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

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Presentation on theme: "Regina Demina, University of Rochester 02/08/2012 Forward-backward asymmetry in top-antitop production in proton-antiproton collisions."— Presentation transcript:

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

2 Asymmetry in top-antitop production Reconstruction level Generator level In early 80s asymmetry observed in e + e -      at sqrt(s)=34.6 GeV<< M Z 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 antiproton are not point-like objects, lab frame is different from rest frame   ee ee _p_p p 01/18/20122Regina Demina, University of Rochester

3 Definitions Asymmetry defined for ee   In proton-antiproton collisions   y  y is invariant to boosts along z-axis Asymmetry based on  y 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    ee ee q _q_q t _t_t 01/18/20123Regina Demina, University of Rochester

4 History of measurements and predictions D0, reconstruction level PRL 100, (2008) ICHEP2010 CDF, generator level PRL 101, (2008) Phys. Rev. D 83, (2011) 01/18/20124Regina 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 01/18/20125Regina Demina, University of Rochester

6 Asymmetry at reconstruction level Using kinematic variables of l+jets events construct a discriminant and fit events with  y>0 and  y<0 for top fraction 01/18/20126Regina Demina, University of Rochester Leading b-jet p T    of kinematic fit k T min M jj Discriminant,  y<0 Discriminant,  y>0

7 Asymmetry dependence on M tt 01/18/20127Regina Demina, University of Rochester

8 Generated asymmetry “ Unfolding” =correcting for acceptance (A) and detector resolution (S) Method 1: 4 bin Likelihood unfolding : Problem with Method 1: migration of events near inner bin edge (  y  0) is underestimated, while for the outer edge it is overestimated Solution: fine bins closer to  y=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 01/18/20128Regina Demina, University of Rochester

9 Method 2: fine bin unfolding with regularization 01/18/2012Regina Demina, University of Rochester9 Migration matrix

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 ) (D0) CDF (5.3 fb -1 ) (CDF) Generator level (experiments can be directly compared) D0 CDF 01/18/201210Regina 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 01/18/201211Regina Demina, University of Rochester

12 Unfolded A FB lep vs A FB 01/18/2012Regina Demina, University of Rochester12

13 Interpretation of the Asymmetry 01/18/2012Regina Demina, University of Rochester13 Coulomb repulsion QED: e +   QCD: quark-top   ee ee q _q_q t _t_t Coulomb attraction QED: e -   QCD: antiquark-top

14 Predicted asymmetry in SM Born(  s 2 ) and box(  s 4 ) Coulomb-like repulsion of top and quark and attraction of antitop and quark in QCD Interference –  s 3 Positive asymmetry Final state with no extra partons  small transverse momentum of the tt system ISR (  s 3 ) and FSR(  s 3 ) Interference –  s 3 Negative asymmetry Final state with extra gluons  large transverse momentum of the tt system Possible extra jets /18/201214Regina Demina, University of Rochester

15 Modeling of gluon radiation p T tt spectrum suggests that gluon radiation might be mismodeled by lower radiation is preferred best agreement with PYTHIA ISR off This suggests a higher contribution from 2  2 processes, e.g. Born+box 01/18/201215Regina Demina, University of Rochester

16 Asymmetry and gluon radiation suggests strong dependence of asymmetry on p T tt 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 p T tt is 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% 01/18/201216Regina Demina, University of Rochester

17 New physics scenarios 01/18/2012Regina Demina, University of Rochester17

18 S-channel: color-octet vectors (axigluons) 01/18/2012Regina Demina, University of Rochester18

19 Experimental constraints on axigluons 01/18/2012Regina Demina, University of Rochester19 Indirect D-mixing M G >200GeV EW precision (Zbb,  Z,  had ) M G >500GeV Direct – dijet resonances LHC pp  G  2 jets Atlas M G >2TeV (  From angular distribution M G >1.7TeV

20 Predicted asymmetries: axigluons 01/18/2012Regina Demina, University of Rochester20

21 t-channel: Z’, W’ 01/18/2012Regina Demina, University of Rochester21 Direct constraint : from like-sign tops at LHC Introduce SU(2) X that places (u t) R in 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

22 Top quark asymmetry at LHC 01/18/2012Regina Demina, University of Rochester22 2 problems compared to Tevatron: Large fraction of top pairs (~90%) are produced in gluon fusion Direction of quark (vs antiquark) is determined from the boost with ~70% accuracy

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 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 01/18/201223Regina Demina, University of Rochester It’s a lovely mystery!

24 Systematics on A 01/18/2012Regina Demina, University of Rochester24

25 Systematics on A l 01/18/2012Regina Demina, University of Rochester25


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