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. 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 86.5%13.5% HERWIG: 458 pb MCatNLO:842 pb PYTHIA:490 pb (nb: no consistent pdf’s!)

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

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

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

13. Reconstructed t-mass 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 Top-mass for top with Pt>250:

15. 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)

16. Commissioning top Extra ‘hard’ gluon can effect reconstruction of 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 between Herwig and MCatNLO

18. Top polarization 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

19. 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.