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Electron identification at STAR and the Barrel Preshower detector

e +. Electron identification at STAR and the Barrel Preshower detector. Texas A&M University Cyclotron Institute APS/DNP: 10/12/2007. BPRS. Matt Cervantes for the STAR Collaboration. Outline. Physics motivation for electron identification Electron identification methods available in STAR

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Electron identification at STAR and the Barrel Preshower detector

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  1. e+ Electron identification at STAR and the Barrel Preshower detector Texas A&M University Cyclotron Institute APS/DNP: 10/12/2007 BPRS Matt Cervantes for the STAR Collaboration

  2. Outline • Physics motivation for electron identification • Electron identification methods available in STAR • Barrel Electromagnetic Calorimeter (BEMC) in STAR • Barrel Preshower (BPRS) in STAR • Electron-based analysis with and without BPRS

  3. Physics motivation for electron ID • Clean electron identification is needed for electron based Heavy Flavor physics analyses… Example:J/Y e+ e-decay channelselected for an invariant mass reconstruction needs strong electron identification to reject hadronic background • STAR has a large geometrical acceptance for data acquisition and subsequent event reconstruction: -1 < h < 1 and DF = 2p • Large acceptance increases ability to study heavier (larger opening angle) vector mesons such as the Y e+ e-decay channel (again, electrons used) Au Au

  4. elec had Electron identification in STAR • Time Projection Chamber (TPC) momentum: charged track curvature dE/dx: Ionization Energy loss of charged particles • High momentum trackslead to convergence of dE/dx particle bands • Project TPC tracks to BEMC and use the energy from BEMC to form ratio p/E • Shower Maximum Detector (SMD) Energy, position and shower profile BPRS

  5. Electron-based analysis • First look at Run 7 AuAu Data Electron ID • Event sample ~ 360 K (Level 2 BEMC “gamma” triggers) • Only rough calibration of this data set currently available • Electron candidates satisfy Etower > 2 GeV • Study the effect of the BPRS cut (ADC signal > MIP as shown in previous talk byR. Clarke) • Method of Electron identification for this study… • TPC : dE/dx • TPC tracks projected to BEMC : p/E • SMD : shower profile • BPRS : preshower( NEW CUT IN DEVELOPMENT )

  6. dE/dx plots from AuAu data See J. Dunkelberger’s DNP 2007 Poster Session for the fitted electron and hadron regions of the dE/dx distribution, which shows a preliminary result of about 2:1 in the electron yield enhancement relative to hadrons when using the BPRS detector. • Projection onto the dE/dx with pT >= 2 cut applied: • Projection onto the dE/dx with no pT cut applied: • The data provides the dE/dx vs. pdistribution: • Standard STAR tight cut of 3.4 x 10-6 < dE/dx < 5.0 x 10-6

  7. p/E plots from AuAu data • The p/E distribution with pT >= 2 and tight electron cut: • The p/E distribution with pT >= 2 cut applied: • The p/Edistribution with no pT cut: 3.4 x 10-6 < dE/dx < 5.0 x 10-6 Note: p/E < 2 cut will provide a small effect on data; (tight electron cut does most of the work in removing hadrons) • Standard STAR cut of p/E < 2 GeV/cis chosen

  8. SMD electron ID (pp data) • SMD distribution for hadrons and electrons in pp data: • The SMD distribution for hadrons has a more narrow profile than that of the distribution for the electrons • Standard STAR electron cut in the number of strips hit is h >= 2and f >= 2 • What is the effect of a SMD cut on dE/dx in pp data?

  9. Effect of the SMD cut on pp dE/dx • We explore the effect of an SMD cut on our dE/dxdistribution at pT >=2 by forming the ratio: (dE/dx) [SMD(on)] / (dE/dx) [SMD(off)] i.e. ~45% reduction in hadron region ~30% rejection difference ~15% reduction in (tight) electron region • SMD effect in hadron vs. electron regions in dE/dx

  10. SMD in AuAu vs. pp events • SMD distributions for the AuAu vs. pp data: • SMD may not be a straight forward cut in AuAu data The SMD distributions in the AuAu data for the electron and hadron regions do not look to be as discriminated SMD appears to be less effective in high multiplicity events! SMD hadrons SMD electrons The SMD distributions in the pp data for the electron and hadron regions does appear to be much more discriminated

  11. Effect of the SMD cut on AuAu dE/dx I.e. Relative to the low multiplicity pp collisions, the effectiveness of the SMD cut appears to be dropping for high multiplicity AuAu collisions in STAR. We should look at what the BPRS does in the high multiplicity AuAu events. • We explore the effect of an SMD cut on our dE/dxdistribution at pT >=2 by forming the ratio: (dE/dx) [SMD(on)] / (dE/dx) [SMD(off)] i.e. ~50% reduction in hadron region Discrimination w/ the SMD seems to be heading in the wrong direction… ~15% rejection difference ~35% reduction in (tight) electron region region suffers an extra ~5% hit relative to pp data region suffers an extra ~20% hit relative to pp data • SMD effect in hadron vs. electron regions in dE/dx

  12. Effect of BPRS cut on AuAu dE/dx I.e. The BPRS cut relative to the SMD cut rejects only an extra ~5% of electron candidates in the tight electron region, while rejecting an extra ~20% of the hadron candidates in the hadron region in the AuAu data. • We explore the effect of a BPRS cut on our dE/dx distribution at pT >=2 by forming the ratio: (dE/dx) [BPRS(on)] / (dE/dx) [BPRS(off)] i.e. ~30% rejection difference ~70% reduction in hadron region ~40% reduction in (tight) electron region • So what is BPRS effect in hadron vs. electron region?

  13. Effect of BPRS on dE/dx with all cuts • We explore the effect of a BPRS cut on our dE/dxdistribution at pT >=2 by forming the ratio: [all cuts](dE/dx) [BPRS(on)] / [all cuts](dE/dx) [BPRS(off)] i.e. By “no difference”, we mean that the BPRS cut relative to the (SMD + p/E) cuts appears to be the strongest cut of the three SMD + p/E cuts ~70% reduction in hadron region (“no difference”) ~40% reduction in (tight) electron region (“no difference”) • Final comments on BPRS effect on dE/dx here…

  14. Conclusions and Future Work • Currently STAR has very good methods in place for electron identification • Additional Hadron discrimination using the Barrel Preshower (BPRS) is a real possibility • Barrel Preshower alone has a stronger effect than the cuts with Shower Maximum Detector and/or p/E < 2 only • Still need to study Shower Maximum Detector performance in high multiplicity events (AuAu collisions) • Need to perform this study on electron rich data (L2 Upsilon) • Strong indications of the possibility to improve the electron identification in STAR by using the Barrel Preshower detector

  15. elec had Old/backup slides… • Dedx calibrations from pp 2006 !! • Tower energy is not calibrated !! • Need upsilon 2007 data !! • Smd not profiling correctly at the moment ! • Status tables for bprs needed !!

  16. Effect of BPRS cut on dE/dx with p/E<2 • We explore the effect of a BPRS cut on our dE/dxdistribution at pT >=2 by forming the ratio: [p/E<2](dE/dx) [BPRS(on)] / [p/E<2](dE/dx) [BPRS(off)] i.e. p/E < 2 60601 small effect ~70% reduction in hadron region (“no difference”) ~40% reduction in (tight) electron region (“no difference”) • What is the complimentary effect of p/E < 2 on BPRS?

  17. BEMC and BPRS in STAR • Layered Sampling Pb-scintillator (5-6 mm thick layer ~X0) • Tower ~21 X0 : slightly less than 1 hadronic interaction length • Interaction probability for hadrons (Pb only) is ~ 3% before layer 1 (~ 6% before layer 2) • BPRS: ~63% of electrons will shower before the scintillator layer 1 (~84% before layer 2) Most hadron interactions with nuclear material develop at depths beyond the first 2 layers! 15 Xo 20 Xo BPRS

  18. Relativistic Heavy Ion Collider Solenoidal Tracking at RHIC

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