1. W+jets and Z+jets studies at CMS Christopher S. Rogan, California Institute of Technology - HCP2009 - Evian-les-Bains Analysis Strategy Analysis Overview: BG scaling in W/Z+jets events Data-driven strategy to study properties of W/Z + jets production in final states with electrons and muons Focus on LHC startup: order Using different jet definitions - in some cases, detector-wise orthogonal Two inter-related analyses: Test of “Berends-Giele” (BG) scaling through the measurement of the Z+n jets / Z+(n+1) jets ratio as a function of n High efficiency signal selection provides “candle” dataset for detector and physics object commissioning studies normalization of irreducible Z( )+jets background in MET+jets new physics searches New physics searches with SM Z bosons in the final state, observed as a deviation from BG scaling Test of BG scaling through the measurement of the W+n jets / W+(n+1) jets ratio as a function of n, along with the double ratio Predict W+ >= 3,4 jet yields from lower jet multiplicities, revealing excesses from new physics processes with leptons, jets and MET Event Reconstruction and Cut-Based W+jets, Z+jets selection High-efficiency event selection to increase S/B ratio to suitable level for ML fit to extract yields Z candle: only minimal isolation and vertex requirements necessary due to discriminating power of di-lepton invariant mass W/Z+jets ratio: synchronized event selection for W and Z events => maximal cancellation of systematic uncertainties in the double ratio. Common selection requirements: –Single non-isolated HLT lepton trigger –Electron/muon reconstruction –Lepton identification –Lepton isolation –Lepton - primary vertex compatibility –Jet clustering –Electron(s) from W(Z) cleaning from jet collections –Jet counting Maximum Likelihood Fits Rather than a “cutting-and-counting”, we use maximum likelihood fits to extract event yields, as a function of the number of jets Additional orthogonal discriminating variable used to validate PDF parameterizations Control samples from data are used to determine distribution shapes and efficiencies required by the fits, minimizing the reliance on Monte Carlo simulation Data control samples Z “candle” dataset The yields from the maximum likelihood fits are used to calculate the ratios related to BG scaling and between the W/Z+jets yields - most systematic uncertainties (PDF’s, jet energy scale, lepton isolation, etc.) cancel in these ratios Additionally, sPlots statistical background subtraction, in conjunction with the maximal likelihood fit, can be used to produce a “pure”, high efficiency, Z( ll )+jets sample - which can be used for a number of applications related to detector and physics object commissioning and background normalization Jet Clustering  calo-jets: jets clustered from the calorimeter (ECAL+HCAL) cells re- projected w.r.t. the primary vertex (standard)  track-jets: jets clustered from tracks consistent with the event primary vertex (lower noise levels relative to calorimeters)  JES corrected calo-jets: synchronized with the above calo-jets definition (mature detector understanding)  Particle Flow jets: synchronized with the above calo-jets definition These types of jets  have orthogonal detector systematics: calorimeter vs tracker  probe different regions of phase space: different cuts in p T, 3.0 vs 2.4 in  p T > 15 GeV/c, |h| < 2.4 p T > 60 GeV/c, |h| < 3.0 In these analyses, each yield measurement is done as a function of inclusive jet multiplicity We consider several types of jet with different experimental constituents (all clustered with the SISCone algorithm and a cone size of 0.5) Maximum Likelihood Fits Maximal likelihood fits are performed for each jet multiplicity in order to extract signal and background yields. For W+jets events, the fit is based on the W transverse mass For Z+jets events, the fit is based on the di-lepton invariant mass Z+jets candle analysis W/Z+jets ratio analysis In the W fit, two different ‘categories’ are defined: a ‘heavy flavor’ (hf) enriched ( ) and depleted region ( ), dominated by signal and single top +, respectively. The hf enriched region is defined by a cut on variables related to the impact parameters of tracks matched to jets in the event, D xy evt and D sz evt. hf variable control samples for signal and background Background control samples for m( ll ) and m T W shapes W/Z+jets ratioBG scaling for W/Z+jets Z+jets candle sample: High signal efficiency with sPlots statistical background subtraction Z( )+jets background normalization MET correction for W(  )+jets events deriived from Z(  )+jets events “Candles” and “Ladders” W/Z+jets have a large cross section at LHC with final states including leptons jets missing transverse energy (neutrinos) Dominant background for SM measurements (ex. and Higgs production) and new physics searches involving jets/leptons/MET final states W/Z+jets @ LHC Overview p T > 30 GeV/c, |h| < 3.0 p T > 60 GeV/c, |h| < 3.0 Deviations from BG scaling Low jet multiplicities can be used to predict the high multiplicity yields. Comparison with measurements of the higher multiplicities can quantify deviations from BG scaling due to new physics in Z and MET+jets+lepton final states W specific requirements: –>= 1 lepton –Z mass veto –extra muon veto (e) –MET > 15 GeV (QCD rejection) – Z specific requirements: –>= 2 leptons –Z mass window orthogonal selection