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\section{Introduction}
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\label{ref:intro}
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The published analysis
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%``A Search For New Physics in Z + Jets + MET using MET Templates'' %AN/old title
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%arxiv title
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``Search for physics beyond the standard model in events with a Z boson, jets,
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and missing transverse energy in pp collisions at $\sqrt{s}$ = 7 TeV''
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(SUS-11-021)
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searches for new physics in the final state of a
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leptonically ($ee$ and $\mu\mu$) decaying Z boson, two or more jets and
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missing transverse energy (\MET)
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\cite{ref:oszpaper} \cite{ref:osznote} \cite{ref:oszpas}.
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This analysis will be referred to throughout this note as the ``nominal''
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analysis. The basic analysis strategy is to select Z bosons and
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use data-driven methods to predict the \MET\ distribution in the signal
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regions.
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The Z+Jets background is predicted using the
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\MET\ templates method \cite{ref:templates1}\cite{ref:templates2}, the
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\ttbar\ background is predicted using opposite flavor ($e\mu$) events,
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and the diboson (WZ, ZZ) background is taken from Monte Carlo.
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The analysis presented in this note is a straightforward extension of
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the nominal analysis in that the analysis strategy and methodology
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remain unchanged.
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The only changes with respect to the nominal analysis are the addition
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of cuts to increase sensitivity to new physics with
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diboson production (WZ and/or ZZ) and \MET .
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An example of one such new physics scenario is the electroweak
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production of SUSY particles.
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In a generic SUSY framework, the neutralinos
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(for example, $\chi_2^0$ or $\chi_1^0$)
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may decay to a Z boson and another neutral SUSY particle such as the LSP.
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%Although SUSY production involving strongly interacting particles
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%(such as gluinos and squarks) is normally targeted due to its expected
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%larger production cross section as compared with electroweak production,
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%such searches have as of yet failed to discover new physics.
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%Another logical search is for electroweak production,
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%and in this case, a final state involving Z bosons is a natural place
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%to start since leptonically decay Zs are an extremely clean signature.
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In the case in which a neutralino is pair produced, the final state
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may be ZZ+\MET. In addition, production of a neutralino and chargino
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may lead to a final state of WZ+\MET.
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When the Z decays leptonically and the other boson (either
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a W or Z) decays to jets, the final state is Z plus two jets plus \MET,
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to which the nominal \MET\ templates analysis is particularly well suited.
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Given that we are now searching for the specific final states WZ plus \MET\ or ZZ plus \MET ,
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rather than the more general Z plus jets plus \MET\ signature,
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we can apply additional cuts to increase the sensitivity.
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In the nominal analysis, the search is performed in the high \MET\ tail.
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\MET\ cuts used for signal regions are 100, 200, and 300 GeV.
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At such high \MET\ cuts, \ttbar\ background in which (the same-flavor
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opposite-sign) dileptons happen to fall in the Z mass window dominate.
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Because all \ttbar\ events contain b jets, a b jet veto is
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very effective in suppressing this background.
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Because the final state targeted involves the decay of W (Z) to jets,
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the dijet mass peaks at the W (Z) mass. In contrast, the jets from the
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background processes Z+jets and \ttbar\ have a very broad distribution.
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The dijet mass is therefore a variable which can further discriminate between
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signal and background (see section \ref{sec:eventSelection}).
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The final sub-leading backgrounds in the nominal analysis are Z plus jets
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and dibosons (WZ and ZZ). In the case of WZ, real \MET\ is produced from
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the leptonically decaying W. In order to suppress this background, we
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introduce a veto on events with three or more leptons (e or $\mu$).
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In summary, we use the same selection as in the approved analysis SUS-11-021 and
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place three additional requirements:
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\begin{itemize}
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\item Veto events containing a b-tagged jet
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\item Require a dijet mass consistent with the hadronic decay of a W/Z boson
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\item Veto events with three or more leptons (e or $\mu$)
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\end{itemize}
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This note is organized as follows.
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In Sec.~\ref{sec:datasets} we review the datasets used.
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In Sec.~\ref{sec:eventSelection} we discuss the event selection.
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In Sec.~\ref{sec:yields} we present the data and MC yields passing the event preselection.
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In Sec.~\ref{sec:sigregion} we define the signal regions.
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In Sec.~\ref{sec:results} we present the results.
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In Sec.~\ref{sec:systematics} we discuss systematics on the background predictions.
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In Sec.~\ref{sec:bsm} we provide a new physics interpretation of the results.
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Additional material is included in the following appendices:
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supplemental results (App. \ref{app:results}),
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supplemental interpretation (App. \ref{sec:app_bsm}),
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kinmatical distributions (App. \ref{sec:appkin}),
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combination of interpretation results (App. \ref{app:combo}),
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and the \MET\ templates (App. \ref{sec:appendix_templates}).
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%OLD NOTE
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\begin{comment}
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In this note we describe a search for new physics in the 2011
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opposite sign isolated dilepton sample ($ee$, $e\mu$, and $\mu\mu$).
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The main sources of high \pt isolated dileptons at CMS are Drell Yan and \ttbar.
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Here we concentrate on dileptons with invariant mass consistent
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with $Z \to ee$ and $Z \to \mu\mu$. A separate search for new physics in the non-\Z
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sample is described in~\cite{ref:GenericOS}.
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We search for new physics in the final state of \Z plus two or more jets plus missing
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transverse energy (MET). We reconstruct the \Z boson
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in its decay to $e^+e^-$ or $\mu^+\mu^-$. Our search regions are defined as
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MET $\ge$ \signalmetl~GeV (loose signal region), MET $\ge$ \signalmett~GeV
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(medium signal region), MET $\ge$ 300~GeV (tight signal region), and two or more jets. We use data driven techniques to predict the
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standard model background in these search regions.
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Contributions from Drell-Yan production combined with detector mis-measurements that
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produce fake MET are modeled via MET templates based on photon plus jets or QCD events.
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Top pair production backgrounds, as well as other backgrounds for which the lepton
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flavors are uncorrelated such as $W^+W^-$, DY$\rightarrow\tau\tau$, and single top, are
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modeled via $e^\pm\mu^\mp$ subtraction.
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As leptonically decaying \Z bosons are a signature that has very little background,
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they provide a clean final state in which to search for new physics.
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Because new physics is expected to be connected to the Standard Model Electroweak sector,
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it is likely that new particles will couple to W and Z bosons.
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For example, in mSUGRA, low $M_{1/2}$ can lead to a significant branching fraction
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for $\chi_2^0 \rightarrow Z \chi_1^0$.
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In addition, we are motivated by the existence of dark matter to search for new physics with MET.
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Enhanced MET is a feature of many new physics scenarios, and R-parity conserving SUSY
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again provides a popular example. The main challenge of this search is therefore to
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understand the tail of the fake MET distribution in \Z plus jets events.
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The basic idea of the MET template method~\cite{ref:templates1}\cite{ref:templates2} is
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to measure the MET distribution in data in a control sample which has no true MET
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and a similar topology to the signal events.
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%Start the qcd vs photon discussion
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Templates can be derived from either a QCD sample (as was done in the original implementation)
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or a photon plus jets sample.
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%In our case, we choose a photon sample with two or more jets as the control sample.
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%Both the control sample and signal sample consist of a well measured object (either a
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%photon or a leptonically decaying $Z$), which recoils against a system of hadronic jets.
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In both cases, the instrumental MET is dominated by mismeasurements of the hadronic system,
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and can be classified by the number of jets in the event and the scalar sum of their transverse
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momenta.
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The prediction is made such that the jet system in the control sample is similar to that of the
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signal sample.
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By using two independent control samples--QCD and photon plus jets--we are able to illustrate
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the robustness of the MET templates method and to cross check the data driven background
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prediction.
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This note is organized as follows.
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In sections \ref{sec:datasets} and \ref{sec:trigSel} we descibe
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the datasets and triggers used, followed by the detailed object definitions (electrons, muons, photons,
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jets, MET) and event selections in sections \ref{sec:evtsel} through \ref{sec:jetsel}.
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We define a preselection and compare data vs. MC yields passing this preselection in
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Section~\ref{sec:yields}.
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We then define the signal regions and show the number of observed events and MC expected
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yields in Section~\ref{sec:sigregion}.
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Section~\ref{sec:templates} introduces the MET template method and discusses its derivation
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in some detail.
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% and is followed by a demonstration in Section~\ref{sec:mc}
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%that the method works in Monte Carlo.
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Section~\ref{sec:topbkg} introduces the top background estimate based on opposite flavor subtraction,
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and contributions from other backgrounds are discussed in Section~\ref{sec:othBG}.
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Section~\ref{sec:results_combined} shows the results for applying these methods in data.
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We analyze the systematic uncertainties in the background predictions and in signal acceptance
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in Section~\ref{app:systematics}.
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We then proceed to calculate upper limit on the BSM physics processes
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in Section~\ref{sec:bsm}.
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%Efficiencies which can be used to test specific models of new physics are given
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%in Section \ref{sec:outreach}.
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%Finally, in Section~\ref{sec:models} we calculate upper limits on the quantity \sta\
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%assuming efficiencies and uncertainties from sample benchmark SUSY processes.
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\end{comment}
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