1. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007 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

2. 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

3. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007 3 Non-Photonic Electron Measurement at RHIC Non-photonic electron energy loss The high p T region non- photonic electron R AA is surprising: Heavy quark R AA has similar magnitude as light quark R AA ! 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

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

5. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007 5 B Versus D Contributions to Electrons Quantitative understanding of features in heavy quark measurements requires experimental 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.

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

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

8. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007 8 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 Major STAR Detectors Used 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 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

9. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007 9 Data Set: --- p+p collisions at  s NN = 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 π 0, η Kaon decays ρ, ω, Φ decays Other possible contributions Data Set, Electron Signal and Background dominant negligible

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

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

12. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007 12 Photonic Background Removal 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. γ e + (e - ) e - (e + ) Tagged Partner

13. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007 13 Photonic Background Removal 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. m<100 MeV/c 2 STAR Preliminary

14. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007 14 All Tracks Inclusive electron Non-photonic electron Photonic electron Reco-photonic electron =OppSign - SameSign Not-reco-photonic electron Pass EID cuts 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 Δφ histogram.

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

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

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

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

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

20. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007 20 Results: B Contribution VS. p T 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 p T region of 2.5-6.5 GeV/c has been observed. The FONLL theoretical calculations are consistent with the measured data.

21. 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 v 2 to decrease at high p T implies that bottom quark may be suppressed in central Au+Au collisions at RHIC in contrast to the theory prediction!

22. 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 p T ~ 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.

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

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

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

26. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007 26 PYTHIA parameters used in this analysis PYTHIA version: v6.22 δ fragmentation function used for both charm and bottom. Parameters for charm: PARP(67) = 4 (factor multiplied to Q 2 ) = 1.5 GeV/c m c = 1.25 GeV/c2 K factor = 3.5 MSTP(33) =1 (inclusion of K factor) MSTP(32) = 4 (Q 2 scale) CTEQ5L PDF Parameters for bottom are the same as for charm except m b = 4.8 GeV/c 2.

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

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

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

30. Xiaoyan Lin SQM 2007, Levoca, Slovakia, June 26, 2007 30 Check on Systematics I 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.

31. 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