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

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

Why we care about 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  Mtop, but  M=1.3 GeV for in RunI CDF analysis [PRD 63, (2001)] ISR is identified as W daughter or 2 nd bjet.  LHC is a top factory, ttbar+njet is a major background to many EWK processes (crucial for Top-Yukawa coupling, ttH) ISR

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. Q 2,  QCD, splitting functions, PDFs –Use DY data (no FSR): study Pt of the dilepton, njets in the DY for different Q 2 region (~different DY mass region) ++ -- qq -> tt(85%)

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 Average Pt of the dilepton has a logarithmic Q 2 dependence ( DGLAP evolution effect )  Measure the slope, this allows us to estimate the size of ISR at M(DY)=2* M t log(M 2 ) (2Mt) 2

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

Datasets SUSY dimuon (4.8.4a): reprocessed with High-pt electron/muon: reprocessed datasets  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) 2Mt Dimuon Dielectron

Analysis cuts 1 st lepton2 nd lepton Hi-pt muon (M>40) Standard CMUP CMX (Pt>20) Standard CMUP CMX CMP (Pt>10*) For Pt>10 EM<1 HAD<3 Low-pt Muon(M<40) Same as hi-pt muon but Pt>10* Same as hi-pt muon Hi-pt electron (M>76) Standard CEM Standard CEM No trilepoton events del Z for two leptons<4cm with opposite charge MET<25

Effect of the QED FSR Willis tuned-Pt(Z) Run I tune M 2 (dimuon) Pt(dimuon) vs M(dimuon)[65-70] M 2 (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)

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

MC datasets CDF standard setting for top/ewk groups Pythia 6.2 (RunI tune) Pythia 6.2 (Willis’ Zpt tune) Herwig 6.4 Q 2 min (cut-off for space-like evol.) 1.25 (D=1) parp(62) (D=0) K factor for space- like evol. scale 1 (=D) parp(64) 0.21 Kt_sigma (intrinsic parton pt in proton) 2.50 (D=1) parp(91) Kt_max (Kt cut-off)15 (D=5) parp(93) 15 + Rick’s underlying evts tunning Ffor Pythia  Top/EWK Pythia and Herwig MC samples

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

Comparison of the Pt (dimuon) at different mass region

Average Pt (dilepton) as function of the dilepton mass 2 M 2 (dilepton) M<40 GeV: SUSY dilepton M>40 GeV: Hi-pt lepton

Corrections for the average Pt (dilepton) Background corrections –very small due to two tight cuts for cental lepton –Mainly DY(  ) contribution Acceptance corrections –Bin by bin correction using Pythia MC (zewk2m,zewk1e) Backgrounds [ DY(  ) ] * 1.2%5.5%< 0.1% Corr due to backgrounds -0.2% -2.5%< 0.01% Corr. due to acceptance *: Other physics backgrounds, WW, top are small but removed by the MET<25 cut

M 2 (dilepton) Evolution of the as function of the dilepton mass 2

M 2 (dilepton) M 2 dependence on the M 2 dependence on the = ( ) + ( )*log (M 2 )

ISR uncertainty ++ -- Q 2 max: K PARP(67) Qmin : PARP(62) Kt 2 = PARP(64)(1-z)Q 2  QCD = PARP (61) Intrinsic Kt: Gaussian Width: PARP(91) Pythia setting ISR Up(*) ISR Down(*) PARP(61) (D=0.192 GeV) PARP(64) (D=1.0) PARP(67) (D=1.0) PARP(91) (D=2.1) *: to see size of variations (not one sigma) 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.

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

ISR variation due to ISR variation due to Intrinsic Kt Though effect of the intrinsic Kt on is big, not much freedom in Kt, because of the constraint from the data. Gaussian Width in Z mass –Data : –Pythia MC : Width: 8.0 vs 0.5

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

ISR uncertainty Lambda QCD: 100 vs 384 More ISR :  QCD = 384, K = 0.5 Less ISR :  QCD = 100, K = 2.0 Run I: no ISR: K = infinite ISR uncertainty samples (conservative) Ttbar: more ISR, less 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)

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 M 2 (dilepton) with the slope of the (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 M 2 (dilepton)., include dijet data at high Et to cover ttbar production region to check.