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benhoob |
1.1 |
\section{Search in the All-Hadronic Final State}
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\label{sec:alphat}
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benhoob |
1.3 |
\begin{figure*}[!ht]
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\centering
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%\begin{center}
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\begin{tabular}{cc}
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\includegraphics[width=0.45\textwidth]{HCPPlots/AlphaT_le3j_prelim.pdf} &
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\includegraphics[width=0.4\textwidth]{HCPPlots/hadronic_2b_le3j_logy.pdf} \\
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\end{tabular}
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\caption{
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The distribution of the \alphat\ variable (left) and the $H_T$ distribution in data, compared to the SM background expectation (right).
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\label{fig:alphat}
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}
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%\end{center}
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\end{figure*}
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benhoob |
1.2 |
The production of bottom squark pairs, followed by the decay $\tilde{b}\to b\lsp$, leads events with
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two b-jets and \met. In this section we report the results from a search in the all-hadronic final state using the
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benhoob |
1.1 |
\alphat\ variable, discussed below, which discriminates between backgrounds with real and fake \met.
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benhoob |
1.2 |
We require at least two jets with \pt\ $>$ 50 GeV. The leading (highest \pt) jet is required to be in the tracker
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acceptance defined by $|\eta|<2.5$, and the leading two jets must satisfy \pt\ $>$ 100 GeV. Events with isolated
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electrons or muons with \pt\ $>$ 10 GeV are vetoed, in order to suppress backgrounds with neutrinos from the decays
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of W bosons. Events with an isolated photon with \pt\ $>$ 25 GeV are vetoed.
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The remaining events are categorized based on the number of b-tagged jets (using CSVM) and the event $H_T$,
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which is required to satisfy $H_T$ $>$ 275 GeV.
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benhoob |
1.1 |
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The background satisfying the above preselection is dominated by QCD multijet production with fake \met\ from mismeasurement effects. To suppress this background,
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we make requirements on the quantity $\alphat$. For dijet events this quantity is defined as $\alphat \equiv E_{T}^{j_2} / M_{T}$, where $E_{T}^{j_2}$ is the $E_T$
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of the second leading jet and $M_T$ is the transverse mass of the dijet system.
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For events with perfectly measured jets, the measured \pt\ values of the two jets are equal, leading to $\alphat=0.5$. The key feature of the \alphat\ variable
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benhoob |
1.2 |
is that mismeasurement effects tend to decrease the value of \alphat, such that it is extremely rare for events with fake \met\ to have \alphat\
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benhoob |
1.1 |
much larger than 0.5; as shown in Fig.~\ref{fig:alphat}a, the \alphat\ distribution for the QCD multijet background falls off extremely rapidly near 0.5.
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For events with three of more jets, an equivalent dijet system is formed by clustering the jets into two pseudo-jets. In our search we strongly suppress the
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QCD multijet background with the requirement \alphat\ $>$ 0.55.
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The background after the \alphat\ requirement is dominated by processes with genuine \met, including \ttljets\ and \wjets\ with a lepton and neutrino from W decay,
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where the lepton is either not reconstructed or is a hadronically decaying $\tau$ lepton.
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These backgrounds are estimated using a $\mu+\rm{jets}$ data control sample.
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benhoob |
1.2 |
The additional background from $\rm{Z}(\nu\nu)+\rm{jets}$ is estimated using two data control samples of $\rm{Z}(\ell\ell)+\rm{jets}$ and $\gamma+\rm{jets}$ events. To estimate these backgrounds, the observed yields in the data control samples are extrapolated to the
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benhoob |
1.1 |
signal region using translation factors derived from MC. The dominant systematic uncertainty in the background prediction is the uncertainty
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in the MC translation factors, which are assessed by performing several closure tests in data, in which the observed yields in one data control region
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benhoob |
1.2 |
are used to predict the yields in another data control region.
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benhoob |
1.1 |
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Events are categorized based on the $H_T$, jet multiplicity, and b-tagged jet multiplicity. For the bottom squark scenario described above, the most sensitive
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category is events with either two or three jets and exactly two b-tagged jets. The $H_T$ distribution for these events is indicated in Fig.~\ref{fig:alphat}b,
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which demonstrates good agreement between the data and the expected background. No evidence for an excess of events is observed.
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benhoob |
1.2 |
The results are interpreted using the model of bottom squark pair production with $\tilde{b}\to b\lsp$ in Fig.~\ref{fig:ss_interpretation}(b).
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benhoob |
1.1 |
These results probe bottom squarks with masses up to approximately 600 GeV. Additional interpretations in models with gluino-mediated
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top and bottom squark pair production are presented in Ref.~\cite{ref:alphat}.
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