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QCD Multi-jet background in W+jets and Rjets

QCD Multi-jet background in W+jets and Rjets. Alessandro Tricoli (CERN) o n behalf of W+jets and Rjets groups. SM W/Z-Physics Group Meeting. 25 th February 2013. Overview. QCD multi-jet background to Z+jets is small and under control Not discussed today

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QCD Multi-jet background in W+jets and Rjets

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  1. QCD Multi-jet background in W+jets and Rjets Alessandro Tricoli (CERN) on behalf of W+jets and Rjets groups SM W/Z-Physics Group Meeting 25thFebruary 2013

  2. Overview • QCD multi-jet background to Z+jets is small and under control • Not discussed today • QCD background to W+jets has shown some unexpected features in both electron and muon channels • Far larger than in 2010 data analysis • Muon channel has as much or higher background than the electron channel • Poor fit quality and some model dependence in electron channel • Discrepancy wrt W inclusive analysis results • A lot of effort in W+jets and Rjets groups to understand it • Some questions answered, some to be further investigated A. Tricoli

  3. Relevance of QCD background • QCD background is an important source of uncertainty for both W+jets and Rjets (no cancellation as QCD is very small in Z+jets) • While Z+jets events are very pure, W+jets events are not, does not cancel in Rjets • QCD is dominant background in 0,1,2,3 jet bins, up to ˜15% of data sample • In 2010 QCD multijet uncertainty was 2nd or 3rd of all syst’s for W+ ≥4 jets (11-20%) • How sure are we that we are measuring features of W+jets in all kinematic regions and not its background? With no bias? • Need to have a precise and robust estimation of the background level to reduce uncertainty and produce accurate cross-section measurements • In 2011 JES has been reduced so must QCD uncertainty • JES 4-5% in W+1 jet, mostly cancels in Rjets(≤1% for 1 jet) • Need precision ≤30% (W+1) and ≤10% (R+1) not to be dominated by QCD • 2011 W+jetshas had some issues in both electron and muon channels • Why multi-jet background ˜3-5x larger in 2011 than 2010? • Why multi-jet background level similar in electron and muon channels? A. Tricoli

  4. 2010 vs 2011 - electrons • Large increase in the QCD fraction between 2010 and 2011 • Same template definition in both years (and same lepton thresholds in both years for this comparison) • Different electron identification tight (2010) -> tight++ (2011) • Different primary and supporting triggers in 2010 and 2011 • Is factor 3-5 difference due to pileup? • Same kinematric selection in both years • MET cut is applied to both fractions A. Tricoli

  5. 2010 vs 2011 - muons • Large increase in the QCD fraction between 2010 and 2011 • The same isolation cut is used in both years • Is difference of 3x-5x between 2010 and 2011 due to the broadening of the MET distribution with more pile-up? MET cut is applied to both fractions A. Tricoli

  6. Technique – electrons (I) • Wen+jetsmethod (similar to W inclusive analysis): • QCD template from data: • use OR of three prescaled loose(1) triggers: • EF_e20(22,22vh)_loose, • EF_e20(22,22vh)_looseTrk • EF_e20(22,22vh)_loose1 • select events with loose electrons passing track quality cuts and failing tight • anti-isolation applied • QCD normalization: • fitting MET • QCD template from data + Signal and other BKGs from MC • Fit done in exclusive jet bins up to last bin which is inclusive • Fit done in two different data-periods: D-K, L-M • Templates extracted for D-K and L-M periods in data and MC A. Tricoli

  7. Technique – electrons (II) • QCD Template definition • Electron identification cut reversal is the same as W inclusive analysis • Pass loose and track quality cuts, • Fail tight, ignoring Conversion cut, Cluster-Track Matching cut in Phi and Tight in Eta • Anti-isolation (to reduce signal contamination) is different • W+jets uses reversal of isolation cut used in signal selection: • not isEiso98Etcone20 || not isEiso97Ptcone40 • W inclusive uses EtCone30/Et > 0.2 constunsigned inttemplate_mask_pass = (0               | 0x1 << egammaPID::TrackPixel_Electron               | 0x1 << egammaPID::TrackSi_Electron               | 0x1 << egammaPID::TrackMatchEta_Electron); constunsignedinttemplate_mask_fail = (egammaPID::ElectronTight               & ~(egammaPID::ElectronLoose)               & ~(0x1 << egammaPID::ConversionMatch_Electron)               & ~(0x1 << egammaPID::TrackMatchPhi_Electron)               & ~(0x1 << egammaPID::TrackMatchEtaTight_Electron)); ((el_isEM->at(qcd_i) & egammaPID::ElectronLoose) == 0)  ((el_isEM->at(qcd_i) & template_mask_pass) == 0) && ((el_isEM->at(qcd_i) & template_mask_fail) != 0)) A. Tricoli

  8. Technique – Comparison with W inc. - electrons • W+jets fit results different from W inclusive: • larger background and worse fit chi2 • A lot of effort put in comparing our results • Cutflow, distributions, MC’s samples, binning, fit range code checks etc. • Coarser binning choice in W inclusive (2 GeVvs 5 GeV) improves chi2 • Reduced fit range as in W inclusive improves fit quality • Missing supporting trigger reduced QCD template statistics in W+jets analysis • Differences in QCD template definition and W MCdataset (small effect) W+jets group Winclusive group • With same binning, MC DS, fit range and triggers we find consistent background fractions for ≥ 0 jets, but chi2/dof is still different (31 vs 3) • differences may be due to different calibration? • W incl. cuts out a small region 1.6 < |η| < 1.7 A. Tricoli

  9. Robert King Addendum • Source of disagreement on χ2 between between W inclusive and W+jets • TFractionFitterdoes not look in the bin error, instead looks at bin content assumes error is √(N) • Should scale N and error such that the fractional error stays the same while the absolute error changes to be equal to √ (N) • This is what Inclusive group does => by applying this method we get lower χ2 • Remaining sources of (small) disagrement (<4%) - do we need to pursue this further? Should we make these changes in W+jets,Rjets analyses? • rescaling W pT • cutting out 1.7>|eta|>1.6 • different calibration package • different background MCs W incl. W+jets A. Tricoli

  10. Technique – Fit stability - electrons • Fit Stability and Optimisation: • QCD fit results have been checked with different • Anti-isolation definitions: etcone20,30,40, ptcone40 / ET >0.20 • W contamination in control sample reduced by 3x or more with anti-isolation • After anti-isolation ≤1% W contamination with any anti-isolation choice • (AlpgenWenu+jetsthrough QCD selection) • Etcone has lower W contamination • QCD fraction with and without anti-isolation changes by 1.5% at most • Both QCD fraction, EW SF and Chi2 are little affected by different isolations • Etcone has EW SF and chi2 closer to 1 • Identification cut reversal • Different combinations of isEM bits to fail or pass • Small effects on QCD background fractions A. Tricoli

  11. Background composition - electrons • Since backgrounds are of the same level in e and m channels, has background composition in e-channel has changed in 2011? Fakes -> H.F.? • Check assumption of multi-jet background dominated by fakes in electron channel • Separate electron fakes from H.F. real electrons in W(ev)+jets • Use 3 components to fit to the data: • EW+Top and 2 QCD templates • (cut reversal+anti-isolation) with addition of • impact param significance >10 • Impact param significance <10 •  Fit quality improves • Fake electrons are dominating contribution • (still to assess contributions from conversions) A. Tricoli

  12. Alternative fitting variables - electrons • Tried Df between electron and vector sum of jets • Dftemplate gives very different results from MET template leading to much worse agreement with data • Df fit gives suspiciously larger EW SF (>7%) • Method abandoned • Exploring now fit of Isolation, extracting W template • from Z sample • Work in progress, promising preliminary results W+1 Period L-M A. Tricoli

  13. Technique - muons • Wmn+jets method: • QCD template from data: with reverted impact parameter cut • d0 significance > 3 • differently from W inclusive analysis that uses anti-isolation, due to bias in jet kinematics • QCD normalization by fitting MET on data with QCD template + Signal and other BKGs (from MC) • Fit done in exclusive jet bins up to • last bin which is inclusive • No splitting in data-periods • Consistent results by Monica and • Andrew Reversed Iso Reversed D0 • Using isolation leads to a bias in the leading jet pT distribution • Shown here for QCD b-bbar MC A. Tricoli

  14. Technique – Comparison with W inc. - muons • W+jets fit results different from W inclusive: • larger background and worse chi2 • Trying to compare our results • Inclusive group uses range of 0-40 GeV to avoid high-MET region that suffers from large shape differences • Tried Reverse Isolation consistently with W inclusive analysis • 1.5% difference in fraction, no big difference in Chi2 • Using Sherpa: 1.5% difference in fraction, no big difference in Chi2 • Chi2 still very different between W+jets and W inclusive analyses • No MET cut applied here A. Tricoli

  15. Fitting Tools - muons • Errors are about ~1.5x smaller with RooFit than with TFRactionFitter. Central values are comparable • No MET cut applied here • Similar effect seen in Top background fit • is TFractionFitter uncertainty realistic? Aren’t they overestimated? • Problem of fit convergence in Top background fit with TFractionFitter while none with RooFit A. Tricoli

  16. Fitting Isolation - muons • Use Isolation as alternative fitting variable to cross check results with MET Binning was too coarse, so perhaps the fit is not the best. Black = Wjets Yellow = QCD Blue = Fit result • Chi2 is not better than with MET • Fit results vary quite a bit from MET fits • up to 2.7% • Further investigation needed MET cut is applied for both A. Tricoli

  17. Pileup – electrons (I) • Big jump in pileup in Period L and M wrt previous periods • <m>~5, 10 in Periods D-K and L-M • Primary and Supporting (background) electron triggers changed during the year 2011 and not always coherently in terms of selection and rates • data period statistical weight is different in signal and background samples: • D-K weigh more than L-M in background sample than in signal sample • Pileup effect in whole 2011 data sets not reflected in QCD background sample • MET distribution is pileup sensitive • Period dependent fits in electron channel: D-K, L-M (fit quality improves) A. Tricoli

  18. Pileup – electrons (II) • Splitting Data and MC sets in D-K and L-M templates • Fit quality improves (chi2/dof and EW SF closer to 1) • QCD fraction increase by 1%-2% overall • Low QCD fraction in D-K, large in L-M => Pileup Effect !? • Results irreproducible on MC due to small statistics • Used in both W+jets and Rjets as default EW Scale Factors Chi2/dof • EW S.F. are at times still large 1%-12% • indication EW process mis-modeling? QCD Fraction • Chi2/dof not yet 1, especially with 0,1,2 jets • QCD Template mis-modeling? A. Tricoli

  19. MC study– electron channel • MC does not show same data-period dependence as in data, probably due to poor statistics • However well reproduces pileup • Cannot use MC to understand data-period dependence A. Tricoli

  20. Pileup – muons (I) • 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? • Still to be explained larger QCD fraction in 2011 than in 2010 Clear period-dependence of isolation distribution (D-K,L-M) QCD control sample Signal sample A. Tricoli

  21. Pileup – muons (II) • Factor up to 3.4x in difference between L-M and D-K • Results to be cross checked No MET cut applied here A. Tricoli

  22. Uncertainties • Electron channel • Preliminary uncertainties, including: • W/Z model dependence (Sherpa vsAlpgen) • Change of fit range • Change of anti-isolation - dominating source • Statistical uncertainty Uncertainties do not cover difference between data-periods • Muon channel • Preliminary uncertainties, including: • Template shape (anti-isolation and changes of cuts) • Change of fit range • Alternative fit variable [Df(l,sumed-jets)] • background dominant systematics on Rjets:˜3% on 0,1,2,3 jets A. Tricoli

  23. On-going Discussion • Ongoing discussion between W+jets and Rjets groups on • should we worry/understand disagreement between 2010 and 2011? • should we worry/understand levels and data-period dependence of background in electron and muon channels? • should we worry/understand remaining disagreement wrt W inclusive analysis? • No evidence of mistakes in 2011 analysis that could lead to such a difference wrt 2010 • Detector levels are in reasonable agreement • “things” have changed between 2010 and 2011 (what?) • Data-period dependence not confirmed by MC but not refuted either • Or maybe 2010 analysis was wrong? • Large chi2/dof and other unknowns will/can eventually translate in larger uncertainties on background estimation A. Tricoli

  24. Conclusions and Outlook • Great progress in the last few week in understanding QCD background in W+jets events – fast pace! • Better understanding now of pileup contribution in the electron channel • Improved agreement with W inclusive analysis results in electron channel • Systematic studies of stability of results undertaken • e.g. Model dependence, change of control sample definition, fitting tools • A few systematics checks remain to finish in electron channel • Trigger bias, optimal control sample definition (e.g. isEM tight++/medium++) • Cross check results with alternative fitting variable: isolation • Reassess W model dependence in fit results • Still a few open questions in muon channel • Source of increase in background fraction in 2011 wrt 2010 • Remaining discrepancies with W inclusive analysis • Debate within W+jets and Rjets group on priorities A. Tricoli

  25. Backup A. Tricoli

  26. W+jets and R+jets Selection (II) object selection • R+jets uses same selections as in W+jets and Z+jets Preselection: -trigger - primary vertex (at least one good vertex with >=3 track ) -MET cleaning and LAr Hole Veto Lepton selection: Z selection: 2 OS leptons, 66 GeV< mll<116 GeV W selection: MET > 25 GeV, mT(W) > 40 GeV Jets selection: Electron Trigger W: e20_medium(D-I), e22_medium (K), e22vh_medium1 (L-M) Z : 2e12_medium(D-I), 2e12T_medium (K), 2e12Tvh_medium (L-M) MuonTrigger: mu18_MG (D-I), mu18_MG_medium (J,M) Electrons: Author 1or 3, Quality flag (OQ&1446 !=0) pT>25 GeV (W),20 GeV (Z) |n|<2.47 (excluding 1.37-1.52) Muons: STACO Combined with cleaning cuts (MCP recommendations), pT>25 GeV (W),20 GeV(Z) |n|<2.4 Electrons: 2 OS medium electrons Muons: 2 OS muons, at least the 1st with d0sign (<3), z0(<10mm), isolation requ. (ptcone20 / pt < 0.1) MET: MET_RefFinal, JES and lepton scale prop with MissingETUtility Muons: 1 muon with d0sign,z0, isolation requ. Electrons: 1 tight++ electron Antikt4TopoEM at EM*JES scale pT > 30 GeV, |y| < 4.4 |JVF| > 0.75 ( for jets with |η| < 2.4) ΔR(jet-lepton) > 0.5 A. Tricoli

  27. Fit Stability Different anti-isolations A. Tricoli

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