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Study of tt production at NLO

Study of tt production at NLO. Stan Bentvelsen Edwin Bos. MC@NLO generator. Since 2 months MC@NLO available in Atlas repository: Using 6.5.0 release of Generators Using tag 00-00-00 of package McAtNLO_i What is MC@NLO? What consequences for top-production and top-reconstruction?.

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Study of tt production at NLO

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  1. Study of tt production at NLO Stan Bentvelsen Edwin Bos

  2. MC@NLO generator • Since 2 months MC@NLO available in Atlas repository: • Using 6.5.0 release of Generators • Using tag 00-00-00 of package McAtNLO_i • What is MC@NLO? • What consequences for • top-production and • top-reconstruction? Developed by S Frixione and B Webber, www.hep.phy.cam.ac.uk/theory/webber/MCatNLO/

  3. The MC@NLO generator • ‘New generation’ event generator • Matching NLO calculations of QCD process with parton shower MC simulation • Fully exclusive events generated • Hard emissions treated as in NLO • Soft emissions handled by MC shower (Herwig) • No ‘double counting’ between these two • Running in ATLAS: • Create event file using ‘runNLO’ program • Interface to Herwig via McAtNLO_i

  4. Negative weights • Born term

  5. 13.5% 86.5% Few distributions • Weights: ±w • ‘unweighted’ events, upto a sign! • Efficient event generation possible • Subtraction cleverly adjusted to obtain • NLO distributions (without MC showering) by itself give nonsence results HERWIG: 458 pb MCatNLO: 842 pb PYTHIA: 490 pb (nb: no consistent pdf’s!)

  6. NLO results by itself give ‘nonsense’ results Pt(tt-system) from NLO small Pt: negativelarge Pt: positive Matched with MC: distribution is positive for all values of Pt NLO and MC matching

  7. Azimuthal opening angle top-topb Pt(tt system) Herwig & MCatNLO agree at low Pt, At large Pt MCatNLO ‘harder’ PYTHIA completely off Comparison LO generators All distributions normalised to 1

  8. Top reconstruction • Use ATLFast package (00-01-24) • StandardNtupleMaker algorithm • Modified slightly to • Get access to event weights • Get access to partons after ISR in Herwig • Get partons from W-decays • Use calibrated jets ‘atlfastB jets’ • Mimic Top reconstruction • Mimic Top reconstruction w/o b-tagging

  9. Kinematic reconstruction t-mass Reconstruct top for 1 b-jet: if R(b-jet,W) < R(b-jet,electron) No spectacular differences! Reconstruct W by considering all jet-jet permutations, take combination with minimum mass distance to 80.3 GeV

  10. Reconstructed t-mass Top-mass for top with Pt>250: Some of the entries are still negative In case of 2 b-tagged jets: Take b-jet which lead to highest Pt of reconstructed top

  11. Top mass • All generators give rather equivalent results • No Pt differences in reconstructed top • No big mass differences Pt distribution of reconstructed top(for 2 b-jet)

  12. Commissioning top Extra ‘hard’ gluon can effect reconstructionof top-mass when no b-tag is assumed (Pessimistic commissioning scenario) Top reconstructed by selecting 3 jets which give the highest resulting Pt Also for resulting top-Pt > 250 GeV Again no significant differences betweenHerwig and MCatNLO

  13. In di-lepton channel Angle between 2 leptons Unsmeared Is this the observable for polarization? (I am not expert!) MCatNLO does not have polarization included Can be implemented according to authors Top polarization

  14. Conclusions • MCatNLO beautiful ‘next generation’ generator • Practicalities: • Negative event weigths: store in full simulation? • Read ‘pre’-generated’ NLO event file • I do not know how to ‘skip’ events in this file • Generated many NLO event files, each 50k events. • No polarization included yet.

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