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The major background to $WZ\to\ell^+\ell^- e\nu_e$ final state is
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$Z$+jets, where one of the jets is misidentified as an electron. The
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focus of this study is to develop a powerful electron identification
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criteria that allows us to suppress jet$\to e$ process. To simplify
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this study, we constrain ourselves to $Z \to \mu^+\mu^-$ decay
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channel, and let $W$ boson to decay into an electron and a neutrino.
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We require an event to satisfy muon triggers which are 99\% efficient
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for $Z\to \mu^+\mu^-$ process. This unbiases the electron
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identification from the trigger and $Z$ boson reconstruction
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requirements, done entirely with muon sub-detector and reconstruction
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algorithms.
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We identify a $Z$ boson candidate by requiring an event to have
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two global muon candidates of opposite electric charge
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that satisfy track and calorimeter isolation criteria defined in the $WZ$
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analysis note~\cite{AnaNote}. The muon candidates are required
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to be in the fiducial volume of the muon sub-detector defined as $|\eta| < 2.4$ and have
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transverse momenta $p_T$ to exceed 15 GeV. The invariant
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dimuon mass is required to be within 40 GeV from the nominal
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$Z$ boson mass.
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The $W$ boson candidate is identified by requiring an event to have
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an electron candidate in the barrel or endcap parts of the ECAL defined
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in pseudorapidity ranges $|\eta| < 1.479$ and $1.55 < |\eta| < 2.5$,
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respectively. The electron candidate must have a $p_T > 20$ GeV.
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If there are more than one candidate, we select the one with the highest $p_T$.
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To keep the selection criteria as close as the one that is going to be
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applied in read data, we impose additional requirements as
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following. We apply a second $Z$ boson veto to suppress $ZZ$
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background. We veto an event if there is more than one pair of leptons
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of the same flavor and opposite electric charge in the window of 40
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GeV around the nominal $Z$ boson mass.
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