1. QCD and Top backgrounds in W+jets and Rjets Alessandro Tricoli (CERN) on behalf of W+jets and Rjets groups 3 rd May 2013 W+jets and Rjets EB Meeting

2. A. Tricoli 2 3 rd May 2013 Overview  Progress made in understanding QCD multi-jet and Top backgrounds  Better understanding of QCD background in W  and e channels, e.g. pileup, trigger and template dependencies  Systematics available for QCD in electron channel  Top background estimations available in both  and e channels with most of systematics included  To do list getting smaller and smaller  big effects already addressed, remaining sub-leading effects to be considered

3. A. Tricoli 3 3 rd May 2013 QCD Methods recap  Both  and e channels fit MET distribution (10-60 GeV) in exclusive jet multiplicity bins, using data-driven template for QCD and other processes from MC  QCD Control samples (Templates); o Mu channel: uses same trigger as for signal and reverts impact parameter significance (d 0 /  (d 0 )>3) o Electron Channel: uses logical OR of a few loose electron triggers and revert some electron identification requirements, plus anti-isolation (etcone30/eT > 0.2). Control sample split in 2 data- periods: D-K and L-M  See my presentation on 25 th February WZ subgroup for more details

4. A. Tricoli 4 3 rd May 2013  Investigation of reasons for increase of QCD background wrt 2010  Is difference of 3x-5x between 2010 and 2011 due to the broadening of the MET distribution with more pile-up?  Previous results were inconclusive  Study QCD fractions in different samples o Low/Medium/High Pileup o D-K and L-M Periods o Low/Medium/High jet pT QCD – W->  +jets Pileup

5. A. Tricoli 5 3 rd May 2013 SampleNjej==0 [%] Njej==1 [%]Njej==2 [%]Njej==3 [%]Njej==4 [%]Njej==5 [%] Reference4.91±0.1814.23±0.2313.23±0.4312.5±0.99.4±1.511±10  =0-4 3.8±1.17.5±1.38.3±2.95.4±3.35.7±5.41.4±5.0  =4-8 3.87±0.2110.91±0.3010.31±0.5810.8±1.27.7±2.010±12  =8+ 5.88±0.2916.67±0.3115.54±0.6313.7±1.210.6±2.112±14 D-K 3.99±0.2111.04±0.2810.47±0.5610.8±1.27.5±1.910±13 L-M 5.81±0.2916.70±0.3215.49±0.6413.5±1.210.9±2.212±14 jet p T >45 GeV 5.33±0.1214.67±0.2810.64±0.648.3±1.48.0±3.33.5±7.7 jet p T >60 GeV 5.55±0.1112.77±0.35 9.64±0.967.3±2.23.3±2.30.3±3.2  QCD Fractions increases when pileup increases, in all three tests  QCD Fractions for  =0-4 close to 2010 values (pileup conditions rather similar)  Pileup is cause of increased pileup in 2011 wrt 2010 QCD – W->  +jets Pileup

6. A. Tricoli 6 3 rd May 2013 QCD – W->  +jets Comparison with W inclusive  W inclusive group uses different definition of QCD control sample  0.1 < ptcone20/pT<0.5  Good agreement on QCD fraction between our results and theirs when same selection is used  Reason for different strategy is bias on jet kinematics by reversing relative isolation Reversed D0Reversed Iso Jet p T biased in all templates tested except our nominal (reversed d0)

7. A. Tricoli 7 3 rd May 2013 QCD – W->  +jets Comparison with W inclusive  Larger W Contamination in QCD control sample found with our selection (reverted impact param significance) wrt W inclusive method  Frac. of expected W  events in QCD control region: 42% vs 13% with 0 jets W+jets W incl  Studying if we should subtract this contamination or try different template definition  Find a balance between contamination and jet p T bias

8. A. Tricoli 8 3 rd May 2013 QCD – W->  +jets Template shape bias  Our method assumes no change of shape in moving QCD events from the control region to the signal region  Test this hypothesis by comparing the shape of QCD MC in the signal selection to the data templates  The heavily signal contaminated templates (i.e. 1 & 3) show large differences in the zero jet bin, others are all more similar  Clear trend: templates have harder met spectrum than MC  Is this a real bias or more general MC-data discrepancy for QCD?

9. A. Tricoli 9 3 rd May 2013 QCD – W->e +jets Comparison with Inclusive group  Believe we have resolved differences with inclusive group  Very good agreement  Inclusive group also see period dependence consistent with us

10. A. Tricoli 10 3 rd May 2013 QCD – W->e +jets Uncertainties  Uncertainties considered so far  Fit range (5-20 GeV)  Varying the anti-isolation requirement (different cone sizes and cut values)  Sherpa instead of Alpgen  Statistical uncertainty  Study of template bias  Different failed tight requirements (different isEM sub-sets) o adding following requirement to pass at a time (in addition to loose+track quality):  Envelope of these then taken for an additional systematic

11. A. Tricoli 11 3 rd May 2013 QCD – W->e +jets Uncertainties  Due to significantly reduced stats in the QCD template for TRACKMATCH bit reversal o Either we increase stat. or eliminate stat fluctuations from sys.

12. A. Tricoli 12 3 rd May 2013  Statistical fluctuations dominate systematics at high Njets  From period L5 new triggers were introduced specifically for QCD background estimations in the electron channel  EF_e22vh_loose_4j15_a4tc_EFFS  EF_e22vh_loose_3j20_a4tc_EFFS  Loosely-identified electrons + 2 and 3 jets respectively (beware of e/j overlap) with 15,20 GeV thresholds on HLT jets (Full Scan)  Increase statistics of QCD Control Sample for 2 and more jets  Statistics increase of factor 2 to 5 2.0x (Njets =2) 3.4x (Njets =3) 3.6x (Njets =4) 4.1x (Njets =5) 5.0x (Njets ≥6)  Need to check bias on distributions as 30 GeV cut offline on jet threshold is not at jet trigger efficiency plateau QCD – W->e +jets Trigger Statistics

13. A. Tricoli 13 3 rd May 2013 QCD – W->e +jets Trigger Statistics and Bias No evidence of bias on MET from these triggers for 2 and more jets  Will check bias on other distributions, e.g. jet pT  Will evaluate impact on overall analysis uncertainty of gain in control sample statistics

14. A. Tricoli 14 3 rd May 2013 QCD – W->e +jets Trigger Bias  Three types of triggers used for collecting control sample 1)e20(22)_loose, e20(22)_looseTrk (= loose + track quality cuts) 2)e20(22)_loose1  No trigger bias on control sample from different types of triggers on jet observables  However bias on electron kinematics (as expected)

15. A. Tricoli 15 Top Background Summary  Have fits from 3-jet to 7-jets for both electron and muon channel  Have evaluated all major systematic uncertainties  But still have several cross checks to do with R+jets code  Remaining outstanding items  Understanding fits using MET and Top mass  Correlations between the QCD fit and top fit 3 rd May 2013

16. A. Tricoli 16 Top - Reminder of Method  Top background is one of the largest uncertainties  Use a full data-driven method  A ttbar template from data  An independent variable to normalize the ttbar template Can select a ‘pure’ ttbar sample by requiring events with one or two b-jets Found using Transverse Sphericity yields the most stable fits, fits converge up to 7 jets Also studying Top mass and MET fits for comparison 3 rd May 2013

17. A. Tricoli 17 Top Fit Example Top template: - B-tagged Data - Wc,b from MC W template - From MC - No b-tagging requirement Data - No b-tagging QCD - Fixed in the fit Njet=4 3 rd May 2013

18. A. Tricoli 18 Top Template Corrections  The b-tagged data- driven sample has slight biases with respect to the non-b-tagged sample  Correct for this using MC  The two distributions are normalised in the ratio so that when the corrections are applied the total area is not changed (correct for bias on shape only) 3 rd May 2013

19. A. Tricoli 19 Top - Wcc, Wbb Contamination  Question was raised in EdBoard about what k-factors to apply to the Wbb and Wcc samples  Estimate of the k-factor in the 1-jet and 2-jet bin where top contribution is small  Has small effect on the fit <4% at 3-jet (compared to 8% statistical uncertainty)  Use K-factor of 1.18 as systematic uncertainty 1-jet2-jet Wcc, Wbb Alpgen 1.111.18 Sherpa0.951.02 3 rd May 2013

20. A. Tricoli 20 Top - Uncertainties  Statistical uncertainties from the fit  W signal model  Fit using W Sherpa sample  Btagging bias correction  Use largest of statistical errors on Alpgen, or difference between Powheg vs Alpgen  Fit range, template shape dependence  Compare the pseudo-data sets to truth as a function of fit range  Wcc, Wbb contamination subtraction  Redo fits using K-factor of 1.18  Standard MC-based uncertainties on W template including  B-tagging efficiency uncertainties (on b, c, and light)  JES, JER, lepton energy scale, etc 3 rd May 2013

21. A. Tricoli 21 Top Fit Results  Take the central value of the 3-jet bin with a grain of salt (sudden change with recent MC version, need to de-bug this)  3-jet bin is dominated by JES (where the W signal is largest)  7-jet bin is dominated by statistical uncertainties from the fit 3 rd May 2013

22. A. Tricoli 22 Top Fit Results  Detector level distributions look good MC Data-driven 3 rd May 2013

23. A. Tricoli 23 Top Open Problems  The Sherpa sample has poor statistics  Using sherpa as a modeling uncertainty has large fluctuations (red curve)  Using Sherpa for both W signal template and W contamination in Top control sample  Will use pseudo-data sets to evaluate if the modeling uncertainty is fully included in the fit range systematic Fractional uncertainty on N. of Top events 3 rd May 2013

24. A. Tricoli 24 Top Open Problems  Fits using top Mass and MET are very unstable and dependent on the fit range.  It would be helpful to have a second variable as a cross check  Need to re-do pseudo- data set fits to evaluate fit ranges for these variables 3 rd May 2013

25. A. Tricoli 25 QCD and Top - To Do List  QCD multi-jet background  Re-evaluate systematics in W->  channel  Study W contamination in QCD control region and alternative control samples (with no kinematic bias) in W->  channel  Study alternative method, e.g. isolation, in both channels  Cross check strategy of systematics with W inclusive group  Top background  Cross check all JES uncertainties, cutflow on MC with R+jets  Add electron, muon systematics  Cross check results with alternative fit methods, i.e. MET and Top Mass, including their systematics  Adding fit range systematic uncertainty and closure tests  Correlation between QCD and Top data-driven estimations  Evaluate Top background on Z (follow Z+jets/CMS method) 3 rd May 2013

26. A. Tricoli 26 Backup 3 rd May 2013

27. A. Tricoli 27 Pileup – muons  No need of period dependence fit in Muon Channel since same trigger used for signal and background samples  pileup effects (e.g. broadening of MET distribution) in signal sample is well reproduced by background sample  Oppositely to electron channel, QCD fraction seems to go down in L-M, by a factor ˜3 wrt D-K  Is this because isolation becomes tighter with more pileup? QCD control sample Signal sample Clear period-dependence of isolation distribution (D-K,L-M) 25 th February 2013 Slide from 25 February

28. A. Tricoli 28 3 rd May 2013 QCD – W->e +jets Systematics – 5 th jet