1. AcerMC and ISR/FSR systematics at ATLAS Liza Mijovic, Borut Kersevan Jozef Stefan Inst. Univ. of Ljubljana ATLAS approach: Generator level studies Parameters treated Interesting examples Top Physics Workshop (Grenoble 2007)

2. 2 Impact of different models Recently a study of top mass reconstruction using tt~ was done using:  MC@NLO (Herwig+Jimmy)  AcerMC (Pythia – new showering and UE model)  Full detector simulation  The observed discrepancy caused quite a few raised eyebrows.. We cannot know offhand which answer is correct!

3. 3 Impact of different models cont’d The first thought was that NLO corrections impact the event shapes more than anyone suspected......But the difference turns out to be purely parton-shower related!  We just plugged the AcerMC events into Herwig and..  The difference becomes really small.. We cannot know offhand which answer - Herwig/Pythia showers and UE - is correct! We need the data!

4. 4 QCD-activity related systematics From the above example one can see that the predictions are by no means unique; using the standard division one needs to have a look at:  Initial state radiation  Final state radiation  Underlying event modeling  PDFs, etc... I think I don’t need to stress that the precision in top measurements at the LHC will be systematics-dominated.. At ATLAS the UE is handled by tuning the available models to the Tevatron data so it was excluded from these studies.

5. 5 Strategy First thing to estimate is the prediction range the models on the marked provide – and the experimentalists can tune on the data.  In this respect HERWIG is rather unflexible: It has excellent theoretical basics but very few parameters in the shower activity that are allowed to have at least some uncertainty..  Pythia is in this respect much more flexible so at least as a start a detailed study was made on how (and how much) we can ‘push around’ the Pythia showering activity. AcerMC + Pythia with varying parameters was thus used to check on the prediction uncertainties w.r.t. the QCD (parton-)showering activity.  Studies first done on generator/truth level only.  In each variation typical/simple analysis cuts and procedures were used for top mass reconstruction and the distributions were compared. Semileptonic decay selection criteria: pT > 40 GeV, |eta| < 2.5 for 2 b-jets and at least two light jets. pT > 20 GeV for the lepton from W, isolation requirement ~cone(lepton, any jet)>0.4 No cuts on missing ET. No jet energy rescaling. W reconstruction: truth / 2 light jets with MJJ closest to MW and MJJ < 120 GeV. The JJB1,2 combination with highest pT chosen as the top candidate.

6. 6 Pythia ISR parameters ATLAS uses the Pythia new pT-ordered showering and UE model! A lot of switches:  MSTP(62): level of coherence  MSTP(70): regularization scheme, pT ->0 PARP(62), PARP(81), PARP(82)  MSTP(72):max. pT for FSR of ISR partons  PARP(61) : Λ(QCD)  PARP(64) : evolution scale factor  ISR master switch, ME corrections...

7. 7 Rregularization scheme: PARP(81) t-tbar rel. pT # of truth-jets Default : PARP(81)=1.9 GeV=D PARP(81)=D/2 PARP(81)=2*D

8. 8 Rregularization scheme: PARP(81) b jet / b quark E ratio light jet / light q. E ratio Default : PARP(81)=1.9 GeV=D PARP(81)=D/2 PARP(81)=2*D

9. 9 Rregularization scheme: PARP(81) top mass: truth W + bjet top mass: W(jets)+bjet Default : PARP(81)=1.9 GeV=D PARP(81)=D/2 PARP(81)=2*D

10. 10 PARP(61): Λ(QCD) t-tbar rel. pt truth-jet number Default : PARP(61)=0.192 GeV PARP(61)=0.1 GeV PARP(61)=0.4 GeV

11. 11 PARP(61): Λ(QCD) high-pt truth jet n. top mass: W(jets)+bjet Default : PARP(61)=0.192 GeV PARP(61)=0.1 GeV PARP(61)=0.4 GeV

12. 12 PARP(64): evolution scale factor t-tbar rel. pt top mass: W(jets)+bjet Default : PARP(64)=1=D PARP(64)=D/2 PARP(64)=2*D

13. 13 Pythia FSR parameters Tunable & relevant parameters for the new showering: PARJ(81): Λ(QCD) (for external processes) (D=0.25 GeV) PARP(71): scale of the hard scattering (D=4) PARJ(82): mass cut-off below which partons don’t radiate (D=1 GeV)

14. 14 PARJ(81): Λ(QCD) Top mass: final W + b jet AcerMC + Pythia: Parj(81) = 0.25 GeV Parj(81) = 0.14 GeV MCatNLO+Herwig Reconstruction: True W from + truth b-jet. M T (AcerMC) < M T (AcerMC-FSR) < M T (MC@NLO) The FSR change in Pythia goes in the right direction!

15. 15 PARP(71) (scale of the hard scattering) b quark pt, number of high-pt jets, pt of the jets, pt of b-jets

16. 16 PARP(71) top mass (final W + bjet) PARP(71) has no/little effect

17. 17 Pythia ISR and FSR parameters PARAMETER TOP MASS ISR low high MSTP(70): reg. scheme 0 : PARP(62)  1 : PARP(81)  2 : PARP(82)  PARP(61) : Λ(QCD)  PARP(64) : evol. Factor  FSR  PARJ(81) : Λ(QCD)   These parameters were then combined into two samples which lower/increase the reconstructed mass to obtain endpoints..

18. 18 Systematics sample proposal AcerMC + Pythia, new showering Minimum top mass: 2*PARJ(81) (Λ(QCD), FSR) 0.5* PARP(61) (Λ(QCD), ISR) 2*PARP(62) (pt -> 0, kt cut-off, ISR) Maximum top mass: 0.5*PARJ(81) (Λ(QCD), FSR) 2* PARP(61) (Λ(QCD), ISR) 0.5*PARP(62) (pt -> 0, kt cut-off, ISR)

19. 19 B-fragmentation studies

20. 20 B-fragmentation studies II

21. 21 B-fragmentation studies III

22. 22 B-fragmentation studies IV The overall effect of varying b-fragm. Parameters gives ~stable results. However: Need to compare with other models like Herwig or EvtGen

23. 23 Further studies/plans I don’t have time to go through all of the things we had done but to summarize other results:  The impact of QCD uncertainties on the ttbar cross-section measurements (efficiency) seems to be small (percent order) so not crucial at the initial measurement stages. Further investigation ongoing...  Studies on the impact of PDF uncertainties is planned.  Impact of other models like EvtGen on b-tagging and top reconstruction needs to be studied.  We need to develop methods on extracting the QCD model parameters from the data and/or validate different showering models.  A lot of work...