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\section{Introduction}
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\label{sec:intro}
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%Even though the Standard Model (SM) provides a very successful
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%description of experimental particle physics data, it is
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%expected that new physics may manifest itself at the TeV energies of
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%the LHC.
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Supersymmetry (SUSY) is a popular extension of the SM that
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postulates for each SM particle the existence of a superpartner with
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the same quantum numbers but differing by one half unit of spin.
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Searches for light top squarks (or stops) are particularly
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interesting because they cancel the quadratically divergent contribution to the Higgs
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mass from the SM top quark, which has a large Yukawa coupling to the
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Higgs.
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Existing searches for direct stop production at the LHC have mainly focused on scenarios where the $\tilde{t}_{1}$ is accessible,
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and the stop decays to $\tilde{t}_{1}\rightarrow t\tilde{\chi}^0_1 \rightarrow b \PW \tilde{\chi}_1^0$ or
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$\tilde{t}_{1}\rightarrow b \tilde{\chi}^+_1 \rightarrow b \PW \tilde{\chi}_1^0$.
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These two decay modes are expected to have large branching fractions
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if kinematically allowed and, in the latter case, if a light
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$\tilde{\chi}^+_1$ exists. These searches have been performed by
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the CMS~\cite{LHCPpas} and ATLAS collaborations~\cite{ATLAS1,ATLAS2,ATLAS3,ATLAS4,ATLAS5} at the LHC,
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and by the CDF \cite{CDFstop} and D0 \cite{D0stop} collaborations at the Tevatron.
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As discussed in Ref~\cite{LHCPpas}, for the $\tilde{t}_{1}\rightarrow
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t\tilde{\chi}^0_1$ decay mode, existing searches have reduced sensitivity
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to the region where the mass difference between the stop and the
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$\tilde{\chi}_1^0$ is very close to the top mass ($m(\tilde{t}_{1}) - m(\tilde{\chi}^0_1) \sim
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m(t)$). This is a very
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challenging kinematic region since the stop decay products are
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produced at rest, giving rise to a signature of $\ttbar$ with little
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additional $\met$, making it difficult to distinguish stop pairs from
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SM $\ttbar$.
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%Another interesting and challenging region is that where
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%the stop decays to an off-shell top.
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The analysis described here seeks to access some of these regions of phase
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space that are unexplored in the existing direct stop searches.
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In particular, it considers the possibility that the $\tilde{t}_{2}$,
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also required to cancel the top quark contributions to the Higgs mass,
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is also accessible.
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The strategy is therefore to target a more complex topology which provides
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additional handles to suppress the $\ttbar$ background.
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%, since it may be more easily observed than $\tilde{t}_{1}$.
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%It should be noted that a light $\tilde{t}_{2}$ is also favored in natural SUSY scenarios.
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The $\tilde{t}_{2}$ may decay to $\tilde{t}_{1}$ and a Higgs boson, leading to the following process of interest:
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$$\tilde{t}_{2}\tilde{t}_{2}^{*} \rightarrow HH
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\tilde{t}_{1}\tilde{t}_{1}^{*} \rightarrow HH \ttbar \tilde{\chi}_1^0
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\tilde{\chi}_1^0.$$ The $\tilde{t}_1$ subsequently decays
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$\tilde{t}_{1}\rightarrow t\tilde{\chi}^0_1 \rightarrow b \PW
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\tilde{\chi}_1^0$.
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The resulting process is shown in Fig.~\ref{fig:T6tthhdiagram}.
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%The presence of two Higgs bosons in the final state can provide powerful discrimination between signal and the dominant $\ttbar$ background.
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This analysis searches for an excess of \ttbar + HH events with little
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additional \met, in order to gain sensitivity to scenarios where $m(\tilde{t}_{1}) - m(\tilde{\chi}^0_1) \sim
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m(t)$. Given the large branching fraction of the Higgs to $\bbbar$,
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this analysis focuses on Higgs decays to final states involving at
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least one $b\bar{b}$ resonance. Finally, the leptonic decay modes of
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$\ttbar$ are considered, either lepton plus jets (\ttbar
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$\rightarrow \ell \nu jjbb$) or dileptons plus jets (\ttbar
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$\rightarrow \ell \nu \ell \nu bb$). In summary, the signature is
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characterized by the presence of one or two leptons, multiple jets and
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high b-jet multiplicities.
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\begin{figure}[hbt]
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\begin{center}
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\includegraphics[width=0.45\linewidth]{plots/T6tthh.pdf}
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\caption{
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\label{fig:T6tthhdiagram}
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Diagram for $\tilde{t}_{2}$ pair production in the $\tilde{t}_{2}
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\rightarrow H \tilde{t}_{1}$ decay mode, where the lightest stop
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subsequently decays $\tilde{t}_{1} \rightarrow \mathrm{t} \tilde{\chi}_1^0$.}
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\end{center}
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\end{figure}
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{\bf FIXME: ADD REFERENCE TO THE ATLAS RESULT WITH Zs SOMEPLACE?}
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