1. 1 Studies into Performance and Systematics of ME-PS matching in W+Jets Andrea Messina, CERN Ben Cooper, QMUL Berkeley VB+Jets Workshop 26th March 2008

2. 2 Overview Used Alpgenv2 to study performance and possible systematics of ME-PS matching in W+Jets: –Dependence on PS/UE –Dependence on P T min generation cut –Dependence on CKKW K T factor –Hadron-Parton level relationship All studies done at particle-level and at 1.96 TeV. Plan to extend this study to other approaches and LHC energies - this is a snapshot! Compare predictions to W+Jets measurement (see Andrea’s talk) but also to each other.

3. 3 Details of Study (i) Cross-section definition for predictions identical to our measurement: –E T ele >20 GeV, |  ele | 30 GeV, M T W > 20GeV/c 2 –Particle-level predictions clustered using JETCLU cone 0.4. Distributions under study: –  (W +  n jet) inclusive cross-section jet E T > 25 GeV. –Differential d  (W +  n jet)/dE T nth jet spectra for jet E T > 20 GeV –Differential d  (W +  2 jet)/d  R jj jet spectra for jet E T > 15 GeV

4. 4 Details of Study (ii) Constants of our generation: –Alpgen v2.13 matrix element - showering with literally identical generation. –PDF: CTEQ5L –Renorm/Factorisation scale Q = Default generation parameters: –Parton P T min 15 GeV –  R jj > 0.4, |  jet | < 2.5 –CKKW K T factor = 1.0 “Matching” parameters are always: –Jet E T > (P T min + 5) GeV –R cone = 0.4, |  jet | < 2.5 –  R(parton-jet) < 0.6

5. 5 Details of Study (iii) Variables: –Parton shower: Pythia(v3.325) or Herwig(v6.510) –For Pythia MSTP(81) = 1 or MSTP(81) = 0 –Parton P T min generation cut –CKKW K T factor –Fully matched or W + np inclusive generation? Keys to different PS/UE generation: –“py” = Pythia 3.325 (calling PYEVNT, MSTP(81) = 1) –“hw” = Herwig 6.510 (default tuning - no Jimmy) –“pynoUE” = Pythia with MSTP(81) = 0 UNLESS OTHERWISE STATED IT IS ALWAYS THE ALPGEN MATCHED PREDICTION.

6. 6 Dependence on parton shower/underlying event

7. 7 Herwig & Pythia vs Data PYTHIAHERWIG

8. 8 Herwig & Pythia vs Data PYTHIAHERWIG Substantial difference in predictions at low E T PY/HW

9. 9 Turning UE “off” in Pythia PYTHIA UE Off HERWIG Impact of UE larger for higher njet. PYnoUE/PY

10. 10 Turning UE “off” in Pythia PYTHIA UE OffHERWIG No significant shape change. PYnoUE/PY

11. 11 W+1p Inclusive Generation PYTHIA W+1p inclusive HERWIG W+1p inclusive Pythia/Herwig differences much smaller than for the matched. PY1p/HW1p

12. 12 Herwig & Pythia vs Data:  R jj HERWIG PYTHIA PYTHIA W+2p inclusive HERWIG W+2p inclusive

13. 13 Dependence on parton P T min cut

14. 14 Lead d  /dE T : Pythia & Herwig HERWIGPYTHIA

15. 15 Herwig Stack Plots: Lead Jet E T P T min 15 P T min 25 P T min 50P T min 8

16. 16 Pythia Stack Plots: Lead Jet E T P T min 15 P T min 25 P T min 50P T min 8

17. 17 Lead Jet E T : Scan of P T min PYTHIA Matched PYTHIA W+1p inclusive

18. 18 Lead Jet E T : Scan of P T min HERWIG Matched HERWIG W+1p inclusive

19. 19 2nd d  /dE T : Pythia & Herwig HERWIGPYTHIA

20. 20 Herwig Stack Plots: 2nd Jet E T P T min 25 P T min 50P T min 8 P T min 15

21. 21 Dependence on CKKW vertex K T factor

22. 22 Comparing different K T factors PYTHIAHERWIG See change in njet rates - greater for larger njets

23. 23 Comparing different K T factors PYTHIAHERWIG

24. 24 Comparing different K T factors PYTHIAHERWIG

25. 25 Hadron-level to Parton-level relationship

26. 26 Herwig Inclusive W+1p Matched

27. 27 Pythia UE turned “off” Inclusive W+1p Matched

28. 28 Work-In-Progress Things we have tried but don’t fully understand yet.

29. 29 MC@NLOMC@NLO Herwig

30. 30 Herwig + Jimmy

31. 31 Future plans What is the source of the difference between matched Pythia and Herwig? To what extent are matched predictions independent of generation cuts? Investigate different UE tunes - Pythia v2 Tunes, v3 tunes, Jimmy. Examine further  R jj distributions. Look at different approaches: MadEvent, MC@NLO, Sherpa.MC@NLO Repeat studies at LHC energies.

32. 32 Backups

33. 33 SMPR vs Alpgen + Pythia

34. 34 SMPR vs Alpgen + Pythia

35. 35 Turning UE “off” in Pythia

36. 36 W +  n jet Pythia & Herwig PYTHIA HERWIG

37. 37 Herwig Stack Plots P T min 15 P T min 25 P T min 50

38. 38 Ptmin 25 vs data PYTHIA HERWIG

39. 39 E T > 20 Inclusive

40. 40 Pythia Inclusive W+1p Matched