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Forward-backward a symmetry in top- antitop production in proton-antiproton collisions

Forward-backward a symmetry in top- antitop production in proton-antiproton collisions. Regina Demina , University of Rochester 02/08/ 2012. Asymmetry in top- antitop production.

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Forward-backward a symmetry in top- antitop production in proton-antiproton collisions

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  1. Forward-backward asymmetry in top-antitop production in proton-antiproton collisions Regina Demina, University of Rochester 02/08/2012

  2. Asymmetry in top-antitop production • In early 80s asymmetry observed in e+e-m+m- at sqrt(s)=34.6 GeV<< MZ was used to verify the validity of EW theory (Phys. Rev. Lett. 48, 1701–1704 (1982) • Similarly, asymmetry in production could give information about new physics • Mediator with axial coupling in s-channel • Abnormally enhanced t-channel production • Complications: • Top is not observed directly, but reconstructed through its decay products • Proton and antiprotonare not point-like objects, lab frame is different from rest frame Reconstruction level Generator level p _ p m+ e- e+ m- Regina Demina, University of Rochester

  3. Definitions m+ t • Asymmetry defined for eemm • In proton-antiproton collisionsqy • Dy is invariant to boosts along z-axis • Asymmetry based on Dy is the same in lab and tt rest frame • Asymmetry based on rapidity of lepton from top decay • Lepton angles are measured with a good precision q e- e+ _ q q _ t m- Regina Demina, University of Rochester

  4. History of measurements and predictions • D0, reconstruction level • PRL 100, 142002(2008) • ICHEP2010 • CDF, generator level • PRL 101, 202001(2008) • Phys. Rev. D 83,112003 (2011) Regina Demina, University of Rochester

  5. Reconstruction of top-antitop signal Leptonic top Hadronic top 1581 events pass the selection requirements in 5.4 fb-1 Regina Demina, University of Rochester

  6. Asymmetry at reconstruction level • Using kinematic variables of l+jetsevents construct a discriminant and fit events with Dy>0 and Dy<0 for top fraction Leading b-jet pTc2of kinematic fit Discriminant, Dy<0 kTminMjj Discriminant, Dy>0 Regina Demina, University of Rochester

  7. Asymmetry dependence on Mtt MC@NLO Regina Demina, University of Rochester

  8. Generated asymmetry Problem with Method 1: migration of events near inner bin edge (Dy0) is underestimated, while for the outer edge it is overestimated Solution:fine bins closer to Dy=0 Problem: statistical fluctuations in data make the fine bin unfolding unstable Solution: employ regularization Bonus: reduced statistical uncertainties Method 2: fine bin unfolding with regularization • “Unfolding” =correcting for acceptance (A) and detector resolution (S) • Method 1: 4 bin Likelihood unfolding : Regina Demina, University of Rochester

  9. Method 2: fine bin unfolding with regularization Migration matrix Regina Demina, University of Rochester

  10. Results for asymmetry, in % • Reconstruction level (experiments cannot be directly compared, only to Monte Carlo after reconstruction and selection) • D0 (5.4 fb-1) • MC@NLO (D0) • CDF (5.3 fb-1) • MC@NLO (CDF) • Generator level (experiments can be directly compared) • D0 • CDF • MC@NLO Regina Demina, University of Rochester

  11. Lepton-based asymmetry, in % • Since lepton direction is defined with a very good precision, lepton based asymmetry is simpler to extract • Lepton from top decay carries information about underlying asymmetry at production • Can be directly compared to theoretical predictions Reconstruction level Generated level Regina Demina, University of Rochester

  12. Unfolded AFBlepvs AFB Regina Demina, University of Rochester

  13. Interpretation of the Asymmetry m+ t • Coulomb repulsion • QED: e+m+ • QCD: quark-top e+ e- • Coulomb attraction • QED: e-m+ • QCD: antiquark-top _ q q _ t m- Regina Demina, University of Rochester

  14. Predicted asymmetry in SM + Born( as2 ) and box(as4 ) • Coulomb-like repulsion of top and quark and attraction of antitop and quark in QCD • Interference – as3 • Positive asymmetry • Final state with no extra partonssmall transverse momentum of the tt system ISR (as3 ) and FSR(as3) • Interference – as3 • Negative asymmetry • Final state with extra gluons large transverse momentum of the tt system • Possible extra jets + Regina Demina, University of Rochester

  15. Modeling of gluon radiation • pTttspectrum suggests that gluon radiation might be mismodeled by MC@NLO+HERWIG • lower radiation is preferred • best agreement with PYTHIA ISR off • This suggests a higher contribution from 22 processes, e.g. Born+box Regina Demina, University of Rochester

  16. Asymmetry and gluon radiation • MC@NLO+HERWIG suggests strong dependence of asymmetry on pTtt • Some PYTHIA tunes suggest even more dramatic dependence while other do not – the main parameter that affects this behavior is angular coherence of ISR • Asymmetry dependence on pTttis a source of systematic uncertainty on the measured value of asymmetry • Higher weight of 22 processes (Born+box) would shift the predicted asymmetry toward more positive and higher values: yet it is hard to make 20% from 5% Regina Demina, University of Rochester

  17. New physics scenarios Regina Demina, University of Rochester

  18. S-channel: color-octet vectors (axigluons) Regina Demina, University of Rochester

  19. Experimental constraints on axigluons • Indirect • D-mixing MG>200GeV • EW precision (Zbb, GZ, shad) MG>500GeV • Direct – dijet resonances LHC ppG2 jets Atlas MG>2TeV (G/M<15%) From angular distribution MG>1.7TeV Regina Demina, University of Rochester

  20. Predicted asymmetries: axigluons Regina Demina, University of Rochester

  21. t-channel: Z’, W’ • Introduce SU(2)X that places (ut)Rin the same doublet • W’ carries “top number” thus suppressing like-sign top production at LHC • Predicted asymmetry due to W’ ~30% • More forward than SM or s-channel production • As a result observed asymmetry reduced to 20% • Least constrained by other experimental data, asymmetries in agreement with observed • Test this hypothesis by using top polarization Direct constraint : from like-sign tops at LHC Regina Demina, University of Rochester

  22. Top quark asymmetry at LHC • 2 problems compared to Tevatron: • Large fraction of top pairs (~90%) are produced in gluon fusion • Direction of quark (vsantiquark) is determined from the boost with ~70% accuracy Regina Demina, University of Rochester

  23. Conclusions • Using 5.4 fb-1 of data D0 measured asymmetry in top-antitop production • Asymmetry in leptons from top decay is • Presently D0 has recorded 10.56 fb-1 • Combination with CDF • Getting more clever in increasing the stat • Soon LHC will have enough qqbarttbar events to probe the asymmetries as well It’s a lovely mystery! Regina Demina, University of Rochester

  24. Systematics on A Regina Demina, University of Rochester

  25. Systematics on Al Regina Demina, University of Rochester

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