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Xiaoyan Lin (for the STAR Collaboration) Institute of Particle Physics Wuhan, P.R. China

Non-Photonic Electron Angular Correlations with Charged Hadrons from the STAR Experiment: First Measurement of Bottom Contribution to Non-Photonic Electrons at RHIC. Xiaoyan Lin (for the STAR Collaboration) Institute of Particle Physics Wuhan, P.R. China. Outline.

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Xiaoyan Lin (for the STAR Collaboration) Institute of Particle Physics Wuhan, P.R. China

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  1. Non-Photonic Electron Angular Correlations with Charged Hadrons from the STAR Experiment: First Measurement of Bottom Contribution to Non-Photonic Electrons at RHIC Xiaoyan Lin (for the STAR Collaboration) Institute of Particle Physics Wuhan, P.R. China Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  2. Outline • Introduction • Data Analysis • Electron Identification • Photonic Background Removal • Electron-Hadron Azimuthal Angular Correlations • Results and Discussion • Summary Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  3. Non-Photonic Electron Measurement at RHIC Non-photonic electron energy loss The high pT region non-photonic electron RAA is surprising: Heavy quark RAA has similar magnitude as light quark RAA! Describing the suppression is difficult for theoretical models. Where is the bottom contribution? Curve-I: M. Djordjevic et.al. PLB632 (2001) 199 Curve-II,V: N. Armesto et.al. PLB637 (2006) 362 Curve-III: S. Wicks et.al. nucl-th/0512076 Curve-VI: H Van Hees et.al. PRC73(2006)034913 Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  4. QM2006 Non-Photonic Electron Measurement at RHIC Non-photonic electron elliptic flow Reduction of v2 at pT > 2 GeV/c. Bottom contribution?? The decay kinematics of D and B mesons are different! The same D and B v2 can lead to very different non-photonic electron v2 ! Y. Zhang, Nucl.Phys.A783:489-492,2007 Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  5. B Versus D Contributions to Electrons • Quantitative understanding of features in heavy quark measurements requiresexperimental measurement of B and D contributions to non-photonic electrons! • Such information should be best obtained from direct measurement of hadronic decays of charm and bottom mesons. This motivates the STAR vertex detector upgrade! • We have proposed an experimental method which uses the azimuthal angular correlations between non-photonic electrons and charged hadrons to measure the relative contributions to non-photonic electrons from D and B meson decays. • Our method is based on the different decay kinematics of D and B mesons. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  6. PYTHIA Simulation: e pT VS. parent pT Charm quark needs to have larger momentum than bottom quark to boost the decayed electron to high pT. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  7. B D PYTHIA Simulation of e-h Correlations Associated pT > 0.3 GeV/c. Significant difference in the near-side correlations. Width of near-side correlations largely due to decay kinematics. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  8. Major STAR Detectors Used • EMC’s Partner Detector: Shower Maximum Detector (SMD) • 5 radiation length depth from the inner surface of the EMC • Providing high spatial resolution • Measuring the position and size of the shower • Time Projection Chamber (TPC) • Coverage: 0 < Φ < 2π, ~ -1.25 < η < 1.25 • Uniform electrical and magnetic field along the beam direction • Tracking mid-rapidity charged particles and particle identification • Electro-Magnetic Calorimeter (EMC) • Coverage: 0 < Φ < 2π, -1.0 < η < 1.0 • 120 calorimeter modules, 40 towers for each module • ¾ of the total barrel was instrumented during RUN V • Providing energy information for electrons/positions Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  9. dominant negligible Data Set, Electron Signal and Background • Data Set: • --- p+p collisions at sNN = 200 GeV in year 5 run. • --- 2.37 million EMC HT1 triggered events with threshold 2.6 GeV. • --- 1.68 million EMC HT2 triggered events with threshold 3.5 GeV. • Electron Signal: • Non-photonic electrons: electrons from semi-leptonic decays of heavy quarks (charm and beauty). • Background • --- Hadron contamination • --- Photonic electron background • Photon conversion • Dalitz decays of π0, η • Kaon decays • ρ, ω, Φ decays • Other possible contributions Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  10. Electron ID Using TPC, EMC and SMD -3σ < z distance < 3σ -3σ <Φdistance < 3σ 3.38 < dE/dx < 4.45 keV/cm 0.3 < p/E < 1.5 # of BSMD hits > 1 Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  11. Purity of Inclusive Electron Sample The purity is above 98% up to pT ~ 6.5 GeV/c. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  12. Photonic Background Removal Tagged e+(e-) γ Partner e-(e+) A pair of photonic electrons are correlated. Their invariant mass should be small. Use invariant mass calculation to reconstruct the photonic background. --- For each tagged e+(e-), we select partner e-(e+) identified only with the TPC and calculate the invariant mass of the pair. (Opposite-sign) --- Combinatorial background: non-photonic electrons may be falsely identified as photonic electrons; reconstructed by Same-sign technique. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  13. Photonic Background Removal m<100 MeV/c2 STAR Preliminary STAR Preliminary The combinatorial background is small in p+p collisions. Reconstructed photonic = OppSign – SameSign. Photonic electron = (reconstructed-photonic)/ε. ε is the background reconstruction efficiency, ~70% from simulation. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  14. Procedure to Extract the Signal of e-h Correlations Semi-inclusive electron • Signal: • non-photonic = (semi-inclusive) + SameSign – (not-reco-photonic) • Advantage: Smaller overall uncertainties. • Each item has its own corresponding Δφhistogram. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  15. Technique to Deal with Not-Reco-Photonic Part • In non-photonic electron yield or v2 analyses, Tagged e Tagged e Partner e missing Partner e found h h h h • However, efficiency correction alone is not enough in e-h correlation analysis. h h h h Not- Reco-Photonic Part Reco-Photonic Part Final equation to extract the signal of e-h correlations: Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  16. e-h Angular Correlations after Bkgd. Subtraction Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  17. B D Fit function: R is B contribution, i.e. B/(B+D), as a parameter in fit function. Use PYTHIA Curves to Fit Data Points Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  18. B/(B+D) Consistent Varying Fit Range Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  19. Chi-square Sensitive to B/(B+D) Ratio Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  20. Results: B Contribution VS. pT Error bars are statistical only! Data uncertainty includes statistic errors and systematic uncertainties from: ----photonic background reconstruction efficiency (dominant). ---- difference introduced by different fit functions. A finite B contribution in the pT region of 2.5-6.5 GeV/c has been observed. The FONLL theoretical calculations are consistent with the measured data. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  21. Discussion: Bottom Suppression M. Djordjevic, Phys. Lett. B632:81-86 (2006) • Radiative energy loss theory: • Bottom significantly less quenched than charm quark and light quarks. • The measured B/(B+D) ratio together with the large suppression of non-photonic electrons and a tendency for the non-photonic v2 to decrease at high pT implies that bottom quark may be suppressed in central Au+Au collisions at RHIC in contrast to the theory prediction! Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  22. Summary The method to estimate D and B contributions is developed in PYTHIA and implemented in real data. We have measured e-h correlations in 200 GeV p+p collisions. The first measured B/(B+D) ratios at RHIC indicate at pT ~ 4-6 GeV/c the measured B contribution to non-photonic electrons is comparable to D contribution based on PYTHIA model. The result of measured B/(B+D) ratios is consistent with the FONLL prediction. The measured B/(B+D) ratios imply that the bottom quarks may suffer considerable amount of energy loss in the dense QCD medium. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  23. Extra Slides Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  24. PYTHIA Simulation: e pT VS. hadron pT The efficiency of associated pT cut is different between D decay and B decay. Therefore, it is better to use lower pT cut on the associated particles in order to avoid analysis bias! Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  25. PYTHIA Simulation: e pT VS. hadron pT Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  26. PYTHIA parameters used in this analysis • PYTHIA version: v6.22 • δfragmentation function used for both charm and bottom. • Parameters for charm: • PARP(67) = 4 (factor multiplied to Q2) • <kt> = 1.5 GeV/c • mc = 1.25 GeV/c2 • Kfactor = 3.5 • MSTP(33) =1 (inclusion of K factor) • MSTP(32) = 4 (Q2 scale) • CTEQ5L PDF • Parameters for bottom are the same as for charm except • mb = 4.8 GeV/c2. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  27. Near-side width due to decay kinematics With δ fragmentation function Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  28. Near-side width does not strongly depend on FF Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  29. Near-side width does not strongly depend on FF Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  30. Allow an overall normalization factor in the fit function to float: • A reflects the uncertainties in the normalization which possibly arises from the counting of the number of non-photonic triggers and tracking efficiency for the associated tracks. • The fit results gives A close to unity and consistent B/(B+D) ratios. Check on Systematics I Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

  31. Add an adjustable constant to the fit function: • C freely adjusts the overall background level and it contains soft particle production. • The fit results gives a value for the constant C close to zero and consistent B/(B+D) ratios. Check on Systematics II Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007

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