W/Z + jets in ATLAS Pierre-Hugues Beauchemin University of Oxford
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Presentation on theme: "W/Z + jets in ATLAS Pierre-Hugues Beauchemin University of Oxford"— Presentation transcript:
1 W/Z + jets in ATLAS Pierre-Hugues Beauchemin University of Oxford On behalf of the ATLAS CollaborationUniversity of OxfordStandard Model Benchmark Workshop at the Tevatron and the LHCFermilab, November 19th-20th, 2010
2 Scope of the presentation Level of precision needed on W/Z+jets measurements for sensitivity to improvement in theoretical understanding of these processesState where W/Z+jets ATLAS measurements stand and show data to MC comparison of various observablesGive a quick overview of how we tackle down some of the important systematics affecting W/Z+jets analysesBring to your attention some issues we are facing in order to start useful discussionsBring feed-back to the collaborationHopefully reach some consensus among the wide HEP community
3 W/Z+jets measurements ATLAS is in commissioning period:Jet & ETmiss resolution and calibrationLeptons energy scale and resolutionTrigger, Pile-up and LuminosityCrucial to study W/Z+jets events to understand our detector and toolsFirst priority with 2010 datamR = HT/4 mF = HT/4mR = HT/4 mF = HTmR = HT mF = HT/4mR = HT mF = HTPerturbativeQCDFragmentationHadronizationUnderlying eventsThe key to reach a better understanding of the Standard Model is:to keep systematic uncertainties low
4 Status of W/Z+jets analyses We studied W+jets and Z+jets with 1pb-1Absolute cross sectionsRelative to inclusive cross sectionsAll possible ratiosUncertainty on W+jets already dominated by systematic uncertaintyMC-based or simplify correction factorsMostly conservative estimate of systematic uncertaintiesPaper under internal reviewZ+jets uncertainty : Dstats DsystPublic note under internal reviewProvide Comparison to LO and NLO (MCFM) calculationsUpdate with 2010 full 45pb-1 recorded datasetUse more data-driven corrections
6 Data vs MCUse 35 to 42 pb-1 (11% ) of data in the following distributionsCollected from 6 GeV to 15 GeV thresholds electron and muon triggersMC used in the following distributionsALPGEN+HERWIG+JIMMY with CTEQ6L1 PDF for W/Z+jets eventsPythia dijet events (PT > 15 GeV) with MRST2007LO* PDF for QCDUse POWHEG with CTEQ6L1 PDF for ttbarAdded <N>=2 pile-up events, reweighted to primary vertices observed in dataATLAS MC09 tune are usedMC events normalized to observed data candidates before jet selectionsRelative normalization of MC samples to NLO cross sections except QCD
7 Selections Lepton kinematic: Eta coverage: Electron ETclus > 20 GeV and muon combined track PT > 20 GeV with PTMS > 10 GeVEta coverage:|hele|<2.47 excluding barrel to end-cap transition region (1.37<|hele|<1.56), |hmuo|<2.4Lepton quality requirements:Tight requirements on electron cluster shape, track quality and matchingMuon cone 4 track isolation SPTID/PT < 0.2 and |PTID – PTMS| < 15 GeVJet selections:AntiKt4 jets with PT > 20 GeV and |h| < 2.8ETmiss selectionsETmiss > 25 GeV, computed from calibrated topoclusters and out-of-cluster energyATLAS standard clean-up cuts are applied
8 Jet MultiplicityQCD and top backgrounds significantly increase with the number of jetsBigger effect at higher centre of mass energy Need to estimate these backgrounds precisely from dataOther electroweak backgrounds can be estimated using MC ratiosElectron channelMuon channelRemember:jet thresholdis ET > 20 GeV
9 Lepton Transverse Momentum MC describes well the detector effects on lepton reconstruction and resolutionCan use MC to estimate electron and muon resolution effects on acceptanceFor better precision, measure resolution in data before correction.Electron channelMuon channel
10 Jet Transverse Momentum Electron channelSimulation are used to unfold detector effects in W/Z+jets measurementsATLAS data well modelled by simulation:Jet transverse momentum in W+jets eventsreconstruction efficiency in QCD dijet eventsjet energy resolution in QCD dijet eventsALPGEN MCWill be measured on Z+jets eventsPYTHIA MCPYTHIA MC
11 Missing ETReasonably good agreement between data and MC in the bulk of the signal regionsome care must be made with missing ET modelpile-up effectsUse data to estimate:Missing ET selection acceptance correctionQCD background predictionElectron channelLow pile-upHigher pile-up
12 W/Z Transverse Momentum Good understanding of vector boson transverse momentum reconstructed from leptons over wide range of kinematicsWill be used to calibrate jets in W/Z+jets eventsUseful to tune soft QCD effects in MCW+≥1-jetElectron channelZ+≥0-jetMuon channel
14 Systematic: QCD background Wen + ≥0-jetQCD bkg predictions proceed from the opposition of two big numbersDijet cross sectionFake rejection Hard to estimate from MCTemplate method (Wen):QCD ETmiss from reversed electron selections, W ETmiss from MCFit both templates to dataIntegrate normalized QCD in signal region► fQCD(W+≥1-jet) ~ 12%, DfQCD/fQCD ~ 30%Zee QCD background estimate:Direct fit to invariant massNumber of same sign leptons under Z peak► fQCD(Z+≥1-jet) ~ 3%, DfQCD/fQCD ~ 30%Zee + ≥0-jet
15 Lepton efficiencyLepton efficiencies don’t depend on the recoiling jet activityCan use precise estimates from inclusive data samplesEfficiencies are measured from MC in first 2010 analysesTag & Probe method and ETmiss preselected events already show promising resultsTrigger fully efficient in 2010 dataUses pseudo-dataEliminated fromdata-driven estimateof efficiencies
16 Systematics: ETmissNeed to correct for detector effects on ETmiss selection acceptanceNon-trivial systematic uncertaintyJet energy scale and resolutionPile-upMaterial modellingNon-cluster energy DAreco ~2-4% from MC estimateETmiss resolution can be measured in dataZnparaData-driven estimate of correction factor to ETmiss acceptanceSelect Z+jets eventsMeasure s(ETmiss) along nperpMeasure m(ETmiss) along nparaApply gaussian smearing to true PTnpTnperppTHadrons
17 Systematics: detector unfolding Jet energy resolution and reconstruction well modelled in MC.To correct measurement up to hadron level, need to solve the reverse problem (unfolding)involve other sources of uncertaintiesMany different techniques on the market More complicated problemSmaller than lepton efficiency correctionNot a dominant source of systematics in 2010 measurementsUsed simple bin-by-bin correctionsNeed to adopt:good unfolding method,generate high MC statistics,identify all sources of systematicsUnfolding workshop at CERNon 20/01/2011
18 Systematics: Jet Energy Scale ATLAS jets are calibrated from MCConservative uncertainty estimateLarge impact of Jet Energy Scale uncertainty on cross sectionsDJES = 5% Dstot ~ 7%Remove statistical effects in DsSmaller effects on ratiosSingle particle measurement will constrain DJES in 2011 analyses
19 Systematics: summary and outlook Dominant systematic uncertainties on W/Z+1-jet ~1pb-1 analysisTotal of ~14/15% systematic uncertainty (excluding luminosity) compared to 3/11% statistical uncertaintyAlready systematic dominated with ~1pb-1 of dataFactor of ~2 reduction on systematic and luminosity uncertainties with full dataset Sensitivity to NLO effects Will soon start to get sensitivity beyond theoretical precisionSources of systematicsDs(W+jets) 1 pb-1Ds(Z+jets) 1 pb-1target for 45pb-1 analysesJet Energy Scale10%6-7%Lepton Adet and e7%4-5%Unfolding + jet ET resolution3%5%QCD background6%2%1-3%ETmiss Adet-1-2%Luminosity11%Effects which were small and neglected must now be treated properly Induce some discussions
21 Jet Energy ThresholdExperimental criteria favour jet PT thresholds above 30 GeV:Calibration below 20 GeV affected by jet reconstruction thresholdJet energy scale uncertainty and pile-up dramatically increase below 30 GeVJet reconstruction efficiency quickly decreases below 30 GeVTheoretical prediction less robust for lower jet ET:Low PT jets are more sensitive to soft QCD effectsLower PT jet thresholds imply higher scale uncertainty for theoretical predictionsQ: Prefer to reach as low jet PT thresholds as possible (input to theorists) or keep thresholds higher (more robust comparisons)?Q: What would be desirable jet thresholds in W/Z+jets physics?
22 Detector acceptanceLepton acceptance corrections depend on theoretical input (generators)True level lepton PT and h cuts, ETmiss cut, mass selections, etc Acceptance correction in measurements make it difficult for theorist to disentangle these effects to test potential improvements in their modelsCorrecting for detector acceptance is needed to:Combine or compare measurements made in muon and electron channelsCompare results from various experiments (detector independent results)Q: Prefer to see publication with visible cross sections only or correct for a full acceptance cross sections?Q: How isolation should be treated?So far, in ATLAS, we are working out both numbers, but publication preferences still on analysis to analysis basis.
23 QED Final State Radiation QED radiation, especially from FSR is:different for electrons and muonssimulated with varying accuracy in different MC programs and kinematic regionsTheory accuracy at the few-per-mille level for inclusive cross section,► this is not true in general for differential or exclusive distributions.Final state electronPythia status code 1Well defined physics final stateInput to unfolding is theory dependentElectron at production vertexPythia status code 3Compare electrons and muonsMeasurement can’t profit from new theory“Dressed” electronFinal state + photon in cone around itPhysics final state close to vertex electronCone size is arbitraryQ: what exactly should we measure and how should we confront it with theory? Correction before or after radiations? Both?Q: How to assign reasonable systematic uncertainty on such effect?Q: Should QED radiation be included in true jet clustering?
24 Jet-Electron overlapElectrons are reconstructed as jets in the calorimeterAffect the energy response and reconstruction efficiency of close-by jetsNo ideal way to experimentally deal with thisRemove jets using DR matchingSmall cone (~0.2)Large cone (~0.5)Small cone + event vetoRemove electron cluster from calorimeter andrerun jet algorithm4-vector subtractionQ: Which is the best approach for comparison to theory and with other experiments?Q: When a DR approach is used, should the decay products of vector boson be included in true jet clustering?
25 Correction to Parton level MC Unfolding brings jets from detector level to hadron levelNLO fixed order calculation programs like MCFM don’t include non- perturbative QCD effects for W/Z+parton(s) processesHadronization, underlying event No hadron level…K-factors obtained from NLO/LO MCFM prediction can’t be applied to predictions involving parton showerQ: Should we compute hadronization + underlying event correction factor from PYTHIA or HERWIG and apply them to MCFM predictions or it is preferable to leave predictions as they are?
26 Conclusion W/Z+jets physics allow to: Study detector performance and validate measurement tools Improve systematic uncertainty on calibrations, efficiencies, unfolding, etcBetter understanding of higher order QCD corrections Require small systematic uncertainty on the measurementsATLAS performed 1 pb-1 W/Z+jets measurementsGood data to MC agreementSet the ground for more precise future measurementsSmall effects will become important as the precision increases► Need already discussions with theorist and other experiments onhow to provide the best handle on these effectsEg: QED FSR, jet energy threshold, jet-electron overlap removal, etcUsing the 2010 full dataset and the yet to come 2011 data:improve jet, ETmiss and lepton performances using Z+jets eventsStart to study Heavy flavourProvide crucial understanding of major background to many new physics searches
27 Some references First Z+jets MC study: CSC book: arXiv: [hep-ex]Paper on the W/Z inclusive measurementCERN-PH-EP , arXiv: [hep-ex]Jet energy resolution and reconstruction efficiency studiesATLAS-CONFJet energy scale uncertainty estimateATLAS-CONFETmiss performance studiesATLAS-CONF