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\Title{Top quark production at ATLAS}

\bigskip
\bigskip

%+\addtocontents{toc}{{\it D. Reggiano}}
%+\label{ReggianoStart}

\begin{raggedright}  

{\it Yuta Takahashi on behalf of the ATLAS Collaboration\\
High Energy Physics Laboratory, Nagoya University, 464-8602 Japan}
\bigskip
\bigskip
\end{raggedright}



\section{Introduction}
%The top-quark ($t$-quark) is the heaviest elementary particle (172.9~GeV) in the particles predicted by the Standard Model (SM).
The top-quark ($t$-quark) is the heaviest elementary particle (173.2~GeV)~\cite{mass} among the Standard Model (SM) particles.
Owing to its largest mass, there are several theoretical predictions that new physics beyond the SM 
could appear in both the production or in the decay process of the $t$-quark.
At the LHC, plenty of the $t$-quarks are produced either as a pair ($t\bar{t}$) or singly, % ($\sigma_{t\bar{t}} = 165$~pb, $\sigma_{t} = 85$~pb),
enough to test the SM by measuring its production cross-section.
Based on the 0.7~fb$^{-1}$ proton-proton ($pp$) collision data recorded by the ATLAS detector~\cite{ATLAS},
precise measurements have been performed of the $t\bar{t}$ production cross-section using the single-lepton ($t\bar{t}\rightarrow \ell^{+}\nu_{\ell} qq^{'}b\bar{b}$, $\ell=e,\mu$)~\cite{atlas_ljet}, and the dilepton ($t\bar{t}\rightarrow \ell^{+} \nu_{\ell} \ell^{-} \bar{\nu_{\ell}} b\bar{b}$)~\cite{atlas_dilepton} channel.
%and all hadronic channel ($t\bar{t}\rightarrow qq^{'}q^{''}q^{'''}b\bar{b}$)~\cite{atlas_allhad}. 
The combined cross-section ($177 \pm 11$~pb) is 
consistent with the theoretical prediction with an uncertainty of 6\%, which is smaller than the theoretical accuracy (10\%).
The $t\bar{t}$ production cross-section in alternative decay channels and the single-top production cross-section are of primary
interest to further investigate new physics effect.
%The $t\bar{t}$ production cross-section in alternative decay channels and the single-top production cross-section are reported.

\section{Single-top production cross-section}
The single-top production cross-section provides
a direct probe of the $Wtb$ coupling, and can constrain the absolute value
of the CKM quark-mixing matrix element $V_{tb}$ without
any assumption on the number of quark generations.
The production cross-section for the two leading processes: the $t$-channel ($64.6^{+2.7}_{-2.0}$~pb) and the
$Wt$ associated production channel (15.7$\pm1.1$~pb) are presented. % are dominant, while small contribution from the $s$-channel process ($4.6\pm0.2$~pb).
%as shown in Figure~\ref{single_top}.
%\begin{figure}[htb]
%\begin{center}
%\epsfig{file=single_top_source.eps,height=1.4in}
%\caption{The production processes of the single-top quark.}
%\label{single_top}
%\end{center}
%\end{figure}


\paragraph{$Wt$-channel}
%The associated $Wt$ production involves massive particles in the final state. The production
The associated $Wt$ production involves more than 250~GeV in the final state. The production
rate at the Tevatron is too small (0.2~pb) to be observed, while it is much
higher at the LHC owing to the larger partonic energy.
%much higher cross-section at the LHC 
%owing to its available partonic energy.
 
Using 2.05~fb$^{-1}$ of $pp$ collision data, 
the analysis exploits the leptonic decays of both the W associated to the $t$-quark and the $W$ from its decay.
Candidate events are extracted by requiring two high-$p_T$ leptons, % (electron with $E_T > 25$~GeV or muon with $p_T > 25$~GeV), 
significant missing transverse momentum ($E_T^{miss} > 50$~GeV),
and at least one jet with $p_T > 30$~GeV. The remaining $Z+$jet background is reduced by requiring the sum of the opening angles between leptons and the $E_T^{miss}$,
$\sum_{i=1,2}\Delta\phi(\ell_i, E_T^{miss}) > 2.5$~rad,
and the dilepton mass ($m_{\ell\ell}$) outside the $Z$ mass window ($m_{\ell\ell} < 81$~GeV or $m_{\ell\ell} > 101$~GeV).
The dominant background comes from $t\bar{t}$ dilepton events.

An approach based on a Boosted Decision Tree (BDT)~\cite{bdt} has been developed
to discriminate $Wt$ events from backgrounds, followed by a template fit to the BDT output
distribution. 
%The resulting cross-section, $16.8\pm 5.7$~pb, 
%shows a good agreement compared to the theoretical prediction, with a significance of 3.3~$\sigma$ (3.4~$\sigma$ is the expected value).
The result is incompatible with the background-only hypothesis at the 3.3~$\sigma$ level, and the cross-section, 
$16.8\pm 5.7$~pb, shows good agreement with the theoretical prediction (3.4~$\sigma$ is the expected value).
This is the first evidence of single-top production via the $Wt$ channel, and the result is used to determine $|V_{tb}| = 1.03^{+0.16}_{-0.19}$, assuming that $|V_{tb}| \gg  |V_{ts}|, |V_{td}|$~\cite{st_wt}.

\paragraph{$t$-channel}
The $t$-channel has the largest cross-section among the single-top processes,
enabling us to measure $|V_{tb}|$, as well as the cross-section separately for the $t$-quark ($\sigma_t$) and the $\bar{t}$-quark ($\sigma_{\bar{t}}$).
The ratio $\sigma_t/\sigma_{\bar{t}}$ is sensitive to the two PDFs ($u$-quark and $d$-quark) as the systematic uncertainties are partially cancelled. 
The enhancement of the ratio may provide an interesting handle to search for new physics.

Based on 1.04~fb$^{-1}$ of $pp$ collision data, 
events are selected by requiring one high-$p_T$ lepton, $E_T^{miss} > 25$~GeV, 
and two or three jets, with exactly one of them identified as originating from a
$b$-quark. 
%The QCD multi-jet events are rejected by requring
%$M_T(\ell, E_T^{miss}) + E_T^{miss} > 60$~GeV, where $M_T$ is the transverse mass between the lepton and the $E_T^{miss}$.
The dominant background consists of $W+$jet events, modeled by the MC simulation and normalized to the data.
The cross-section is extracted by fitting the distribution of a multivariate discriminant based on a
neural network, and turns out to be $83 \pm 20$~pb.
Assuming that the $t$-quark related CKM matrix elements obey the relation $|V_{tb}| \gg  |V_{ts}|, |V_{td}|$, 
the coupling strength at the $Wtb$ vertex is determined to be $|V_{tb}| = 1.13^{+0.14}_{-0.13}$.
Under the assumption that $|V_{tb}| \leq 1$, a lower limit of $|V_{tb}| > 0.75$ is obtained with the 95\% confidence level~\cite{st}.

The ratio $\sigma_t/\sigma_{\bar{t}}$ is measured using 4.7~fb$^{-1}$ data with a slightly optimized event selection.
The cross-section is extracted separately according to the lepton charge by performing a binned maximum likelihood fit to the output distribution of the neural
networks. The resulting cross-section is $\sigma_t = 53.2 \pm 10.8$~pb and 
$\sigma_{\bar{t}} = 29.5{}^{+7.4}_{-7.5}$~pb, with its ratio measured to be $R_t = 1.81{}^{+0.23}_{-0.22}$. 
The measurement is in agreement with the predictions, that vary between 1.86 and 2.07 depending on the assumed PDFs~\cite{st2}.



\section{$t\bar{t}$ production cross-section}
The measurement of the $t\bar{t}$ production cross-section 
in different decay channels is of primary importance to test the SM.
In particular, the cross-section using final states including a $\tau$-lepton
is sensitive to new physics such as the charged Higgs boson, predicted by the supersymmetric model.
In the following, the analysis in the $\tau$ plus lepton channel ($t\bar{t}\rightarrow \tau^{+} \nu_\tau \ell^{-} \bar{\nu_\ell} b\bar{b}$)~\cite{atlas_taulepton}, in the $\tau$ plus jet channel ($t\bar{t}\rightarrow \tau^{+} \nu_\tau q q^{'} b\bar{b}$)~\cite{atlas_taujet},
and in the all-hadronic channel ($t\bar{t}\rightarrow qq^{'}q^{''}q^{'''}b\bar{b}$)~\cite{atlas_allhad} are shortly described.

\paragraph{$t\bar{t}$ production in the $\tau$ plus lepton final state}
Candidate events are extracted from 2.05~fb$^{-1}$ data, 
by requiring one high-$p_T$ lepton (electron or muon), at least one hadronically decaying $\tau$ candidate,
$E_T^{miss} > 30$~GeV, and two or more energetic jets, 
with at least one of them identified as a $b$-jet.
Since the most important background is due to jets mimicing a $\tau$ candidate,
multivariate technique based on a BDT
is used to reconstruct the $\tau$ candidate, relying on its large separation power between signal and background.
In order to estimate the number of background events in a data-driven way, 
the data have been split according to the charge correlation between a lepton and a reconstructed $\tau$ candidate, 
followed by the subtraction of the same-sign events from the opposite-sign events to reduce part of the backgrounds.
This enables to model the background in a data-driven way, which leads to the reduction of the systematic uncertainty.
The cross-section is extracted by the template fitting to the BDT output distribution, which yields $\sigma_{t\bar{t}} = 186\pm25$~pb.
With a total uncertainty of 13\%, the obtained result is consistent with the theoretical prediction (164$^{+11}_{-16}$~pb).
%The method developed in this analysis will be utilized for the new physics search, such as the charged Higgs boson.

\paragraph{$t\bar{t}$ production in the $\tau$ plus jet final state}
%This channel is a complementary channel with the $\tau$ plus lepton channel. analysis exploits the event topology with a hadronically decaying tau lepton and jets,
%which corresponds to 10\% of all $t\bar{t}$ decays.
Candidate events are selected from 1.67~fb$^{-1}$ data,
by requiring at least five jets, with two of them identified as $b$-jets,
a large $E_T^{miss}$ significance ($E_T^{miss}/\sum E_T > 4$, where $\sum E_T$ is the scalar sum of the transverse energy released by all the visible particles).
One of the remaining jets is selected as the $\tau$ candidate based on the event topology.
%The choice of the $\tau$ candidate 
%is performed by rejecting three jets that can reproduce the $t$-quark mass, and selecting remaining no $b$-tagged jet
%with $p_T > 40$~GeV. 
The remaining backgrounds come from the QCD multi-jet events.

The cross-section is obtained by fitting the distribution of the number of tracks associated to the $\tau$ candidate
with templates. Signal events produce a peak in correspondence to an odd number of tracks coming from the $\tau$'s.
The signal template is derived from MC simulation, while the background templates 
for the multi-jet and the $t\bar{t}$ combinatorics are constructed in a data-driven way.
The obtained cross-section of $200\pm 47$~pb, is in agreement with the theoretical prediction.


\paragraph{$t\bar{t}$ production in the all-hadronic channel}
The $t\bar{t}$ production cross-section in all-hadronic channel 
is sensitive to new physics including a multi-jets final state, such as the one predicted by the low scale gravity and the weakly-coupled string theory.
Although there is the big advantage of a largest branching ratio (46\% of all $t\bar{t}$ decays), 
the analysis is challenging due to the large multi-jet backgrounds.

Candidate events are selected from 4.7~fb$^{-1}$ data by requiring at least six jets, with at least two of them being identified as $b$-jet,
and a small $E_T^{miss}$ significance less than three.
A kinematic likelihood fit is performed to find the correct association of jets that can reconstruct the $t$-quark mass.
The number of signal events is extracted by using an unbinned likelihood fit to the $t$-quark mass.
The shape of the background, which consists of multi-jet events, is modeled by the events passing the event selection without the requirement of the presence of $b$-tagged jets. The signal is modeled using a MC simulation.
The measured cross-section, $168\pm62$~pb, is found to be compatible with the theoretical prediction.

\section{Summary}
Figure~\ref{fig:magnet} shows the summary of the $t\bar{t}$ and the single-top production cross-section measurements, with the corresponding theoretical cross-sections.
All the results are consistent with the theoretical prediction, showing the validity of the perturbative QCD at the LHC energy scale.
Further improvement on the systematic uncertainty will lead to better measurements which will allow to probe new physics.
\begin{figure}[htb]
\begin{center}
\epsfig{file=ST.eps,height=2.75in}
\epsfig{file=TT.eps,height=2.85in}
\caption{Summary of the $t\bar{t}$ and the single-top cross-section measurement.}
\label{fig:magnet}
\end{center}
\end{figure}
\begin{thebibliography}{99}

%%
%%  bibliographic items can be constructed using the LaTeX format in SPIRES:
%%    see    http://www.slac.stanford.edu/spires/hep/latex.html
%%  SPIRES will also supply the CITATION line information; please include it.
%%

\bibitem{mass}
The Tevatron EW Group for the CDF and D0 Collaborations, arXiv:1107.5255


\bibitem{ATLAS}
ATLAS Collaboration, JINST 3 (2008) S08003

\bibitem{atlas_ljet}
ATLAS Collaboration, ATLAS-CONF-2011-121
%Measurement of the $t\bar{t}$ production cross-section in $pp$ collisions at $\sqrt{s} = 7$~TeV using kinematic information of lepton$+$jets events

\bibitem{atlas_dilepton}
ATLAS Collaboration, JHEP 1205 (2012) 059
%Measurement of the cross-section for top-quark pair production using final states with two high-pt leptons, 
\bibitem{bdt}
B. P. Roe et al., Nucl. Instrum. Meth. A 543, (2005) 577


\bibitem{st_wt}
ATLAS Collaboration, Phys. Lett. B 716 (2012) 142-159

\bibitem{st}
ATLAS Collaboration, arXiv:1205.3130 [hep-ex]

\bibitem{st2}
ATLAS Collaboration, ATLAS-CONF-2012-056


\bibitem{atlas_taulepton}
ATLAS Collaboration, CERN-PH-EP-2012-102, arXiv:1205.2067
%Measurement of the top quark pair production cross-section with ATLAS in $pp$ collisions at $\sqrt{s} = 7$~TeV using final states with an electron or a muon and a hadronically decaying tau, arXiv:1205.2067

\bibitem{atlas_taujet}
ATLAS Collaboration, ATLAS-CONF-2011-032
%Measurement of the $t\bar{t}$ production cross-section in the final state with a hadronically decaying tau lepton and jets using the ATLAS detector, 
% arXiv:1205.2067
 
\bibitem{atlas_allhad}
ATLAS Collaboration, ATLAS-CONF-2012-031
%Measurement of $t\bar{t}$ production in the all-hadronic channel in 4.7~fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 7$~TeV with the ATLAS detector
 
%\bibitem{copy}
%ATLAS Experiment $\copyright$ 2012 CERN 

\end{thebibliography}

 
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