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A symmetries in top- antitop production

A symmetries in top- antitop production. Regina Demina , University of Rochester 04 /1 1 /2013. Outline. Introduction of asymmetry Asymmetry measurement at the Tevatron Interpretation Within Standard Model Beyond Standard Model Axigluons Experimental evidence Charge asymmetry at LHC

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A symmetries in top- antitop production

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  1. Asymmetries in top-antitop production Regina Demina, University of Rochester 04/11/2013

  2. Outline • Introduction of asymmetry • Asymmetry measurement at the Tevatron • Interpretation • Within Standard Model • Beyond Standard Model • Axigluons • Experimental evidence • Charge asymmetry at LHC • Ttbar resonances • Same sign tops • Dijet resonances Regina Demina, University of Rochester

  3. 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

  4. 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

  5. Measured Forward-Backward Asymmetry Regina Demina, University of Rochester

  6. Lepton Asymmetry from top decay Regina Demina, University of Rochester

  7. AFBlepvs AFB AFB and AFBlep provide independent information and help to distinguish between models. D0, 5.4fb-1 Axigluons M=500 GeV, G=100 GeV Left – pTdep Right– pTdep Axial – pTdep D0 SM PT (lepton) dependence provides further discrimination Regina Demina, University of Rochester

  8. More Definitions – dilepton system • Asymmetry based on the direction of both leptons (two measurements per event) • Asymmetry based on Dy between leptons e+ _ q q t _ t m- same as in single lepton case Regina Demina, University of Rochester

  9. Asymmetries in dilepton system Different quantities Regina Demina, University of Rochester

  10. 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

  11. 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

  12. AFBvs PT (ttbar) • Gluons from initial radiation are not always reconstructed as jets. • PT of the ttbar system is another way to be sensitive to gluon radiation. Need full as4 prediction for asymmetry. So far the total cross section is available to that order:CERN-PH-TH-2013-056, TTK-13-08 Michal Czakon, Paul Fiedler, Alexander Mitov. Mar 25, 2013. Regina Demina, University of Rochester

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

  14. Axigluons – a bit of history Original paper on the subject Phys. Lett. BV190, Issues 1–2, 21 May 1987, 157–161 : Chiral color: An alternative to the standard model P. H. Frampton, S. L. Glashowa SU(3)L ×SU(3)R -8 heavy gluons with chiral structure Alternative Version of Chiral Color as Alternative to the Standard Model P. H. Frampton PhysRevD.81.095005 10/2009 SU(3)_L × U(1) – simpler model, cancellation of triangular divergences. Axigluonas Possible Explanation for ttbar Forward-Backward Asymmetry P. H. Frampton, J. Shu, K. Wang Phys. Lett. B 683, 294 (2010) Axigluonscannot explain the observed top quark forward-backward asymmetry R. S. Chivukula,E. H. Simmons, C.-P. Yuan http://arxiv.org/pdf/1007.0260v3.pdf Light axigluon explanation of the Tevatronttbar asymmetry and multijet signals at the LHC C. Gross, G. M. Tavares, M. Schmaltz, C. Spethmann10/12, http://arxiv.org/pdf/1209.6375v2.pdf Open windows for a light axigluon explanation of the top forward-backward asymmetry M. Gresham, J. Shelton, K. M. Zurek 9 Feb 2013 http://arxiv.org/abs/arXiv:1212.1718 50<M(axigluon)<450 GeV Regina Demina, University of Rochester

  15. Axigluons: Dijet resonances LHC limits are not applicable for low mass (<400GeV) and large width (G/M>15%) Regina Demina, University of Rochester

  16. 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

  17. How to compare to charge asymmetry at LHC • 2 problems compared to Tevatron: • Large fraction of top pairs (~90%) are produced in gluon fusion, in addition asymmetric qg provides a sizable fraction of ttbar events • Anti quark direction is not known in case of positive asymmetry tops tend to be more forward (because of the boost in the direction of valence quark) than antitops Regina Demina, University of Rochester

  18. Charge asymmetry at LHC CMS, 5.0fb-1, l+jets AC= 0.4±1.0 (stat.)±1.1(syst.)% Atlas, l+jets(1.0 fb-1)+dileptons(4.7 fb-1) AC=5.7±2.4(stat)±1.5(syst)% Regina Demina, University of Rochester

  19. What needs to happen to have a (more) complete story? • Complete Tevatron analyses • Full asymmetry, lepton based, all as a function of PT(lepton) and/or Mttbar • Combine CDF and D0 • LHC analyses – try to isolate the qqbarttbar process • SM theory - Need full as4 prediction for the asymmetry. • BSM – reexamine low mass dijet resonances, maybe combine with asymmetry (e.g in bbar system) – hard! • BSM for full comparison between Tevatron and LHC – agree on the framework (e.g. axigluons) Regina Demina, University of Rochester

  20. Instead of conclusion: Personal remarks • Results are consistent between Tevatron experiment and correspond to ~20% asymmetry at production level • Combination of full asymmetry with lepton-based one provides a better discrimination between different models • LHC results are reaching the precision compared to the Tevatron data • In agreement with the SM, but there is a class of BSM models consistent with the LHC and Tevatron data It’s a lovely mystery, Stay tuned Regina Demina, University of Rochester

  21. Back up Regina Demina, University of Rochester

  22. Systematics - Tevatron CDF: Systematic uncertainties on the parton level AFB measurement, % Background shape 1.4 Background normalization 1.3 Hadronization 1.0 Jet energy scale 0.5 Initial- and nal-state radiation 0.5 Correction procedure0.3 Colorreconnection0.1 Parton-distribution functions 0.1 Total systematic uncertainty 2.3 D0 Regina Demina, University of Rochester

  23. Systematics - LHC Atlas CMS Systematicuncertainty JES 0.003 JER 0.002 Lepton ID/sel. efficiency 0.006 Generator 0.001 Hadronization 0.001 Q2 scale 0.002 PDF 0.002 Pileup <0.001 W + jets 0.004 Multijet 0.001 Migration matrix 0.002 Model dependence 0.007 Total 0.011 Regina Demina, University of Rochester

  24. 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

  25. 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

  26. Dijet resonances Regina Demina, University of Rochester

  27. Regina Demina, University of Rochester

  28. Regina Demina, University of Rochester

  29. Indirect • D-mixing MG>200GeV • EW precision (Zbb, GZ, shad) MG>500GeV • Direct – dijet resonances LHC ppG2 jets Regina Demina, University of Rochester

  30. Reconstruction of top-antitop signal Leptonic top Hadronic top Regina Demina, University of Rochester

  31. D0: 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

  32. Towards “true or 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

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

  34. D0: 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

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