| 4 |
|
This section presents the results of a dedicated search for the direct pair production of top squarks, based on an integrated luminosity of 9.7~fb$^{-1}$. |
| 5 |
|
The decay of the top squark depends on the difference between its mass and that of the \lsp\ LSP, |
| 6 |
|
$\Delta m = m_{\tilde{t}}-m_{\lsp}$. If $\Delta m > m_{t}$, the decay $\tilde{t}\to t\lsp$ is expected |
| 7 |
< |
to have a large branching fraction. If there is a light chargino \chip, the decay |
| 7 |
> |
to have a large branching fraction. If there is a light chargino \chipm, the decay |
| 8 |
|
$\tilde{t}\to b\chip\to b W \lsp$ is expected to be significant, especially in the $\Delta m < m_{t}$ region. |
| 9 |
|
The pair production of top squarks decaying to either of these channels leads to events with two b-jets, two W bosons, |
| 10 |
|
and two LSPs. Our signal thus resembles SM $t\bar{t}$ production but with larger \met\ from |
| 44 |
|
where $p_{T}^{\ell}$ is the lepton transverse momentum and $\Delta\phi$ is the difference in azimuthal angles between the lepton |
| 45 |
|
and \met. This requirement strongly suppresses the background from \ttljets\ and \wjets, which have a kinematic endpoint |
| 46 |
|
at \mt\ $=$ $M_W$ since the lepton and neutrino (which produces the \met) are produced together in the decay of the W. |
| 47 |
< |
For signal events, as well as the \ttll\ background, the presence of more than one invisible |
| 47 |
> |
For signal events, as well as for the \ttll\ background, the presence of more than one invisible |
| 48 |
|
particle in the final state leads to events with \mt\ $>>$ $M_W$. |
| 49 |
|
In addition to the \mt\ requirement, we make several |
| 50 |
|
\met\ requirements to achieve sensitivity to signals with different mass spectra. |
| 57 |
|
requirements. The latter two categories are suppressed by vetoing events that contain, in addition to the selected lepton, |
| 58 |
|
a charged particle with \pt\ $>$ 10 GeV that is isolated in space from other energetic charged particles. Furthermore, additional jets |
| 59 |
|
from initial state or final state radiation (ISR/FSR) are required to satisfy the jet multiplicity requirement $n_{jets}\geq4$. |
| 60 |
< |
To validate and correct the MC modeling of jets from radiation, the MC is compared to data in a dilepton control region dominated by \ttll. The MC distribution of $n_{jets}$ is reweighted to |
| 61 |
< |
match the corresponding data distribution, resulting in corrections of (1--7)\%. |
| 60 |
> |
To validate and correct the MC modeling of jets from radiation, the MC is compared to data in a dilepton control region dominated by \ttll. |
| 61 |
> |
The MC distribution of $n_{jets}$ is reweighted to match the corresponding data distribution, resulting in small corrections of (1--7)\%. |
| 62 |
|
|
| 63 |
|
The SM backgrounds are estimated from events simulated with Monte Carlo (MC) techniques, which are validated and |
| 64 |
|
(if necessary) corrected using comparisons to data in control regions. The MC expectation is normalized to data in the \mt\ peak region, |
| 74 |
|
%\subsection{Results} |
| 75 |
|
|
| 76 |
|
The results of the search are summarized in Table~\ref{tab:stop}, which displays the SM background expectations and the observed data yields |
| 77 |
< |
in the signal regions. The distribution of \met\ after the requirement \mt\ $>$ 120 GeV for the SM background expectations is compared to |
| 78 |
< |
data in Fig.~\ref{fig:stop}. Good agreement between the data and the expected background is observed. We find no evidence |
| 77 |
> |
in the signal regions. The distribution of \met\ after the requirement \mt\ $>$ 120 GeV is displayed in Fig.~\ref{fig:stop}. |
| 78 |
> |
Good agreement between the data and the expected background is observed. We find no evidence |
| 79 |
|
for the pair production of top squarks. |
| 80 |
|
|
| 81 |
|
\begin{figure} |
| 98 |
|
%We consider $x=0.5$ and $x=0.75$ (we do not have sensitivity to the $x=0.25$ scenario). |
| 99 |
|
|
| 100 |
|
To interpret the results of our search, we consider top squark pair production where both top squarks decay according to |
| 101 |
< |
$\tilde{t}\to t\lsp$ as depicted in Fig.~\ref{fig:diagrams}(a). |
| 101 |
> |
$\tilde{t}\to t\lsp$, in Fig.~\ref{fig:stop_interpretation}. |
| 102 |
|
The model is parameterized by the masses of the top squark and \lsp. We place upper limits on the signal |
| 103 |
|
production cross section using, for each model point in the 2-dimensional parameter space, the signal region with the best expected |
| 104 |
|
sensitivity. A region of the parameter space is excluded by comparing these cross section upper limits with the theoretical predictions |
| 109 |
|
Our results probe top squarks with masses up to 430 GeV. For comparison, the requirement that SUSY |
| 110 |
|
provides a natural solution to the hierarchy problem suggests top squarks with masses not exceeding 500--700 GeV~\cite{ref:naturalsusy}. |
| 111 |
|
We also interpret our results assuming the top squark decays according to $\tilde{t}\to b\chip\to b W \lsp$, |
| 112 |
< |
as depicted in Fig.~\ref{fig:diagrams}(b)~\cite{ref:stop}. |
| 112 |
> |
as depicted in Fig.~\ref{fig:diagrams}(b); see Ref.~\cite{ref:stop}. |
| 113 |
|
|
| 114 |
|
The ATLAS experiment has presented a similar search for top squark pairs in the single lepton final state~\cite{ref:atlasstop}. |
| 115 |
|
The constraints from ATLAS on the top squark mass are more stringent than those presented here. The ATLAS model assumes large |
