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MC study of W polarization and ttbar spin correlation

MC study of W polarization and ttbar spin correlation. Liu Minghui, Zhu Chengguang April 27, 2008. Outline. 1. Physics motivation 2. Study of W polarization and ttbar spin correlation 3. Summary. Physics Motivation (1). top production:

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MC study of W polarization and ttbar spin correlation

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

  2. Outline • 1. Physics motivation • 2. Study of W polarization and ttbar spin correlation • 3. Summary

  3. Physics Motivation (1) • top production: At LHC, ttbar produced mainly from g-g fusion, , means 8M ttbar events produced each year at low luminosity (1033cm-2s-1) The spin of t and tbar are correlated Study of ttbar spin correlation

  4. top decay: SM predicted: Almost all of the top quarks decay to W boson and b quark Physics Motivation (2) • BR(tWb)~100% • No top hadronization Study of W polarization

  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. Study of W polarization and ttbar spin correlation(1) • tt spin correlation tt spin correlation parameters • t and t not polarised in tt pairs, but correlations between spins of t and t A=0.33 SM prediction AD=-0.24 AD=-0.29 Add restriction Mtt<550 GeV A=0.42

  7. Study of W polarization and ttbar spin correlation(2) • tt spin correlation tt spin correlation parameters with final objects Angle between t (t ) and spin analysers Angle betweenspin analysers α1and α2 are spin analyzing power of the spin analyzers

  8. Study of W polarization and ttbar spin correlation(3)  spin=1/2 1/2 1 angle between lepton direction in W rest frame and W direction in top rest frame • W polarization There are 3 kinds of W polarization: left handed, right handed and longitudinal. There components correspond to FL, FR, F0 F0=0.695 FL=0.304 FR=0.001 SM

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

  10. Study of W polarization and ttbar spin correlation(5) • Event selection After selection: For W polarization, angle wasreconstructed For tt spin correlation, 1, 2 and  werereconstructed

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

  12. Study of W polarization and ttbar spin correlation(7) • Systematics in our studies—semileptonic channel

  13. Study of W polarization and ttbar spin correlation(8) • Systematics in our studies—dileptonic channel

  14. Study of W polarization and ttbar spin correlation(9) • Final results Statistical errors corresponding to integral luminosity of 1/fb (scaled from lower statistics artificially) Semileptonic channel ttbar spin correlation: A = 0.67±0.08(stat)±0.15(syst) AD= -0.40±0.05(stat)±0.08(syst) 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) Dileptonic channel W polarization parameters are got with a sample of 770pb-1, while ttbar spin correlation with a sample of 220pb-1 ttbar spin correlation: A = 0.490±0.228(stat)±0.282(syst) AD= -0.347±0.157(stat)±0.167(syst) 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)

  15. Summary • 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.

  16. Thanks！

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