We investigate the discovery potential for the $pp \to WZ \to \ell^{\pm}\nu\ell\ell (\ell=e,\mu)$ process at CMS, using full detector simulation of the signal and of the various physics and instrumental backgrounds. We propose data driven methods to extract backgrounds and estimate that we can reach 5$\sigma$ significance of WZ signal at CMS with less than 350 pb$^{-1}$ at 95 \% C.L.

Brigljevic, Vuko

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Available on CMS information server CMS CR -2009/031The Compact Muon Solenoid ExperimentMailing address: CMS CERN, CH-1211 GENEVA 23, SwitzerlandConference Report14 January 2009 (v2, 19 January 2009)Measurement of diboson production in early LHCdataVuko Brigljevicon behalf of the CMS CollaborationAbstractWe investigate the discovery potential for the pp → WZ → `±ν``(` = e, µ) process at CMS, usingfull detector simulation of the signal and of the various physics and instrumental backgrounds. Wepropose data driven methods to extract backgrounds and estimate that we can reach 5σ significance ofWZ signal at CMS with less than 350 pb−1 at 95 % C.L..Presented at Physics at LHC,29.9-4.10.2008,Split,Croatia,15/01/2008Measurement of diboson production in early LHCdataVuko Brigljevic´ ∗Rudjer Boskovic InstituteE-mail: vuko.brigljevic@irb.hrWe investigate the discovery potential for the pp →WZ0 → `±ν`+`−(` = e,µ) process at CMS,using full detector simulation of the signal and of the various physics and instrumental back-grounds. We propose data driven methods to extract backgrounds and estimate that we can reach5σ signicance of WZ0signal at CMS with less than 350 pb−1at 95 % C.L..2008 Physics at LHCSeptember 29 - 4 October 2008Split, Croatia∗On behalf of the CMS Collaborationc© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/Measurement of diboson production in early LHC data1. IntroductionThe study of multiple gauge-boson production at the TeV scale constitutes a unique opportu-nity to test the Standard Model of Electroweak interactions at the highest possible energies. Pro-duction cross sections for all diboson processes, W Z0, Z0Z0, WW , Wγ and Z0γ are relatively largeand these processes are expected to be observed already with luminosities of a few hundreds inversepicobarns at the LHC.A study of W Z0 production in pp collisions at a center of mass energy of 14TeV is presentedin the following. The production of W Z0 events allows probing triple gauge-boson couplings and,therefore, non-Abelian gauge symmetry of the Standard Model at energies never before attained.Any deviation of the strength of these couplings from their Standard Model expectations indicatesnew physics.We consider the four leptonic final states involving electrons and muons, labeled in the fol-lowing as 3e, 2e1µ , 2µ1e and 3µ .We use signal and background Monte Carlo samples that are processed with the full simulationof the CMS detector [1]. The backgrounds are processes with three or more genuine leptons ormisidentified jets in the final state. The largest instrumental backgrounds to all four signatures isdue to misidentified jets from associated production of jets and heavy bosons and production oftop quarks. Among these backgrounds, the main contribution to the W Z 0 signal is from Z0+jetsprocesses. The only irreducible backgrounds to the WZ0 final state are the Z0Z0 production withone of the leptons being mis-reconstructed or lost and Z0γ process where a photon converts andproduces a genuine electron.2. Event SelectionA key element for this study is an effective selection of electrons and muons. The selectioncriteria must have high efficiency for true isolated leptons from heavy boson decays, while atthe same time effectively suppressing leptons from heavy quark decays as well as other objectsmisidentified as leptons.The identification of electrons is based on matching of a charged track reconstructed in the cen-tral tracker with an energy deposition (supercluster) in the electromagnetic calorimeter (ECAL) [2].Suppression of fake electrons is achieved by applying requirements on the shape of the ECALenergy deposition, on the matching in position and energy between the charged track and the su-percluster. Muon candidates are identified by matching a track reconstructed in the muon detec-tors with a track reconstructed in the central tracker. To further suppress backgrounds originatingmainly from genuine muons from heavy quark decays, the significance of the impact parameterin the transverse plane of a muon candidate must also be consistent with a track originating fromthe primary interaction point. Furthermore, requirements on calorimetric and track isolation areapplied for both electron and muon candidates.Events stemming from the three-lepton final states of W Z0 production are collected by electronand/or muon triggers. Events are accepted if they contain at least three charged leptons, eitherelectrons or muons, with pT > 15GeV and |η | < 2.5 (2.4) for electrons (muons). To select W Z02Measurement of diboson production in early LHC datacandidates we form all possible Z0-boson candidates from same-flavour, opposite-charge leptonpairs.Events are retained if the invariant mass of the Z0 boson candidate is between 50 and 120 GeV.The event is rejected if a second Z0 boson candidate is found with an invariant mass within 20 GeVaround the nominal Z0 mass. If two Z0 boson candidates are found which share one of the leptons,we select the candidate with the invariant mass closest to the nominal Z 0 mass.After the Z0 boson candidate is identified, we search for a third lepton in the event to be associ-ated to the W boson decay. This lepton is required to carry a transverse momentum above 20 GeV.If more than one lepton candidate is available, the one with the highest transverse momentum ischosen.Further suppression of the background can be achieved by applying W identification require-ments. We define a W boson candidate transverse mass asMT (W ) =√2 ·MET ·E`(1− cos∆φMET,`), (2.1)where MET is the missing transverse energy, E` is the energy of the W lepton candidate, and∆φMET,` is the azimuthal separation between the MET direction and the W lepton. We require thetransverse mass to be larger than 50 GeV. The event yield for the signal and background eventsafter the full selection is summarized in Table 1.all 3e 2e1µ 2µ1e 3µWZ0 34.9±0.5 7.9±0.3 8.0±0.3 8.9±0.3 10.1±0.3Z0 + jets 3.9±1.0 1.9±0.8 < 0.1 1.8±0.7 0.1±0.1bbll 2.9±0.3 1.2±0.2 0.1±0.1 1.3±0.2 0.3±0.1tt¯ + jets 2.2±0.6 0.6±0.3 0.6±0.3 0.6±0.3 0.3±0.2W + jets 0.4±0.4 0.4±0.4 < 0.1 < 0.1 < 0.1Z0Z0 2.8±0.3 0.8±0.1 0.6±0.1 0.7±0.1 0.7±0.1Z0 + γ 1.4±0.1 0.7±0.1 < 0.1 0.7±0.1 < 0.1Total non genuine Z0bkg 2.6±0.7 1.0 ±0.5 0.6±0.3 0.6±0.3 0.3±0.2Total genuine Z0instrumental bkg 6.8 ±1.0 3.1±0.8 0.1±0.1 3.1±0.5 0.4±0.1Total genuine Z0physics bkg 4.2 ± 0.3 1.5±0.1 0.6±0.1 1.4±0.1 0.7± 0.1Table 1: Expected number of selected events for an integrated luminosity of 300 pb−1for the signal andbackground with 81 GeV < MZ < 101 GeV obtained using MC truth information. The uncertainties arestatistical.3. Signal ExtractionWe separate backgrounds into three categories: physics background from Z 0γ and Z0Z0 pro-duction, processes without a genuine Z0 boson from tt¯ + jets and W + jets production, and pro-cesses with a genuine Z0 boson from Z0 + jets production.The physics background from Z0Z0 and Z0γ production is estimated from Monte Carlo sim-ulation. We assign a conservative 100% uncertainty on the estimated Z 0Z0 and Z0γ contribution3Measurement of diboson production in early LHC datadue to modeling of kinematics of the decay products and the photon conversion probability. Wealso use the Monte Carlo simulation to estimate the background from from t t¯ + jets and W + jetsprocesses, and we assign an additional 100% systematic uncertainty on that contribution. Withincreasing luminosity, this contribution can be estimated from the side-bands of the Z 0 candidateinvariant mass distribution.The remaining and also the largest instrumental background is from processes with real Z 0boson and jets where one of the jets is misidentified as a lepton. We estimate this background indata by applying a method, commonly referred to as a “matrix method” (see for example [3]). Theidea of the method is to apply a “loose” identification criteria on the lepton associated to the Wboson decay, after a Z0 boson candidate is identified and count the number of observed events,Nloose. These events contain events with real isolated leptons N` and events with misidentified jetsN j:Nloose = N` +N j. (3.1)We then apply a “tight” selection on the third lepton, the number of observed events Ntight changesas followingNtight = εtight N` + p f akeN j, (3.2)where εtight and p f ake are efficiency of “tight” criteria with respect to “loose” requirements for trueisolated leptons and misidentified jets, respectively. The “tight” selection uses the final selectioncriteria applied on the third lepton as used in the analysis, while some of these requirements areloosened for the “loose” sample.As Nloose and Ntight are directly observable, to extract the number of Z + jets events in the finalsample, one needs to measure εtight and p f ake in control data samples. To estimate εtight we applythe “tag-and-probe” method [4] using Z0 → e+e− or Z0 → µ+µ− events. The determination ofp f ake is done on a control-sample of jets misidentified as leptons. The jet composition (heavy vslight flavour, quark vs gluon jets) of the control sample must be similar to the one of the Z 0 + jetsevents passing the signal selection. Thus, we propose to measure p f ake in a W + jets sample whichhas a similar jet composition as in Z0 + jets. The obtained p f ake is further cross-checked with p f akemeasured in multijet events. We performed both measurements and found the p f ake values to agreewithin uncertainties.Using the values of εtight and p f ake obtained from the methods described above, we estimatedthe backgrounds from genuine Z0decays by solving Eqs. 3.1 and 3.2 for N j. The results are givenin Table 2. They agree well with the true Monte Carlo yields.4. ResultsThe distribution of the Z0candidate invariant mass for all four channels combined after apply-ing the final selection is shown in Fig. 1.We estimated full systematic uncertainties for each of the four individual signature, theyamount to 34%, 25%, 35% and 21% for the 3e, 2e1µ , 2µ1e and 3µ final states, respectively.The largest contributions come from the luminosity determination and the requirement on MT (W ).It is comparable to the statistical uncertainty which is roughly 30% for each channel.4Measurement of diboson production in early LHC data3e 2e1µ 2µ1e 3µN - ZZ -Zγ - W+jets - tt¯ 11.1 ± 1.3 8.2 ± 0.9 12.1 ± 1.2 10.5±0.8Ngenuine Z (matrix method) 3.2 ± 1.7 0.6 ± 0.8 4.6 ± 2.0 0.6 ± 0.9NWZ0 7.9 ± 2.1 7.6 ± 1.2 7.5 ± 2.3 10.0 ± 1.2WZ0from MC 7.9 8.1 9.0 10.1Table 2: Expected number of events for an integrated luminosity of 300 pb−1for the signal and estimatedbackground for 81 GeV < MZ < 101 GeV using data-driven methods. Uncertainty is systematic associatedwith the background subtraction method only. (GeV)llM40 50 60 70 80 90 100 110 120Events / 2.5 GeV02468101214WZZ+jetsγZZbbZZttbar+jetsW+jetsCMS PreliminaryEvents / 2.5 GeVFigure 1: Z0candidate invariant mass for all fourchannels combined, normalized to integrated lu-minosity of 300 pb−1.)−1Luminosity (fb0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5LS02468101214LS95% C.L68% C.LCMS PreliminaryFigure 2: Expected signal signicance forWZ0production as a function of integrated lu-minosity. We use a frequentist approach to es-timate variation of expected signal and back-ground events. The corresponding 68% and95% C.L. regions are displayed as red and greenbands, respectively.The expected signal significance SL of a W Z0 production as a function of integrated luminosityfor all four categories combined, after applying final selection criteria and requirement on the Z 0invariant mass to be within 10 GeV from the nominal Z0 boson mass, taking into account fullsystematic and statistical uncertainties is shown in Fig. 2. We expect to achieve SL = 5 with lessthan 350 pb−1of integrated luminosity at 95% C.L..References[1] R. Adolphi et al. [CMS Collaboration], “The CMS experiment at the CERN LHC,” JINST 3 (2008)S08004[2] S. Bafoni et al., Electron Reconstruction in CMS, CMS Note 2006-40 (2006).[3] D0 Collaboration, V. M. Abazov et al., Zγ production and limits on anomalous ZZγ and Zγγcouplings in p flp collisions at√s = 1.96- TeV, Phys. Lett. B653 (2007) 378386.[4] CMS Collaboration, Measuring Electron Efciencies at CMS with Early Data, CMS PASEGM-07-001 (2007).5

Measurement of diboson production in early LHC data

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