| 1 |
|
\section{Search for Direct Top Squark Pair Production in the Single Lepton Final State} |
| 2 |
|
\label{sec:stop} |
| 3 |
|
|
| 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 mass difference between the top squark and the LSP, |
| 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, |
| 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 |
| 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, followed be either of these decays, leads to events with 2 b-jets, 2 W bosons, |
| 10 |
< |
and \met\ from the invisible LSPs. Our signal thus resembles SM $t\bar{t}$ production but with larger \met. |
| 11 |
< |
The presence of two W bosons leads to a significant branching fraction to the single lepton final state, |
| 12 |
< |
which we focus on here because it has smaller SM backgrounds than the all-hadronic final state. |
| 13 |
< |
The analysis strategy is thus to select events with a single lepton and jets, and discriminate between |
| 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 \met\ from the invisible LSPs. Our signal thus resembles SM $t\bar{t}$ production but with larger \met\ from |
| 11 |
> |
the invisible LSPs. |
| 12 |
> |
We focus on the single lepton final state, which has a significant branching fraction due to the two W bosons, |
| 13 |
> |
and smaller SM backgrounds than the all-hadronic final state. |
| 14 |
> |
We thus select events with a single lepton and jets and discriminate between |
| 15 |
|
signal and background using \met\ and the transverse mass \mt, discussed below. |
| 16 |
|
|
| 17 |
< |
\subsection{Event Selection} |
| 17 |
> |
%\subsection{Event Selection} |
| 18 |
|
|
| 19 |
|
We require the presence of exactly one well-identified and isolated lepton (e or $\mu$) with transverse |
| 20 |
< |
momentum \pt\ $>$ 30 GeV and $|\eta|<1.44$ ($|\eta|<2.1$) for electrons (muons). |
| 21 |
< |
We select events with at least four jets with \pt\ $>$ 30 GeV and $|\eta|<2.5$, |
| 22 |
< |
which must be separated by $\Delta R \equiv \sqrt{(\Delta\eta)^2+(\Delta\phi)^2} < 0.4$ from the selected leptons. |
| 20 |
> |
momentum \pt\ $>$ 30 GeV. |
| 21 |
> |
We select events with at least four jets with \pt\ $>$ 30 GeV, |
| 22 |
> |
which must be well-separated from the selected leptons. |
| 23 |
|
At least one of these jets is required to be consistent with coming from the decay of heavy flavor hadrons, as |
| 24 |
|
identified by the Combined Secondary Vertex Medium Point (CSVM) b-tagging algorithm~\cite{ref:btag}. |
| 25 |
< |
The (b-)jet requirements suppress SM backgrounds from W bosons produced in association with jets from initial state |
| 25 |
> |
The jet requirements suppress SM backgrounds from W bosons produced in association with jets from initial state |
| 26 |
|
radiation (ISR), referred to as the \wjets\ background. |
| 27 |
|
The \met\ is required to exceed 50 GeV, suppressing the background from QCD multijet production. |
| 28 |
|
|
| 29 |
< |
\subsection{Backgrounds and Estimation Strategy} |
| 29 |
> |
%\subsection{Backgrounds and Estimation Strategy} |
| 30 |
|
|
| 31 |
|
The SM background satisfying the above requirements is dominated by $t\bar{t}$ production where |
| 32 |
|
one W boson decays hadronically and the other leptonically (\ttljets), or where both W bosons decay leptonically (\ttll). |
| 33 |
|
There is a small contribution from \wjets, as well as a variety of SM |
| 34 |
|
processes with small cross section, including $t\bar{t}$ produced in association with a vector boson |
| 35 |
< |
($t\bar{t}W$, $t\bar{t}Z$, $t\bar{t}\gamma$), processes with two (WW, WZ, ZZ) and three (WWW, WWZ, WZZ, ZZZ) elec- |
| 36 |
< |
troweak vector bosons, and single top production in the tW-channel mode (these small contributions are collectively |
| 35 |
> |
($t\bar{t}W$, $t\bar{t}Z$, $t\bar{t}\gamma$), processes with two (WW, WZ, ZZ) and three (WWW, WWZ, WZZ, ZZZ) electroweak |
| 36 |
> |
vector bosons, and single top production in the tW-channel mode (these small contributions are collectively |
| 37 |
|
referred to below as the ``rare'' background). |
| 38 |
|
|
| 39 |
< |
We define signal regions by requiring the events to have large transverse mass, defined as |
| 39 |
> |
To define signal regions, we require the events to have large transverse mass, defined as: |
| 40 |
|
|
| 41 |
|
\begin{equation} |
| 42 |
|
M_T = \sqrt{ 2 p_{T}^{\ell} \met ( 1-cos(\Delta\phi))}, |
| 43 |
|
\end{equation} |
| 44 |
|
|
| 45 |
|
where $p_{T}^{\ell}$ is the lepton transverse momentum and $\Delta\phi$ is the difference in azimuthal angles between the lepton |
| 46 |
< |
and \met. This requirement strongly suppresses the background from \ttljets\ and \wjets. For both of these background sources, the lepton |
| 47 |
< |
and neutrino which gives rise to the \met\ are produced together in the 2-body decay of the W, leading to the kinematic |
| 48 |
< |
endpoint \mt\ $<$ $M_W$. For signal events, as well as the \ttll\ background, the presence of more than one invisible |
| 49 |
< |
particle in the final state leads to events with \mt\ $>$ $M_W$. In addition to the \mt\ requirement, we make several |
| 46 |
> |
and \met. This requirement strongly suppresses the background from \ttljets\ and \wjets, which have a kinematic endpoint |
| 47 |
> |
at \mt\ $=$ $M_W$ since the lepton and neutrino (which produces the \met) are produced together in the decay of the W. |
| 48 |
> |
For signal events, as well as the \ttll\ background, the presence of more than one invisible |
| 49 |
> |
particle in the final state leads to events with \mt\ $>>$ $M_W$. |
| 50 |
> |
In addition to the \mt\ requirement, we make several |
| 51 |
|
\met\ requirements to achieve sensitivity to signals with different mass spectra. |
| 52 |
< |
Signal regions with a large \met\ requirement are more sensitive to signals with large $\Delta m$. |
| 52 |
> |
Signal regions with large (small) \met\ requirements are more sensitive to signals with large (small) values of $\Delta m$. |
| 53 |
|
|
| 54 |
|
The dominant background in our signal regions is \ttll, which may produce events with large \met\ and \mt\ due to the presence of |
| 55 |
|
two invisible neutrinos. In order for \ttll\ events to pass the signal region selection, one of the two W leptons must not be identified, |
| 56 |
< |
which occurs if one of the leptons is outside the acceptance, is a hadronic $\tau$ decaying to three charged particles (3-prong decay), |
| 56 |
> |
which occurs if it is outside the acceptance, is a hadronic $\tau$ decaying to three charged particles (3-prong decay), |
| 57 |
|
is a hadronic $\tau$ decaying to a single charged particle (1-prong decay), or is an electron or muon that fails the lepton identification |
| 58 |
< |
requirements. The latter two categories are suppressed by vetoing events that contain, in addition to the selected lepton, a charged particle |
| 59 |
< |
with \pt\ $>$ 10 GeV that is isolated in space from other energetic charged particles. Furthermore, additional jets from initial state or |
| 60 |
< |
final state radiation (ISR/FSR) are required to satisfy the jet multiplicity requirement $n_{jets}\geq4$. To validate and correct the MC modeling |
| 61 |
< |
of jets from radiation, the MC is compared to data in a control dominated by \ttll, obtained by selecting events with two reconstructed leptons, |
| 62 |
< |
moderate \met, and at least one b-jet. The MC distribution of $n_{jets}$ is reweighted to match the corresponding data distribution, resulting |
| 63 |
< |
in correction factors of a few \%. |
| 58 |
> |
requirements. The latter two categories are suppressed by vetoing events that contain, in addition to the selected lepton, |
| 59 |
> |
a charged particle with \pt\ $>$ 10 GeV that is isolated in space from other energetic charged particles. Furthermore, additional jets |
| 60 |
> |
from initial state or final state radiation (ISR/FSR) are required to satisfy the jet multiplicity requirement $n_{jets}\geq4$. |
| 61 |
> |
To validate and correct the MC modeling of jets from radiation, the MC is compared to data in a control region dominated by \ttll, obtained |
| 62 |
> |
by selecting events with two analysis leptons, moderate \met, and at least one b-jet. The MC distribution of $n_{jets}$ is reweighted to |
| 63 |
> |
match the corresponding data distribution, resulting in corrections of (1--7)\%. |
| 64 |
|
|
| 65 |
|
The SM backgrounds are estimated from events simulated with Monte Carlo (MC) techniques, which are validated and |
| 66 |
|
(if necessary) corrected using comparisons to data in control regions. The MC expectation is normalized to data in the \mt\ peak region, |
| 67 |
< |
in order to remove systematic uncertainties from integrated luminosity and $t\bar{t}$ cross section, and then extrapolated to the large \mt\ region. |
| 68 |
< |
Correction factors and corresponding systematic uncertainties on the MC extrapolation factors are evaluated by comparing MC to data in dedicated |
| 69 |
< |
control regions dominated by \wjets\ (obtained by vetoing events with b-jets), \ttll\ (obtained by requiring two selected leptons), |
| 70 |
< |
and a mixture of \ttll\ and \ttljets\ (obtained by requiring a selected lepton and an isolated track). |
| 67 |
> |
in order to remove systematic uncertainties from integrated luminosity and $t\bar{t}$ cross section, and then extrapolated to the |
| 68 |
> |
large \mt\ region. Correction factors and corresponding systematic uncertainties on the MC extrapolation factors are evaluated by |
| 69 |
> |
comparing MC to data in dedicated control regions dominated by \wjets\ (obtained by vetoing events with b-jets), \ttll\ |
| 70 |
> |
(obtained by requiring two selected leptons), and a mixture of \ttll\ and \ttljets\ (obtained by requiring a selected lepton and |
| 71 |
> |
an isolated track). The dominant systematic uncertainty in the background prediction is due to the limited statistical precision in |
| 72 |
> |
the data control samples used for these tests. |
| 73 |
|
|
| 74 |
|
\input{results_table.tex} |
| 75 |
|
|
| 76 |
< |
\subsection{Results} |
| 76 |
> |
%\subsection{Results} |
| 77 |
|
|
| 78 |
|
The results of the search are summarized in Table~\ref{tab:stop}, which displays the SM background expectations and the observed data yields |
| 79 |
|
in the signal regions. The distribution of \met\ after the requirement \mt\ $>$ 120 GeV for the SM background expectations is compared to |
| 80 |
< |
data in Fig.~\ref{fig:stop}. Good agreement between the data and the expected background is observed, and we thus do not find evidence |
| 81 |
< |
for the production of top squarks. |
| 80 |
> |
data in Fig.~\ref{fig:stop}. Good agreement between the data and the expected background is observed. We find no evidence |
| 81 |
> |
for the pair production of top squarks. |
| 82 |
|
|
| 83 |
|
\begin{figure} |
| 84 |
|
% Use the relevant command for your figure-insertion program |
| 85 |
|
% to insert the figure file. |
| 86 |
|
\centering |
| 87 |
< |
\includegraphics[width=7cm,clip]{HCPPlots/stopmet.pdf} |
| 87 |
> |
\includegraphics[width=0.4\textwidth]{HCPPlots/stopmet.pdf} |
| 88 |
> |
%\includegraphics[width=7cm,clip]{HCPPlots/stopmet.pdf} |
| 89 |
|
\caption{The \met\ distributions data and expected backgrounds for the top squark pair search. The data is compared to the sum of the |
| 90 |
|
expected backgrounds. The \met\ distributions expected in two example signal scenarios are indicated. The numbers in parentheses |
| 91 |
|
indicate the top squark mass, the LSP mass, and the chargino mass parameter $x$ defined in the text.} |
| 92 |
|
\label{fig:stop} % Give a unique label |
| 93 |
|
\end{figure} |
| 94 |
|
|
| 95 |
< |
\subsection{Interpretation} |
| 95 |
> |
%\subsection{Interpretation} |
| 96 |
|
|
| 97 |
|
%To interpret the results of our search, we consider two signal scenarios of top squark pair production, followed by the decays |
| 98 |
|
%$\tilde{t}\to t\lsp$ and $\tilde{t}\to b\chip\to b W \lsp$. In the first scenario, the only SUSY particles which participate |
| 100 |
|
%the chargino mass is also relevant, and we introduce a third parameter $x$, defined as $m_{\chip} = x m_{\lsp} + (1-x) m_{\tilde{t}}$. |
| 101 |
|
%We consider $x=0.5$ and $x=0.75$ (we do not have sensitivity to the $x=0.25$ scenario). |
| 102 |
|
|
| 103 |
< |
To interpret the results of our search, we consider top squark pair production where both top squarks decay according to $\tilde{t}\to t\lsp$ |
| 104 |
< |
(Fig.~\ref{fig:stop_interpretation}). |
| 105 |
< |
The model is parameterized by the masses of the top squark ($m_{\tilde{top}}$) and \lsp ($m_{\lsp}$. We place upper limits on the signal |
| 103 |
> |
To interpret the results of our search, we consider top squark pair production where both top squarks decay according to |
| 104 |
> |
$\tilde{t}\to t\lsp$ as depicted in Fig.~\ref{fig:diagrams}(a). |
| 105 |
> |
The model is parameterized by the masses of the top squark and \lsp. We place upper limits on the signal |
| 106 |
|
production cross section using, for each model point in the 2-dimensional parameter space, the signal region with the best expected |
| 107 |
|
sensitivity. A region of the parameter space is excluded by comparing these cross section upper limits with the theoretical predictions |
| 108 |
< |
for the signal cross section, computed at next-to-leading order including the resummation of soft gluon emission at next-to-leading-logarithmic |
| 109 |
< |
accuracy (NLO+NLL)~\cite{ref:nlonll}. Our results probe top squarks with masses up to 430 GeV. For comparison, the requirement that SUSY |
| 108 |
> |
for the signal cross section. |
| 109 |
> |
%, computed at next-to-leading order including the resummation of soft gluon emission at |
| 110 |
> |
%next-to-leading-logarithmic |
| 111 |
> |
%accuracy (NLO+NLL)~\cite{ref:nlonll}. |
| 112 |
> |
Our results probe top squarks with masses up to 430 GeV. For comparison, the requirement that SUSY |
| 113 |
|
provides a natural solution to the hierarchy problem suggests top squarks with masses not exceeding 500--700 GeV~\cite{ref:naturalsusy}. |
| 114 |
< |
We also interpret our results assuming the top squark decays according to $\tilde{t}\to b\chip\to b W \lsp$, see Ref.~\cite{ref:stop}. |
| 114 |
> |
We also interpret our results assuming the top squark decays according to $\tilde{t}\to b\chip\to b W \lsp$ |
| 115 |
> |
depicted in Fig.~\ref{fig:diagrams}(b)~\cite{ref:stop}. |
| 116 |
|
|
| 117 |
|
The ATLAS experiment has presented a similar search for top squark pairs in the single lepton final state~\cite{ref:atlasstop}. |
| 118 |
|
The constraints from ATLAS on the top squark mass are more stringent than those presented here. The ATLAS model assumes large |
| 123 |
|
% Use the relevant command for your figure-insertion program |
| 124 |
|
% to insert the figure file. |
| 125 |
|
\centering |
| 126 |
< |
\includegraphics[width=7cm,clip]{HCPPlots/stop_interpretation.pdf} |
| 126 |
> |
\includegraphics[width=0.5\textwidth]{HCPPlots/stop_interpretation.pdf} |
| 127 |
> |
%\includegraphics[width=7cm,clip]{HCPPlots/stop_interpretation.pdf} |
| 128 |
|
\caption{Interpretation of the results of the top squark pair search in the $\tilde{t}\to t\lsp$ scenario. The color scale indicates the |
| 129 |
|
cross section upper limits. The solid black contour and dashed black contours indicate the observed excluded region and variation in this |
| 130 |
|
excluded region due to the $\pm1\sigma$ uncertainties in the theoretical prediction of the signal cross section. The dashed blue |
