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% ATLAS-SCT-Intro.tex: introductory part for SCT
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% RN: 14/01/13: New start for FB13
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\subsubsection{The Inner Detector} 
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Tracking detectors at the LHC have to face three major challenges: high
occupancy, severe radiation damage and a short bunch crossing interval. A
typical hard interaction like $t\bar{t}$ production creates several hundred
charged particles within a rapidity range of $|\eta|\le 2.5$.
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In ten years of operation a flux of up to $5\cdot 10^{14}$
n$_{\mathrm{eq}}$/cm$^2$ (1\,MeV neutron equivalent) particles cause substantial
radiation damage to the SemiConductor Tracker (SCT).
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Furthermore the short bunch crossing interval of 25\,ns demands fast sensors
with fast readout electronics.
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The layout for the silicon part of the ATLAS Inner Detector (ID) is described
in~\cite{atl_det_paper}. A sketch of the ID, Fig.~\ref{fig:ATLAS_SCT_ID}, shows
the separation of the ID into a central (barrel) part and two endcaps.
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\begin{figure}[phtb]
\begin{center}
\includegraphics[width=0.9\linewidth]{./ATLAS_SCT_ID}
\caption{Sketch of the ATLAS Inner Detector}
\label{fig:ATLAS_SCT_ID}
\end{center}
\end{figure}
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The complete ID is operated in a 2\,T axial field. Detailed descriptions and
specifications of the ID, its components and the expected performance can be
found in~\cite{atl_det_paper,ATLAS_ATLASPerf}.  % ATLAS_DETECTOR
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It is designed to provide a resolution in transverse momentum, in the plane
perpendicular to the beam axis, of
$\sigma_{p_\mathrm{T}}/p_\mathrm{T}=0.0005p_\mathrm{T}/{\mathrm{GeV}}\oplus0.01$
and a transverse impact parameter resolution of 10\,\micron\ for high momentum
particles in the central detector region~\cite{atl_det_paper}.

Occupancy and radiation damage vary strongly within the ID volume. To cope with
this, depending on the distance to the beam line, $R$, the ID is segmented into
three parts, each using a different detector technology:
\begin{itemize}
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\item In the innermost region, $5\mathrm{cm}\le R\le 12\mathrm{cm}$ 
a high granularity silicon pixel detector with about 80\,M readout channels is
used. It offers an excellent space point resolution and low occupancy. The high
resolution of this detector is essential for secondary vertex reconstruction,
needed \eg for $\tau$-lepton and b-quark identification.
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\item In the intermediate region, $30\mathrm{cm}\le R\le 51\mathrm{cm}$, 
a silicon micro-strip detector, the SCT, is used. The strips are oriented to
measure with high precision the track coordinates transverse to the beam. A
small stereo angle between two faces of a detector module allows measuring the
orthogonal coordinate, albeit with less precision. The detector measures up to
four space points per track. For the SCT the total area of silicon is about
61\,m$^2$.
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\item Finally, for $56\mathrm{cm}\le R\le 107\mathrm{cm}$, the transition 
radiation tracker (TRT) uses straw tubes. The large number of straws allows
continuous tracking with on average 36 hits per track, which, despite the
moderate space resolution of single hits, results in a high precision
measurement at large radii. In addition, the ability to detect transition
radiation improves the electron/pion separation. The challenge for this detector
is to cope with the high occupancy.
\end{itemize}
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The MPP made major contributions to the design of the pixel- and micro-strip
silicon detectors, and at MPP we have built about 30$\%$ of all SCT endcap
modules.
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The actual performance of the ID after the final installation including the is
discussed in~\cite{ATLAS_ATLASIDPerf}.
