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ISR studies on the Drell-Yan for top-pair production

ISR studies on the Drell-Yan for top-pair production. Un-ki Yang, Young-kee Kim University of Chicago. Top group meeting, April 22, 2004. Reference. Document: cdf note 6804 Recent talks: Status reports in top mass meeting: Mar. 17 (2004), Dec. 10 (2003).

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ISR studies on the Drell-Yan for top-pair production

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  1. ISR studies on the Drell-Yanfor top-pair production Un-ki Yang, Young-kee Kim University of Chicago Top group meeting, April 22, 2004

  2. Reference • Document: cdf note 6804 • Recent talks: • Status reports in top mass meeting: Mar. 17 (2004), Dec. 10 (2003). • Proposals: Nov. 11 (2003) in the 2nd top mass workshop, Dec. 4 (2003) in MC workshop. • Reviewers: • Joey Huston, Mario Martinez-Perez , Eva Halkiadakis • Web site • http://wwwcdf.fnal.gov/internal/physics/top/ISR/ISR.html • Plots for preblessing: boxed with red color

  3. Why we care about ISR? ISR • Initial State Radiations (ISR) gluon plays important roles in many physics measurements. • W mass: more ISRs, higher Pt of W • Top mass: RunII goal 2-3 GeV for dMtop, but dM=1.3 GeV for in RunI CDF analysis [PRD 63, 032001 (2001)] ISR is identified as W daughter or 2nd bjet. • LHC is a top factory, ttbar+njet is a major background to many EWK processes (crucial for Top-Yukawa coupling, ttH)

  4. How to control ISR in ttbar system? • In Run I: switch ISR on/off using PYTHIA • Good for understanding size of the effect, but we don’t use what we have learned, and no clear one sigma definition • In Run II: systematic approach • ISR is governed by DGALP eq. Q2, LQCD, splittingfunctions, PDFs • Use DY data (no FSR): study Pt of the dilepton, njets in the DY for different Q2region (~different DY mass region) m+ m- qq -> tt(85%)

  5. Average Pt of the dilepton for the different DY mass region Pt of the dilepton from Herwig (M=30 vs 90 GeV) Average Pt of the dilepton at generator: Ptcut=50 (2Mt)2 log(M2) • Average Pt of the dilepton has a logarithmic Q2 dependence • ( DGLAP evolution effect ) • Measure the slope, this allows us to estimate the size of ISR at M(DY)=2*Mt

  6. ISR jets in ttbar system Incoming parton E for top pair production • Many soft-ISR jets are easily larger than 8 GeV in top pair production, because of very high incoming parton energy

  7. Datasets Dielectron Dimuon 20 40 76 106 200 2Mt • Very important to have pure DY data sample (normalization is not important) • SUSY dilepton (only dimuon): 67 pb-1 (not full) • High-pt electron: 193 pb-1 for CEM18 • High-pt muon: 193pb-1 for CMUP18, 175 for CMX18 • Analysis tool: UCntuples ( used in T. Maruyama’s analysis)

  8. Analysis cuts • No trilepoton events • del Z for two leptons<4cm with opposite charge • MET<25

  9. Effect of the QED FSR Pt(dimuon) vs M(dimuon)[65-70] Willis tuned-Pt(Z) Run I tune M2(dimuon) M2(Z/g) • Many QED FSR photons (mainly collinear) • Need to remove them (we don’t care about normalization) • Prediction for collinear photons in Pythia is not well controlled (effect on kinematic acceptance, ISO variable: under study: but not subject of this talk)

  10. Photon cuts EM (Et>5) ISO ISO Central pio Central FSR chisq ISO Plug pio ISO Plug FSR Cut effect: CEM FSR 90% (313/345) CEM pio 17% (50/284)

  11. MC datasets • Top/EWK Pythia and Herwig MC samples + Rick’s underlying evts tunning Ffor Pythia

  12. Comparison of the Pt(dimuon) at Z mass [76-106] Log scale Willis tuned-Pt(Z) Run I tune

  13. Comparison of the Pt (dimuon) at different mass region

  14. Average Pt (dilepton) as function of the dilepton mass2 M2(dilepton) M2(dilepton) M<40 GeV: SUSY dilepton M>40 GeV: Hi-pt lepton

  15. Corrections for the average Pt (dilepton) • Background corrections • very small due to two tight cuts for cental lepton • Mainly DY(tt) contribution • Acceptance corrections • Bin by bin correction using Pythia MC (zewk2m,zewk1e) *: Other physics backgrounds, WW, top are small but removed by the MET<25 cut

  16. Evolution of the <Pt (dilepton)> as function of the dilepton mass2 M2(dilepton)

  17. M2 dependence on the <Pt (dilepton)> M2(dilepton) <Pt(M2)> = (-6.43+3.10) + (1.96+-0.35)*log (M2)

  18. ISR uncertainty Kt2 = PARP(64)(1-z)Q2 LQCD= PARP (61) • ISR uncertainty is only due to uncertainty in shower processing, • PDF uncertainty is not treated as a part of the ISR uncertainty. • Factorization scale uncertainty is not. m+ m- Q2max: K PARP(67) Qmin : PARP(62) Intrinsic Kt: Gaussian Width: PARP(91) *: to see size of variations (not one sigma)

  19. ISR variations Lambda QCD: 100 vs 384 Q2max K factor: 4.0 vs 0.25 Kt2 K factor: 4.0 vs 0.25 Width: 8.0 vs 0.5

  20. ISR variation due to Intrinsic Kt Width: 8.0 vs 0.5 • Though effect of the intrinsic Kt on <Pt> is big, not much freedom in Kt, because of the constraint from the data. • Gaussian Width in Z mass • Data : 3.14 +- 0.20 • Pythia MC : 2.86 +- 0.05

  21. ISR variation due to LQCD, K factor Kt2 K factor: 4.0 vs 0.25 Lambda QCD: 100 vs 384 Lambda QCD: 100 vs 384 Lambda QCD: 100 vs Kt2 K factor: 4.0 GEN GEN

  22. ISR uncertainty ISR uncertainty samples (conservative) Lambda QCD: 100 vs 384 • More ISR : LQCD = 384, K = 0.5 • Less ISR : LQCD = 100, K = 2.0 Run I: no ISR: K = infinite Issue: Q2max kfactor : PARP(67)=4 from Rick’s tune A for more ISR, but no effect on the DY. So EWK changed another K factor, PARP(64)=0.2 (almost same effect but not good) Ttbar: more ISR, less

  23. Conclusion and plans • We find that both Pythia tuned version and Herwig describe the ISR activities reasonably well over a wide range of the DY mass region. • Average Pt of the dilepton: shows a good logarithmic dependence on M2(dilepton) with the slope of the 1.96+-0.35 (useful for LHC too). • With this information, we setup the conservative ISR systematic samples for ttbar (ISR more, less) • In future, plan to study Njet wrt M2(dilepton)., include dijet data at high Et to cover ttbar production region to check.

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