Study of MC@NLO production at NLO: Implications for top reconstruction

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

2. MC@NLO generator • Since few 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 • Contains kinematic of hard NLO process • Interface to Herwig via McAtNLO_i

4. Real gluon emission in NLO Real gluon emission in parton shower Problem of ‘double counting’! Weights and double counting • Born term (LO) • Virtual NLO corrections • Real NLO corrections Virtual and real NLO corrections both infinite → their sum is finite. MC@NLO • Matching NLO with parton shower • Concequence: events weights can become negative Clever subtraction terms, slightly different for real and virtual NLO correctionscompensate for additional parton shower gluon emission: avoid double counting

5. 13.5% 86.5% Weights ‘standard’ tt production process -1706 • Weights: ±w • ‘unweighted’ events, up to a sign!(practically weight ±1) • Efficient event generation possible • NLO distributions (without MC showering) are non-physical tt production cross section MCatNLO: 842 pb HERWIG: 458 pb PYTHIA: 490 pb (nb: no consistent pdf’s!)

6. Matched with MC: distribution is positive for all values of Pt NLO results by itself give ‘nonsense’ results Pt(tt-system) from NLO small Pt: negativelarge Pt: positive NLO and MC matching Pt(tt-system) non-zero due to real gluon emission (both via NLO and PS)

7. Comparison to LO generators • Pt(tt system) • Herwig & MCatNLO agree at low Pt, • At large Pt MCatNLO ‘harder’ • PYTHIA completely off Same distribution on linear scale All distributions normalised to 1

8. Comparison to LO generators • Azimuthal opening angle top-topb • Rapidity of tops(2 entries/event)

9. Pt distributions topidentical for all MC Before ISR/FSR After ISR/FSR Pt – distribution individual top

10. 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 ‘atlfastB jets’ • Mimic Top reconstruction • Mimic Top reconstruction w/o b-tagging

11. Pt-distribtion of jet with highest Pt Very hard gluon jets in MC@NLO Second ‘hardest’ Pt jet:effect of hard emission gone Reconstructed jets

12. 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

13. 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

14. Top mass • All generators give rather equivalent results • No Pt differences in reconstructed top • No big mass differences • Quantitative differences have not been determined yet Pt distribution of reconstructed top(for 2 b-jet)

15. ‘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 between Herwig and MCatNLO

16. Reconstructed W-mass Top-pt: ‘Commissioning’ top Top-pt

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

18. Conclusions • MCatNLO beautiful ‘next generation’ generator • Practicalities: • Negative event weights: 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. • Spectrum for pt of lepton has problems: • Maybe problem with Ntuples, investigating… • In previous plots spikes are removed.

19. AlpGen generator • tt production – with extra n-jets (‘light jets’) • Exact (LO) matrix element • Matching n-jets for various n left to user (?) • Only started very recently… • Alpgen generates file with hard scattering • To be fed into Herwig/Pythia shower MC’s • Generation of (unweighted) events time consuming • Problems: (Herwig_i-00-01-18) • Top decays done in Alpgen (incl polarization) – • Event record not recognized in Herwig interface • Top decays as well done in Herwig • For now force alpgen to leave top stable

20. tt-system alpgen affected by extra gluon Extra jet: Pt-min = 10 GeV |η| < 2.5 R>0.4 AlpGen tt+1-jet production Need a bit more study….

21. Outlook • Proceed with backgrounds to top • MC@NLO for background studies?? • Use AlpGen for background, W+ n-jet • Estimation of QCD background • NIKHEF has broader interest in top: • Single top (theory and measurements) • Top cross section at ‘commissioning stage’ • Top and extra dimensions • Top analyses at D0