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MC validation: comparison ALPGEN vs. MC@NLO

MC validation: comparison ALPGEN vs. MC@NLO. Activity in ATLAS. Osamu Jinnouchi (KEK) S. Asai (Tokyo) Special thanks to Michelangelo Mangano (CERN). Oct. 20, 2005 LCG MC Validation meeting. preface.

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MC validation: comparison ALPGEN vs. MC@NLO

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  1. MC validation: comparison ALPGEN vs. MC@NLO Activity in ATLAS Osamu Jinnouchi (KEK) S. Asai (Tokyo) Special thanks to Michelangelo Mangano (CERN) Oct. 20, 2005 LCG MC Validation meeting

  2. preface • This study is not based on the atlas detector simulation, but based on the MC truth information so this is not ATLAS specific (I believe this fits better to the purpose of this meeting) • This work is from one of the recent activities in SUSY WG in ATLAS about experience in ALPGEN and MC@NLO (this talk is not the review of all the activities in ATLAS) O.Jinnouchi (LCG MC validation)

  3. Outline • Introduction • ME and PS • MC samples • ALPGEN, MC@NLO in ATLAS Athena framework • Comparison using top-pair samples • Top • Jets • Available samples for the production O.Jinnouchi (LCG MC validation)

  4. ME (matrix element) and PS (parton shower) Parton shower model can describe well about the jets in collinear region However it has a problem with jets in high Pt range In the SUSY background study, we have to deal with high-pt / high multiplicity jets  ME approach is important Two major MC codes (ALPGEN, MC@NLO) are compared • ALPGEN • Leading order calculations for ME • ME used for all jets • Exclusive  the contributions (N=0, 1, 2, …) have to be summed later (see cartoon in next slide) • MC@NLO • Order as calculation for ME (1st jet is well described) • Parton Shower for additional jets • Inclusive  no user operation needed O.Jinnouchi (LCG MC validation)

  5. Alpgen approach in practice If you want inclusive 3-jets samples in ALPGEN un-weighted event files (4-vector files) 0-jet 1-jet 2-jets 3-jets exclusive exclusive exclusive inclusive Pyhia / Herwig (PS matching some fraction of events are vetoed) 2-jet exclusive 1-jet exclusive 0-jet exclusive 3-jets inclusive sample PS jets : 4+5+6+… 3-jet inclusive higher order inclusive sample is better but, it gets more time consuming to generate 4-vector By definition, MC@NLO is equivalent to 1-jet inclusive sample O.Jinnouchi (LCG MC validation)

  6. MC samples( tt + Njets SM background) Top is a major background in the SUSY search The samples are made within ATLAS Athena framework • ALPGEN • Event generation – Alpgen ver 2.05 • Hadronization – Pythia 6.323 (on Athena Rel.10.5.0) - upveto correctly works with this combination - user defined matching condition • MC@NLO • Event generation – MC@NLO ver 3.1 • Hadronization – Jimmy4.0 (Herwig 6.5) (on Athena 10.0.4) about 100k events for both are used in this study Q2ren=Q2fac= m2(top)+<PT(top)2> (default definition in MC@NLO) O.Jinnouchi (LCG MC validation)

  7. parton-jet matching in Alpgen • Event generation (ALPGEN) performed with light jets Pt>20GeV • Additional PS jets (Pythia) must have lower Pt than ME jets (reference http://mlm.home.cern.ch/mlm/alpgen/ ) Some fraction of events vetoed in Pythia (matching at Pt>20GeV Rcone=0.7 ) Total Cross section is different from MC@NLO output log (773.4pb) by factor 1.2~1.3 O.Jinnouchi (LCG MC validation)

  8. Comparison on MC Truth top • No kinetic cut applied on top production at event generation  good clue to check the matching performance (reason is in next slide) dots ALPGEN (N=0,1) + Pythia_i line MC@NLO + Jimmy_i (GeV) |y|<1 • ALPGEN has high Pt tail which needs to be understood • Discrepancy in low Pt region is observed, it may be come from the difference btw Pythia/Herwig parton shower scale ( additional info in next slide) 10GeV 100GeV Log10 Pt (GeV) Scale factor 1.28 for ALPGEN O.Jinnouchi (LCG MC validation)

  9. Comparison on top (Cont.) Thanks to Michelangelo Mangano • The same plot created by M. Mangano • He uses • MC@NLO + Herwig • ALPGEN + Herwig • So the parton shower contributions are exactly the same (better agreement in low Pt region) • High Pt tail is still observed |y|<1 Scale factor 1.28 We should try ALPGEN + Jimmy_i (Herwig_i) combination Careful study is necessary Technical problem in Herwig_i is fixed in Herwig_i-00-01-80 Now we can try direct comparison of Pythia_i vs. Herwig_i O.Jinnouchi (LCG MC validation)

  10. Comparison between leading jets Compare the TruthJets (cone0.7) for three data sets - Jets not originated from top are selected - Histograms are normalized (by total cross sections for each) - MC@NLO and ALPGEN (N=0+1) are supposed to be equivalent - Concentrate on cone7 truth jet, lnu+qq • Pt distributions (of three) look almost identical • ALPGEN has more central in h h Pt (GeV) O.Jinnouchi (LCG MC validation)

  11. The second leading jets • The second leading jet is PS for MC@NLO, and ALPGEN (N=0,1) • For ALPGEN (N=0-3) it is still generated from ME • ALPGEN (N=0-3) has harder in Pt > 200GeV • h distributions are getting wider h Pt (GeV) O.Jinnouchi (LCG MC validation)

  12. The third leading jet • Cross section for the 3rd jet is somewhat lower in MC@NLO • Difference in h distribution needs to be understood • Could be difference in Pythia/Herwig • MC@NLO and ALPGEN (N=0,1) have similar Pt distributions (slope) • ALPGEN (N=0-3) has harder jets h Pt (GeV) • - Behavior of 1-hard jet configuration compared (MC@NLO vs. ALPGEN(N=0,1)) • The difference among ALPGEN seen in higher order jets (N=0,1 vs. N=0-3) • Need much higher statistics for better understanding and quantitative estimation O.Jinnouchi (LCG MC validation)

  13. Available samples (1) in Univ. of Tokyo Towards the MC data validation / production, tons of events are being prepared O.Jinnouchi (LCG MC validation)

  14. Available samples (2) in Univ. of Wisconsin Thanks to S. Padhi (Univ. of Wisconsin-Madison) We need huge amount of samples to estimate SM background for SUSY ! O.Jinnouchi (LCG MC validation)

  15. Summary • ALPGEN, MC@NLO are compared in tt+Njets samples • ALPGEN (N=0,1) resembles the Pt distribution of MC@NLO • ALPGEN (N=0-3) has harder jets in higher order (2nd, 3rd Pt leading) jets • It would be better to use ALPGEN for SUSY study?? • Pythia_i and Herwig_i needs to be compared carefully • Studies on Q scale, Matching condition dependences are important • Huge amount of samples (ALPGEN and MC@NLO) are being prepared in ATLAS (waiting for validation and production) O.Jinnouchi (LCG MC validation)

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