1. Liu Minghui Nanjing 2008-4-27 1 MC study of W polarization and ttbar spin correlation Liu Minghui, Zhu Chengguang April 27, 2008

2. Liu Minghui Nanjing 2008-4-27 2 Outline 1. Physics motivation 2. Study of W polarization and ttbar spin correlation 3. Summary

3. Liu Minghui Nanjing 2008-4-27 3  top production: At LHC, ttbar produced mainly from g-g fusion,, means 8M ttbar events produced each year at low luminosity (10 33 cm -2 s -1 ) Physics Motivation (1) Study of ttbar spin correlation The spin of t and tbar are correlated

4. Liu Minghui Nanjing 2008-4-27 4 Physics Motivation (2)  top decay: SM predicted: Almost all of the top quarks decay to W boson and b quark  BR(t  Wb)~100%  No top hadronization Study of W polarization

5. Liu Minghui Nanjing 2008-4-27 5 Physics Motivation (3) W polarization ttbar spin correlation Precisely predicted by the Standard Model  Validate the SM  Test of V-A structure  Search for anomalous coupling  Search for new physics

6. Liu Minghui Nanjing 2008-4-27 6 tt spin correlation parameters  t and t not polarised in tt pairs, but correlations between spins of t and t A D =-0.29 M tt <550 GeV A D =-0.24 A=0.42 A=0.33 Study of W polarization and ttbar spin correlation(1) SM prediction  tt spin correlation Add restriction

7. Liu Minghui Nanjing 2008-4-27 7 tt spin correlation parameters with final objects Angle between spin analysers Angle between t (t ) and spin analysers Study of W polarization and ttbar spin correlation(2)  tt spin correlation α 1 and α 2 are spin analyzing power of the spin analyzers

8. Liu Minghui Nanjing 2008-4-27 8 angle between lepton direction in W rest frame and W direction in top rest frame  spin=1/211/2 Study of W polarization and ttbar spin correlation(3)  W polarization There are 3 kinds of W polarization: left handed, right handed and longitudinal. There components correspond to F L, F R, F 0 SM F 0 =0.695 F L =0.304 F R =0.001

9. Liu Minghui Nanjing 2008-4-27 9 Study of W polarization and ttbar spin correlation(4)  Used channels in our study  Full hadronic : jets  Dileptonic : 2 l + 2 b + 2   Semileptonic : l + + 2b + jets Number of events (10 fb -1 ) 0.4M 2.5M tt final states We studied the W polarization and ttbar spin correlation in both ttbar semileptonic channel and dileptonic channel

10. Liu Minghui Nanjing 2008-4-27 10 Study of W polarization and ttbar spin correlation(5)  Event selection channelcut1cut2cut3 semileptonic 1 isolated lepton pt>20GeV | η |<2.5 2 b-jets, ≥2 light jets E T miss>20GeV dileptonic 2 isolated lepton pt>20GeV | η |<2.5 exactly 2 b-jets E T miss>40GeV After selection: For W polarization, angle  was reconstructed For tt spin correlation,  1,  2 and  were reconstructed

11. Liu Minghui Nanjing 2008-4-27 11 Reconstruction and cuts efficiency function : Fit with pol3 in range [-0.9;0.8] parton level cos  1/N dN/dcos  After events selection and reconstruction, the cosΨ distribution is changed, we use an efficiency function to correct this difference. Study of W polarization and ttbar spin correlation(6)  Our correction to selection and reconstruction

12. Liu Minghui Nanjing 2008-4-27 12 Study of W polarization and ttbar spin correlation(7)  Systematics in our studies—semileptonic channel

13. Liu Minghui Nanjing 2008-4-27 13 Study of W polarization and ttbar spin correlation(8)  Systematics in our studies—dileptonic channel Source of uncertaintyFLFL F0F0 FRFR AADAD Q-scale Structure function ISR FSR b fragmentation Hadronization scheme 0.011 0.002 0.000 0.016 0.002 0.010 0.004 0.001 0.008 0.003 0.002 0.003 0.001 0.008 0.002 0.011 0.008 0.001 0.005 0.007 0.005 0.003 0.005 0.001 0.000 0.004 0.003 b tagging efficiency(5%) b jet energy scale(3%) backgrounds uncertainty(5%) Input top mass(5GeV) 0.063 0.039 0.007 0.011 0.040 0.051 0.002 0.026 0.022 0.023 0.002 0.014 0.231 0.161 0.006 0.121 0.115 0.006 Total0.0790.0760.0390.2820.167

14. Liu Minghui Nanjing 2008-4-27 14 Study of W polarization and ttbar spin correlation(9)  Final results Semileptonic channel W polarization: FL= 0.29±0.02(stat)±0.03(syst) F0= 0.70±0.03(stat)±0.02(syst) FR= 0.01±0.02(stat)±0.02(syst) ttbar spin correlation: A = 0.67±0.08(stat)±0.15(syst) A D = -0.40±0.05(stat)±0.08(syst) Dileptonic channel W polarization: FL= 0.297±0.026(stat)±0.079(syst) F0= 0.709±0.044(stat)±0.076(syst) FR= -0.005±0.027(stat)±0.039(syst) ttbar spin correlation: A = 0.490±0.228(stat)±0.282(syst) A D = -0.347±0.157(stat)±0.167(syst) Statistical errors corresponding to integral luminosity of 1/fb (scaled from lower statistics artificially) W polarization parameters are got with a sample of 770pb -1, while ttbar spin correlation with a sample of 220pb -1

15. Liu Minghui Nanjing 2008-4-27 15 With MC samples, W polarization and tt spin correlation are studied in both tt semi-leptonic channel and dileptonic channel. A experimental method is given to extract these parameters; Systematics are estimated in our study; Deviation of A measurement indicates possibly an unknown systematics needing further investigation; The systematics are large for the limited statistics of MC samples. Summary

16. Liu Minghui Nanjing 2008-4-27 16 Thanks ！