syst) μb • Charm: integration after cocktail subtraction • σcc = 544 ± 39 (stat) ± 142 (syst) ± 200 (model) μb Excellent agreement with Cocktail Filtered in PHENIX acceptance

5. Dilepton Pair Analysis in Au+Au • 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? arxiv: 0912.0244v1 [nucl-ex] Signal/Background  1/500 – 1/100 depending on pt cut and mass.

6. The PHENIX Hadron Blind Detector HBD concept: Windowless Cherenkov detector (L=50cm) CF4 as radiator and detector gas Proximity focus: detect circular blob not ring CF4 Gas 50 cm e+ Create a field free region close to the vertex to preserve opening angle of close pairs. Identify electrons in the field free region reject close pairs. e- 5 cm Opening angle can be used to cut out photon conversion and Dalitz decays MUST BE ABLE TO DISTINGUISH SINGLE AND DOUBLE HITS Cherenkov “blobs”(rBLOB~3.36cm) Dilepton pair Beam Pipe Triple GEM stacks (10 panels/side)

7. Separating Signal from Background Opening angle can be used to cut out photon conversion and Dalitz decays Identify electrons with p > 200 MeV/c in Central Arms, project back and match to HBD. Reject if there is another electron within θ < 200 mrad

8. Single vs Double Electron Clusters (Run 9) Use reconstructed Dalitz pairs (Mee < 150 MeV/c) in PHENIX Central Arms Match to single or double clusters in HBD I.Ravinovich ~ 22 p.e. per single electron track ~ 40 p.e. per two electron track Agrees with our expected yield taking into account p.e. collection efficiency and transmission loss in the gas.

9. Electron Efficiency (Run 9) • Measured using well identified electrons in low mass region (0.025 < m < 0.050 GeV) measured in the PHENIX Central Arms and matched with hits in the HBD • Single electron efficiency > 90% • Pair efficiency ~ 80%. • Meets requirements for achieving 90 % rejection of photon conversions and Dalitz decays Opening angle I.Ravinovich

10. **Very** preliminary rejection numbers: matching to HBD ~2 - 7 double hit cut ~6.5 single pad cluster cut ~2 Total Rejection Factor ~26 First look at Run9 p+p 200 GeV dielectron data I.Ravinovich

11. Improvements in S/B with HBD in Run 10 ~ 25% decrease due to scintillation in central collisions Npe single = 22 pe Run 4 statistics Run 4 statistics Z.Citron With these numbers we can Expect an increase in effective Signal by ~70! Run 4 = 0.8 x 109 MB events Run 7 = 5.4 x 109 MB Run 10  ~ 6.0 x 109 MB f ~ 7.5

12. Collaboration • Brookhaven National Lab: B. Azmoun, A.Milov, R. Pisani, T. Sakaguchi, A. Sickles, C. Woody • Columbia University: C.-Y. Chi • UC Riverside: • Rich Seto, Sky D. Rolnick • Stony Brook University: W. Anderson, Z. Citron, J. M. Durham, T.Hemmick, J. Kamin, V. Pantuyev, J. Sun, B. Lewis • Weizmann Institute of Science: A. Dubey, Z. Fraenkel, A. Kozlov, A. Milov, • M. Naglis, I. Ravinovich, D. Sharma, I. Tserruya

13. Backup Slides

14. Detector Occupancy 90% Centrality I.Ravinovich

15. Detector Occupancy 30% Centrality I.Ravinovich

16. Detector Occupancy 10% Centrality I.Ravinovich

17. Scintillation makes life Difficult! I.Ravinovich • Curves show: Blue = Double • Red = Single • Magenta= Scintiallation

18. HBD Clustering Algorithms Wis Clusterizer (pad seeded) HnS Clusterizer (track seeded) Finds clusters using neighbor pads and associates clusters to nearest track. Scintillation in pp identified easily by single pad clusters with low charge response. Higher multiplicities often cause problems. Searches around region of track for cluster. Uses a three-tuples as primitive cluster object. In AuAu has the advantage that it is much less sensitive to scintillation.

19. Summary • The HBD is a first of its kind, very high performance Cherenkov counter that has been very challenging to build an operate. • The detector performed well in Run 9 in p-p and gave the expected level of performance in terms of p.e. yield, electron efficiency and hadron rejection • It seems to be performing well again in Run 10 and should give the required level of efficiency and rejection in Au+Au collisions • Given this level of performance and the added benefit of the HBD in terms of suppressing Dalitz pairs and photon conversions, it should enable a much more sensitive and precise measurement of low mass dilepton pairs in heavy ion collisions at RHIC.