| 1 |
|
| 2 |
This note presents a search for the production of supersymmetric (SUSY) stop quark pairs in events with a
|
| 3 |
single isolated lepton, several jets, missing transverse energy, and large transverse mass. We use the full
|
| 4 |
2011 data sample, corresponding to an integrated luminosity of \lumi.
|
| 5 |
This search is of theoretical interest because of the critical role played
|
| 6 |
by the stop quark in solving the hierarchy problem in SUSY models. This solution requires that the stop quark
|
| 7 |
be light, less than a few hundred GeV and hence within reach for direct pair production. We focus on two decay modes
|
| 8 |
$\tilde{t}\rightarrow t\chi^0_1$ and $\tilde{t}\rightarrow b \chi^+_1$ which are expected
|
| 9 |
to have large branching fractions if they are kinematically accessible, leading to:
|
| 10 |
|
| 11 |
\begin{itemize}
|
| 12 |
\item $pp\rightarrow\tilde{t}\bar{\tilde{t}}\rightarrow t\bar{t}\chi^0_1\chi^0_1$, and
|
| 13 |
\item $pp\rightarrow\tilde{t}\bar{\tilde{t}}\rightarrow b\bar{b}\chi^+_1\chi^-_1 \rightarrow b\bar{b}W^+W^-\chi^0_1\chi^0_1$.
|
| 14 |
\end{itemize}
|
| 15 |
|
| 16 |
Both of these signatures contain high transverse momentum (\pt) jets including two b-jets, and missing transverse
|
| 17 |
energy (\met) due to the invisible $\chi^0_1$ lightest SUSY particles (LSP's). In addition, the presence of
|
| 18 |
two W bosons leads to a large branching fraction to the single lepton final state. Hence we require the presence
|
| 19 |
of exactly one isolated, high \pt electron or muon, which provides significant suppression of several backgrounds
|
| 20 |
that are present in the all-hadronic channel. The largest backgrounds for this signature are semi-leptonic \ttbar\
|
| 21 |
and \wjets. These backgrounds contain a single leptonically-decaying W boson, and the transverse mass (\mt)
|
| 22 |
of the lepton-neutrino system has a kinematic endpoint requiring \mt $<$ $M_W$. For signal stop quark events,
|
| 23 |
the presence additional LSP's in the final states allows the \mt to exceed $M_W$. Hence we search for an excess
|
| 24 |
of events with large \mt. The dominant background in this kinematic region is dilepton \ttbar\ where one of the
|
| 25 |
leptons is not identified, since the presence of two neutrinos from leptonically-decaying W bosons allows the
|
| 26 |
\mt\ to exceed $M_W$. Backgrounds are estimated from Monte Carlo (MC) simulation, with careful validation
|
| 27 |
and determination of scale factors and corresponding uncertainties based on data control samples.
|
| 28 |
|
| 29 |
The expected stop quark pair production cross section (see Fig.~\ref{fig:stopxsec}) varies between O(10) pb
|
| 30 |
for $m_{\tilde{t}}=200$~GeV and O(0.01) pb for $m_{\tilde{t}}=500$~GeV. The critical challenge of this analysis
|
| 31 |
is due to the fact that for light stop quarks with a mass close to the top quark, the production cross section is
|
| 32 |
large but the kinematic distributions, in particular \mt, are very similar to SM \ttbar\ production, such that it becomes very
|
| 33 |
difficult to distinguish the signal and background. For large stop quark mass the kinematic distributions differ
|
| 34 |
from those in SM \ttbar\ production, but the cross section decreases rapidly, reducing the signal-to-background
|
| 35 |
ratio.
|
| 36 |
|
| 37 |
\begin{figure}[hbt]
|
| 38 |
\begin{center}
|
| 39 |
\includegraphics[width=0.5\linewidth]{plots/total_scale_pdf_LHC.eps}
|
| 40 |
\caption{
|
| 41 |
\label{fig:stopxsec}\protect
|
| 42 |
The stop quark pair production cross section in pb, as a function of the stop quark mass.}
|
| 43 |
\end{center}
|
| 44 |
\end{figure}
|
| 45 |
|
