%%%%%  The following statement gives section #5 to the MDT-TGC,
%%%%%  independent of the number of previous sections
%
\setcounter{section}{4}
%
\newpage


%%% ==== 5   ============================================================
%%%
\section{Detector concept 1 : Integrated MDT-TGC System}
%%%
%%% =====================================================================

\if{0}

{\it Introduction to this concept:
This section should describe the technology concept, details
up to the level of a single chamber should be given in the appropriate
appendix.}

\fi

%%% ==== 5.1 ============================================================
%%%
\subsection{Introduction}
\label{Introduction}
%%%
%%% =====================================================================

The dominant motivation behind the present proposal is to  considerably increase the
performance and rate capability of the triggering and precision tracking system in the
Small Wheel in order to match the higher background levels in the Phase~II upgrade
scenario of the LHC.

For this purpose, we propose the following layout for the upgrade of the Small Wheel.

\begin{enumerate}

\item[(a)] a new type of TGC trigger chamber, providing considerably improved spatial resolution
in the bending plane but preserving the basic rugged structure together with the operating
parameters of the proven TGC system, as presently installed in the muon end-cap.

\item[(b)] a new type of MDT chamber, made from tubes of half the diameter of the presently
used tubes, otherwise operating under the same conditions with respect to gas gain, gas
composition and pressure. The reduction of the tube diameter from 30 to 15\,mm leads to an
increase of the rate capability of the tubes by a factor of seven, sufficient to cope with the
level of background hits expected at the SLHC.

\end{enumerate}

%
% --- Fig.  1 ----------------------------------------------------------------
%

\begin{figure}[!htb]

   \centering

\includegraphics[width=0.6\linewidth,clip]{Figures/TGC_MDT_package_BB}

\caption{Cross section through the proposed arrangement of trigger TGCs and precision MDTs. The
MDT, made out of two multilayers (with 4 or 6 tube layers) is sandwiched between two packages of
4-layer TGCs. The distance of 300\,mm between the TGC packages is instrumental for the angular
resolution of the triggering muon track.}

\label{TGC_MDT_package}

\end{figure}


With this upgrade of the existing chamber technology towards higher performance the uniformity
of the present muon spectrometer (MDT as tracking detectors, RPC as trigger detectors in the
barrel and TGCs in the end-cap) is preserved while taking advantage of the existing technical
infrastructure and the experience in constructing such an assembly at the relevant institutions.

For the trigger chambers we propose to maintain the basic operating concept of TGC chambers,
complementing them with a new, high-performance trigger system.  The pulseheight distribution  of
signals induced into the strips of the TGC cathodes (measuring $\eta$) are processed to  give the
precision coordinate with an accuracy of $<$\,100\,$\mu m$. To distinguish the new TGCs, proposed for
the New Small Wheel (NSW) from the existing ones in the Big Wheel, we will refer to them as to
''sTGCs'' throughout this article.

In the proposed detector layout, one package of four sTGC chambers is placed in front and one behind
the  MDT chambers (Fig.~\ref{TGC_MDT_package}). A sTGC package will contain four wire layers (wires
running radially, perpendicular to the strips), four cathode layers for centroid finding and another
four cathode layers, segmented into pads, for fast selection of a trigger tower of interest.
Centroids are found for those strips in the trigger tower.
Also, in the offline
analysis the pads will  be an important tool for resolving ambiguities in the x-y pairing of
coordinates, when more than one track is present in a chamber.

One important limitation of the present L1-trigger in the end-cap is due to the fact that a
majority of tracks, crossing the Small Wheel, do not originate from the primary vertex and should
therefore not be considered candidates for a high-p$_T$ muon by the triggering system. The  present
trigger, however, can not determine the slope of the candidate track in the r-z plane and for this
reason is unable to discard tracks not coming from the primary vertex. With a  distance of about
300 mm between the sTGC packages in front and behind the MDT, an angular resolution of $\sim
0.4$\,mrad  (including multiple scattering) can be achieved, which allows to discard tracks not
pointing to the primary vertex from the L1 trigger decision. In addition to the precise measurement
of the slope in the $\eta$-direction, the slope of in the $\phi$ direction will also be determined,
though with less accuracy, to further reduce the number of false triggers. A detailed presentation
of the method is given in section~\ref{TGC_technology}.

The limitations of the MDT precision chambers at high luminosity are mainly caused by
isolated hits in the tubes (''fake hits''), coming from neutron and gamma conversions in
nearby support structures, tube walls, chamber gas and tungsten wires. The presently
used 30\,mm diameter tubes (''Large tubes'') start to lose efficiency if hit rates go
beyond about 300\,kHz per tube and, therefore, a new MDT chamber type with half the tube
diameter is proposed, leading to seven times higher rate capability.

Both new chamber systems, the sTGCs and the MDTs, can be operated with the same chamber gas
as in the present system. Power requirements for the electronics of both systems will be
higher due to higher channel count. Whether this requires active cooling, as is the case
for the electronics of the CSC chambers in the region $\eta$ $>$ 2 is under study. Aim is
to have services arranged in such a way as to be able to use the existing service
infrastructure like e.g. the flexible chains (''cable Schlepps'').

In the present architecture of the Muon spectrometer the precise tracking coordinates of the MDT
are not used for the Level-1 trigger, precision and trigger chambers being read out separately and
in an asynchronous way. The MDT information is only used at Level-2, where it is instrumental in
discarding the majority of Level-1 muon triggers, which may either be caused by muons with too low
p$_T$ or by tracks not pointing to the vertex. To capitalize on the Level-2 selection method
already at Level-1, it has been proposed to implement a fast readout path into the MDT readout
architecture, making the accurate MDT coordinates available to the trigger logic, early enough for
use in the L1 trigger decision. A more detailed discussion of the proposed concepts for L1 trigger
sharpening with MDTs is given in section~\ref{MDT_triggering} and in appendix F. 

%% ----- 5.1.1-----------------------------------------------------------
%%
\subsubsection{Upgrade of the TGC System}
\label{New_TGC}
%%
%% ----------------------------------------------------------------------

The basis of the proposed concept is to have as many as possible accurate measurements in
the trigger layers, to allow discarding a high percentage of measurements where $\delta$-rays,
additional $\gamma'$s or neutrons are present and still obtain a position
measurement from each of the two packages with a precision of 100\,$\mu$m.  To obtain such a
precision using the 3.5\,mm pitch strips present in each gas-gap, while keeping simple
electronics, a method using Time-Over-Threshold as an estimate of
charge has been developed. The achieved performance using this method for
large size sTGCs is discussed in detail in section~\ref{Detector_technology}.

Achieving this high accuracy for the bending coordinate in each gas gap provides a valuable
complement to the MDT position measurements, similar in precision, reducing the required number of
tube layers in the MDT necessary for an unambiguous track identification at high background rates.
In particular, in the high background rate region of the Small Wheel, having a total of 20
potential measurement points, of which up to 30\% may have to be discarded, ensures that an
accurate particle vector will be provided in front of the End-Cap Toroidal field, even if some of
the layers become non-operational during the exploitation of ATLAS at SLHC. The  overall signal
processing to produce the trigger is schematically  shown in figure~\ref{appendix:TGC1} of
appendix~(\ref{appendix:Latency}).  A demonstrator to study the signal path, logics and latency in
detail has recently been completed.


\if{0}
\begin{figure}[!htb]
   \centering
\vspace{-2 pt}

%% \includegraphics[width=16cm]{Figures/TGC1}

\includegraphics[width=0.7\linewidth,clip]{Figures/TGC1}

\caption{Schematics of the internal layout of a 4-layer TGC package,
showing the pad-strip and wire readout for each gas-gap.}
\label{TGC1}

\end{figure}
\fi


By making a coincidence among the pads in a tower of successive layers (within a 1\,degree aperture), one
is able to substantially reduce the number of channels that have to be read out for a fast
trigger decision. Furthermore, the pad-coincidence provides a unique 3-D address, where a
corresponding angle and displacement comparison can be made with the trigger input from the
existing muon end-cap trigger, originating from the ATLAS Big Wheels.
To simplify the logic of what channels to read for trigger purposes, one might loose some of the
potential muon candidates.
%, which can be recuperated by the measurements of the MDT.
%(<<<???? Not at the trigger level! RR>>>).

The MDTs provide, furthermore, an accurate alignment that can be used for precisely aligning
the TGC positions by means of tracks.

\newpage


 %% ----- 5.1.2-----------------------------------------------------------
%%
\subsubsection{Upgrade of the MDT System}
\label{New_MDT}
%%
%% ----------------------------------------------------------------------

The large majority of hits in the Monitored Drift Tube (MDT) chambers are
caused by  Compton scattering of $\gamma'$s which, in turn, come from neutron capture in the
material of the chambers and adjacent support material. The resulting hit densities strongly
increase towards the inner radius of the Small Wheels, while, on the other hand, the length of the tubes in
the trapezoidal geometry of the SW decreases proportional to the distance from the beam line (r).
Hit densities, however, are increasing considerably faster than with 1/r, and therefore the highest
tube hit rates are at the innermost radii of the chambers.

%
% --- Fig.  2 ----------------------------------------------------------------
%

\begin{figure}[!htb]
\begin{center}
\includegraphics[width=0.35\linewidth,clip]{Figures/Large_small_tubes}

\caption
{The tracking performance of 15\,mm with respect to 30\,mm diameter drift tube chambers at high n/$\gamma$
background rates. An occupancy of 50\% in
30\,mm diameter tubes (left) due to background hits (red dots) corresponds to 
only 6\% occupancy in the 15\,mm diameter tubes (right) due to the 3.9 times shorter drift time 
and the 2 times smaller cross section.
}

\label{Large_small_tubes}

\end{center}
\end{figure}


Reduction of the drift-tube diameter from 30\,mm to 15\,mm, while keeping the other parameters
unchanged (gas composition and gain, sense wire  diameter and tube wall thickness) leads to a
vast improvement of the rate capability of the drift-tube chambers, exceeding the 
requirements for operation in the NSW up to the highest conceivable background rates 
(see illustration in Fig.~\ref{Large_small_tubes}).


%
% --- Table  1 -------------------------------------------------------------------
%

\begin{table}[!htb]   %%% Table:  1
\centering
\begin{tabular}{|l|c||c|c|c|c|c|c|c|} \hline
%% \small  -- doesn't work
  Chamb. & Tube  & Location & Tube    & Expected      & Count rate & Occup-   & Tube effi- & Segment \\
%\hline
  type   & layers& in r     & length  & hit rate      & per tube   & ancy     & ciency   & efficiency \\
\hline
         &       & {\it cm}& {\it cm}& {\it Hz/cm$^{2}$}& {\it kHz}& {\it \%} & {\it \%} & {\it \%} \\
\hline
  EIL0 &2$\times$6& 93  &   56 &14000  & 1176  & 23,5 & 79,0 & 99.9 \\
  EIL1 &2$\times$4&208  &  125 & 5150  &  965  & 19,3 & 82,4 & 99.9 \\
  EIL2 &2$\times$4&318  &  179 & 1978  &  530  & 10,6 & 90,0 & 99.9 \\ \hline
  \end{tabular}

\caption{\small Expected hit rates and efficiencies for 15\,mm tubes in a large sector of the NSW.
		 In col.~3 the radial location of the innermost tube of each chamber and in cols.~4 to 8
		 the tube lengths, the expected hit rates, the counting rates per tube, the occupancies
		 and  the $3\sigma$ single-tube and the track segment reconstruction efficiencies of a
		 chamber are given for the tubes with highest counting rate  at a luminosity of 5 $\times$
		 nominal (times a factor of 1.4, see text).}

  \label{NSW_efficiencies}
\end{table}



Table~\ref{NSW_efficiencies} presents tube hit rates, occupancies and efficiencies for the
innermost tubes of the three chambers in a Large Sector of the NSW. Chamber EIL0 covers the area
now taken by the CSC chambers. The numbers in col.\ 4 are based on hit rate measurements at a luminosity of
$9\cdot10^{32}$  cm$^{-2}$s$^{-1}$, scaled up linearly to $5\cdot10^{34}$ cm$^{-2}$s$^{-1}$. Rates
in the CSC  region (1 m$<$r$<$2 m) are extrapolated from larger radii, using an exponential
law~\cite{ref_Conti}. In addition, these rates have been multiplied by a factor of 1.4 in order to
increase rates to the 14 kHz/cm$^2$ specification of this Technical Proposal. This factor of 1.4
could also be considered an additional safety factor in case the real rates will increase faster
than predicted by the  extrapolation. The efficiencies are calculated from exp(-$\tau\cdot$f)
$\approx$  1-$\tau\cdot$f, where $\tau$ is the dead time after a hit (we use as the minimal adjustable value 
the maximum drift time of 200 ns) and f the hit rate per tube.

Being able to use the drift tubes as tracking devices at very high background rates allows to preserve the
main advantages of MDT concept:

\begin{enumerate}

\item[(a)] High pattern recognition and tracking efficiency up to high backround rates. 

\item[(b)] High position and angular resolution, independent of the angle of incidence onto the
chamber plane. This property of drift tube chambers with their {\it{circular}} drift geometry is a
decisive advantage over drift chambers with {\it{rectangular}} drift geometry.

\item[(c)] Operational independence of the drift tubes, where a malfunction of any individual tube
can only generate a negligible inefficiency.

\item[(d)] Modularity and mechanical robustness of chamber construction.

\item[(e)] Direct connection of the high intrinsic mechanical precision of the chambers
to the global optical alignment system.

\item[(f)] Extensive experience in the construction, quality assurance and operation of the MDT
chambers in ATLAS; easy integration into the DAQ, trigger and muon reconstruction chains. 

\end{enumerate}

For the standard MDT gas mixture of Ar:CO$_2$ (93:7) at 3 bar absolute pressure, the gas gain  of
$2\cdot 10^4$ is achieved at an operating voltage of 2730~V for 15\,mm diameter drift tubes
(Table~\ref{Sml_MDT_parameters}). Under these operating conditions, the maximum drift time is
reduced by a factor of 3.8 from about 700~ns to 185~ns~\cite{ref_bittner1}. In addition, the
background flux, hitting a tube of given length is reduced by a factor of two. Altogether, the
drift-tube occupancy is thus reduced by a factor of 7.6. At the benchmark background rate 
of 14 kHz/cm$^2$ at the inner bore of the NSW, corresponding to a
counting rate of 1176~kHz in 0.55~m long tubes, the maximum drift tube occupancy is 23\% 
(cf. Table~\ref{NSW_efficiencies}). 

The proposed layout with 2$\times$6 drift-tube layers at
radii $R<220$~cm in the NSW and with 2$\times$4 layers at larger radii provides very robust tracking
with track segment reconstruction efficiencies above 99\% 
~\cite{ref_bittner2} and a sMDT chamber spatial resolution of better than $40~\mu$m 
at the highest background rates corresponding to 
14 kHz/cm$^2$ at the inner bore of the NSW (see Fig.~\ref{fig_nswresol}). The deterioration
of the spatial resolution due to background radiation is negligible.  

The mechanical structure and the alignment system will be very similar to the 
present Small Wheel architecture. The sTGC and sMDT chambers will be mechanically aligned
in the integrated chamber modules with an accuracy of better than $20~mu$m which will be verified
with cosmic ray measurements with MDT referen 
Like in the case of the trigger chambers, new readout electronics
with higher rate capability has to be installed (see section~\ref{chapter:MDT_electronics}).


%% ----- 5.1.3-----------------------------------------------------------
%%
\subsubsection{Combination of MDT and TGC information to sharpen the trigger}
\label{MDT_triggering}
%%
%% ----------------------------------------------------------------------

In the present architecture of the Muon spectrometer, trigger and precision chambers operate
independently. The information of both chamber systems is read out separately, and the precise
tracking coordinates of the MDT are only used at Level-2, where the majority of candidate triggers
is discarded, being either caused by tracks with p$_T$ below threshold or by non-projective tracks
(not pointing to the vertex). To capitalize on this trigger sharpening already at Level-1, it has
been proposed to implement a second, fast readout path, parallel to the existing one, into the MDT
readout architecture. This ''fast lane'' should supply MDT tracking information in a synchroneous
way with BX, making the accurate MDT coordinates available to the trigger logic at a very early
stage. 


%
% --- Fig.  3 ----------------------------------------------------------------
%

\begin{figure}[!htb]

   \centering

\includegraphics[width=0.4\linewidth,clip]{Figures/Tagged_triggering_phase_II}


\caption{The tagged triggering method uses the RoI of trigger chambers in the same tower to
define a search road inside which to look for hits of the candidate track. Track segments of
several MDT can be combined to give a precise estimate for the p$_T$ of the track.}

\label{Tagged_method}

\end{figure}

To simplify the drift time measurement for the fast readout, the clock frequency of
1.28\,GHz, used internally by the TDC and corresponding to a least significant bit (lsb) value of
0.78\,ns, needs to be relaxed. For the fast readout a clock frequency of 80 or 160\,MHz may be
sufficient, corresponding to lsb values of 12.5\,ns and 6.25\,ns, respectively. Given the drift
velocity of 37.5\,ns/$\mu m$ in the Small drift tubes, the corresponding RMS position errors are 135
and 68\,$\mu m$. With a lever arm of about 150 mm between the sMDT multilayers, this leads to an
angular RMS resolution of 0.9 and 0.45\,mrad, respectively, both values being compatible with the
required angular resolution of $<$\,1\,mrad.  

Two methods have been proposed for the implementation of a sMDT based L1 triggering scheme. The
''tagged'' method (see Fig.~\ref{Tagged_method}) uses the early information ("Level-0") on a
candidate track (RoI), obtained from close-by trigger chambers, as a search road for track hits in
a narrow corridor of the sMDTs along the track. This way most of the background hits, located
outside the search road, can be safely ignored. On a Level-0 request, local processors start to
search for hits in the RoI and derive the resulting tracking angle and momentum, if applicable. 
The tagged method requires very limited output bandwidth, as in any given trigger tower the rate of
L1 candidates is very small ($<$ \,100\,Hz). 

The communication path between trigger and MDT chambers leads to additional latency. Timing
estimates show that a latency in the range 3.2--4.5\,$\mu s$ is required, too long for the
available latency of 2.55\,$\mu s$ in phase I, but well below the limit of 6.4\,$\mu s$, foreseen
for phase II of the LHC upgrade. A detailed discussion of this method is given
in~\cite{Trigger_Upgrade}. 

%
% --- Fig.  4 ----------------------------------------------------------------
%

\begin{figure}[!htb]

   \centering

\includegraphics[width=0.65\linewidth,clip]{Figures/Untagged_triggering_phase_I}

\caption{The untagged tagged triggering is looking for track segments in a MDT which point to the
primary vertex.}

\label{Untagged_method}

\end{figure}

The ''untagged'' method (see Fig.~\ref{Untagged_method}) uses all hits from all MDT tubes and looks
for hit patterns pointing to the primary vertex. It does not make use of a preceeding Level-0 tag
from the trigger chambers. Consequently, the fast MDT readout needs to transfer all incoming hits
to the processors in USA15, leading to bandwidth requirements of up to about 2.5 Gbit/s per group
of 24 tubes. Timing estimates for the untagged method lead to a latency of $<$\,2.55\,$\mu s$,
compatible with the phase I limit by a narrow margin of 160\,ns. More details about this method are
given in appendix F.

In both methods the information is fed into the L1 trigger logic at the level of the sector logic
(SL).



%% =======================  section 5.2  ==============================
%%
% ==== 5.2 ============================================================
%%%
\subsection{Detector technology and layout}
\label{Detector_technology}
%%%
%%% =====================================================================

\if{0}

{\it
    Description of detector concept (a detailed description of the
    working principle of each detector should be given as an
    appendix).\\
    If more than 1 technology is used motivate the decision and detail
    how they work together (e.g.\ can services be shared, is the
    information of the trigger chambers used in the precision
    chambers, how are combined chambers assembled etc.)

    List of all operating parameters\\
    Detailed layout: acceptance, description of chamber overlap
    and dead areas, drawings\\
    Tables with chambers sizes, number of channels\\
    Internal alignment scheme (the overall common endcap alignment scheme
    is described in the previous chapter)\\
    Calculations about mechanical stability and expected
    deformations due to gravity, temperature changes, (magnetic field) etc.\\
    Requirements for mount points\\
    Details of service points and other positions where access is needed\\
    Concept for chamber replacement (what needs to be dismounted etc.)
}

\fi

%% ----- 5.2.1-----------------------------------------------------------
%%
\subsubsection{Detector technology of the sTGC chambers}
\label{TGC_technology}
%%
%% ----------------------------------------------------------------------

The structure of the new sTGC chambers is shown in Fig.~\ref{TGC_details}, while the schematic
layout of the proposed NSW trigger chamber system is shown in Fig.~\ref{TGC3}. This layout should
be consider as one of the possible arrangements of identical small-large sectors. The 4+4-layer
Small tube MDTs are sandwiched between two sTGC packages, each 5\,cm thick, containing four
gas-gaps each. Each gas-gap provides pad, strip and wire readout. Since the trigger chambers,
unlike the MDT, do not need an overlap region for relative alignment, that part of the MDT surface
is used to implement the alignment connection (purple boxes) to the projective alignment bars.

%
% --- Fig.  5 ----------------------------------------------------------------
%

\begin{figure}[!htb]

   \centering

\includegraphics[width=0.85\linewidth,clip]{Figures/TGC_details_BB}

\caption{Schematics of the internal layout of a 4-layer 
TGC package, showing the pad-strip and wire readout for each gas-gap.}
\label{TGC_details}

\end{figure}

The internal structure of each sTGC package is the same as that shown in Fig.~\ref{TGC_details}.
The innermost radial sTGC packages are subdivided into two stair-like packages (to provide full
coverage in 3 out of 4 layers), in order to keep short wires in the high background region. The
readout of the wires is done as an OR of the segments, flagged by the corresponding pad coincidence.
The preliminary dimensions of each sTGC package can be seen in  Fig.~\ref{TGC3}.
This corresponds to a total of 192 packages, each of them containing 4 gas-gaps, 
corresponding to a total of 768 gas-gaps.

The corresponding number of electronics channels is given in Table~\ref{tab:TGC1}, where the pad
surfaces increase from 7.5$\times$7.5\,cm$^2$ to 20$\times$20\,cm$^2$  at the outermost radial
layers. The dimensions of the "small", "medium" and "large" pads are determined with respect to the corresponding background
rates. The pads are shifted every two layers (Fig.~\ref{TGC4}) to reduce the size of the
corresponding Tower-of-Interest, which defines those strips and wires to be
used for the trigger decision and readout.

The internal alignment achieved by the production of the strip patterns in the sTGC gas gap is
precise to better than 50\,$\mu$m, however, the transfer of this precision from gap-to-gap has only
been achieved to within 150$~\mu$m. For this reason, the final precision alignment between gaps will
be done using tracks from the sandwiched MDT. This requires few tracks, since the internal
strip pattern within a gap is extremely precise.

%
% --- Fig.  6 ----------------------------------------------------------------
%

\begin{figure}[!htb]
   \centering
%%  \vspace{-2 pt}
%%%   \includegraphics[width=16cm]{Figures/TGC3.pdf}
\includegraphics[width=0.85\linewidth,clip]{Figures/TGC3}

\caption{Schematic of the detector layout for the case of equal  small-large sectors.
Insert A shows the stairs-type arrangement for the innermost radial sTGC package. The
purple boxes represent the alignment elements for the MDT detectors.}

\label{TGC3}

\end{figure}

The mechanical stability of a TGC package is very good for vertically mounted detectors, based on
ATLAS experience. The use of composite materials allows for very small sensitivity to temperature
changes in the range expected for the Small Wheel. To remove any mechanical sensitivity to magnetic
field in the Small Wheel region, the mounting needs to be kinematic (one fixed point, one sliding
point and one plane), of the same type as presently used in ATLAS.

%
% --- Fig.  7 ----------------------------------------------------------------
%

\begin{figure}[!htb]

\centering

\includegraphics[width=0.55\linewidth,clip]{Figures/TGC4}

\caption{Schematics of Pad and strips arrangement (only 2 layers shown).}

\label{TGC4}

\end{figure}

The processing of the sTGC signals is considerably more complex than for the existing TGCs. An
accurate determination of the track position in the two chamber packages is required, and the
resulting vector has to be extrapolated to the corresponding trigger sector in the Big Wheel. In
this process, the trigger latency must remain below the limit defined by the present ATLAS trigger
system, a limit which will also be valid for Phase~I. Details of the proposed trigger and readout scheme are
given in~\ref{chapter:TGC}.


Each package will be connected to four HV lines, three LV lines and the corresponding
optical fibres that transmit the trigger/data information. The HV lines will be
connected to one corner of the chamber, while the LV and fibres will be connected to
a trigger box, where the logic will be installed. In case of a major failure, the
full TGC-MDT-TGC package will be replaced. This means that two such spare packages
for each of the station types need to be constructed.

%
% --- Table  2 -------------------------------------------------------------------
%

 \begin{table}[!htb]
    \centering
    \begin{tabular}{lr}
      Component     & Number of channels\\
      \hline
      Strips / side & 135,000 \\
      Pads / side   &  30,000 \\
      Wires / side  &  27,500 \\
      For two sides detector & 385,000 \\
      \hline
    \end{tabular}

	\caption{Channel counts for the three readout streams for the 192 packages with
     768 gas gaps.}

    \label{tab:TGC1}
  \end{table}

\newpage

%% ----- 5.2.2-----------------------------------------------------------
%%
\subsubsection{Detector technology and layout of the sMDT chambers}
\label{MDT_technology}
%%
%
% --- Fig.  8 ----------------------------------------------------------------
%

\begin{figure}[!htb]

   \centering
%% \vspace{-2 pt}


\includegraphics[width=0.75\linewidth,clip]{Figures/NSW_structure_layout_3D}

\caption{Structure of the NSW (left) and 3D layout of the sMDT-sTGC chamber system (right) 
         with the global alignment rays indicated as red tubes.}

\label{fig_structure_layout_3D}

\end{figure}

%
% --- Fig.  9 ----------------------------------------------------------------
%

\begin{figure}[!htb]

   \centering
%% \vspace{-2 pt}

\includegraphics[width=0.75\linewidth,clip]{Figures/layout_small_large}

\caption{Layout of the integrated sMDT and sTGC chambers in the small (left) and large (right)
sectors of the NSW.}

\label{fig_layout_small_large}

\end{figure}


The layout of the sMDT chambers for the NSWs follows exactly the present layout of the MDT
chambers in the Small Wheels with fully projective small and large chamber towers  in combination
with the EM and EO layers.
(see Figs.~\ref{fig_structure_layout_3D} and \ref{fig_layout_small_large}).  
The chamber parameters are summarized in Table~\ref{sMDT_layout_parameters}.  There are only three
radial chamber segments in each sector.  The sTGC chambers are precisely positioned and
kinematically mounted on the outer tube layers of the sMDT chambers on both sides. The integrated
sMDT-sTGC chamber packages are connected by light rays to the global optical alignment system and
with the neighbouring chambers via sensors mounted precisely on the outer tube layers of the inner
multilayers of the  sMDT chambers while the in-plane alignment system is integrated into the sMDT
spacer structure which also carries the common chamber supports. The well understood alignment
system implementation guarantees precise absolute chamber  alignment from the beginning which is
essential for the performance of the muon spectrometer. 

%
% --- Table  3 -------------------------------------------------------------------
%

\begin{table}[htb!]

\centering

\begin{tabular}{|l||c|c|c|c|c|c|} \hline

sMDT chambers                & EIL0     & EIL1     & EIL2     & EIS0    & EIS1    & EIS2    \\
\hline
Number of chambers           & 2 x 8    & 2 x 8    & 2 x 8    & 2 x 8   & 2 x 8   & 2 x 8   \\  
Radial extension (mm)        & 1095     & 1095     & 1457     & 1215    & 1276    & 1095    \\
Minimum chamber width (mm)   & 740      & 1428     & 1968     & 678     & 1056    & 1434    \\     
Maximum chamber width (mm)   & 1380     & 1878     & 2418     & 1002    & 1380    & 1704    \\
Minimum tube length (mm)     & 560      & 1248     & 1788     & 498     & 876     & 1254    \\
Maximum tube length (mm)     & 1200     & 1698     & 2238     & 822     & 1200    & 1524    \\
Spacer height (mm)           & 68       & 120      & 120      & 68      & 120     & 120     \\
Thickness in z, tubes (mm)   & 228      & 228      & 228      & 228     & 228     & 228     \\ 
Thickness in z, chamber (mm) & 300      & 300      & 300      & 300     & 300     & 300     \\
Weight/chamber (kg)          & 150      & 130      & 180      & 140     & 120     & 130     \\ 
Number of tube layers        & 2 x 6    & 2 x 4    & 2 x 4    & 2 x 6   & 2 x 4   & 2 x 4   \\ 
Number of tubes/layer        & 72       & 72       & 96       & 80      & 84      & 72      \\
Number of tubes/chamber      & 864      & 576      & 768      & 960     & 672     & 576     \\
\hline
Total number of tubes        & 13824    & 9216     & 12288    & 15360   & 10752   & 9216    \\ 
Gas volume/chamber (l)       & 122      & 162      & 219      & 102     & 110     & 127     \\
\hline     

\end{tabular}

\caption{\small Parameters of the sMDT chambers in the NSWs. The total number of drift tubes is 70656
and the total gas volume in each NSW 6750 l.}  

  \label{sMDT_layout_parameters}
  
\end{table}


The design of the muon drift-tube chambers with 15~mm diameter tubes for the new Small Wheels
follows as closely as possible the current ATLAS MDT chamber design in the endcap region of the muon
spectrometer (Fig.~\ref{fig_smdt_chamber_design}).  The chambers consist of two multilayers of
four or six layers of drift tubes each in densest package. The aluminum tubes of 15~mm outer
diameter and 0.4~mm wall thickness are produced according to industry standard and are chromatized
for cleaning and reliable electric contact. The tubes are separated by 0.1 mm wide glue gaps during
chamber assembly. The two multilayers are separated by a spacer frame carrying the supports of the
integrated drift-tube and trigger chambers on the Small Wheel structure and the in-plane optical
alignment system monitoring the planarity of the chambers. The aluminum spacer frame is of 
similar design as for the present Small Wheel MDT chambers. The main changes in the chamber
design are due to the four times denser packaging of the drift tubes and their gas and electrical 
connections.

%
% --- Fig. 10 ----------------------------------------------------------------
%

\begin{figure}[!htb]  

\begin{center}

\includegraphics[width=0.32\linewidth,clip]{Figures/smdt_chamber_design3D}
\hspace{2mm}
\includegraphics[width=0.65\linewidth,clip]{Figures/smdt_chamber_design1D}

\caption{Small drift-tube chamber design for the new Small Wheels: 3D view (left) and cross sections
		 in the chamber middle planes parallel and perpendicular to the tube layers (right). The
		 mounting of the trigger chambers and of the external alignment sensor is also shown.}

\label{fig_smdt_chamber_design}
% 
\end{center}

\end{figure}

Central to the chamber design is the design of the drift tubes and of their endplugs (see
Fig.~\ref{fig_smdt_tube_design}).  The aluminum tubes have 15~mm outer diameter and 0.4~mm wall
thickness and are produced according to industry standards (DIN). 
The endplugs insulate the sense wire from the tube wall, center the wire in the tubes, position 
it with respect to the  external reference surface on the endplug with an accuracy of only a few micron 
and provide high-voltage-safe connections to the gas distribution manifold  (see
Figs.~\ref{fig_gasmanifold_design}) and to the  24-channel readout and high-voltage interface boards 
(''hedgehog cards'', Figs.~\ref{fig_hedgehogs}). 

The modular gas distribution system consists of injection moulded plastic tubes made of
Pocan\textsuperscript{\textregistered} without glass fiber which interconnect adjacent tubes in
the direction perpedicular to the multilayer and connect them to an aluminum gas distribution bar
on the readout and the high-voltage end of the multilayer.

%
% --- Fig. 11 ----------------------------------------------------------------
%

\begin{figure}[!htb] 
\begin{center}
\includegraphics[width=0.7\linewidth,clip]{Figures/smdt_tube_design_3D} 
%
\caption{Exploded view of a drift tube with gas, signal and ground connections.}
\label{fig_smdt_tube_design}
% 
\end{center}
\end{figure}

The readout and high-voltage supply scheme follows closely the one of the present MDT chambers
using the same parameters and specifications. The high-voltage decoupling capacitors on the readout
side and the terminating resistors on the high-voltage supply end are individually encapsulated in
plastic containers for high-voltage protection. The plastic containers are integrated with the
hedgehog cards plugged onto the gold-plated pins of the signal caps and ground connectors. The
ground pins are screwed into the gaps formed by three adjacent tubes 
In order to during chamber assembly  to connect the tube walls to the ground of the hedgehog cards. This scheme
allows for the high-voltage-free design of the readout hedgehog cards. Each readout hedgehog card
carries a mezzanine card with  the new radiation hard active readout electronics (see below). The
mezzanine cards follow the shape of the hedgehog cards and are currently  under development. The
electronics boards are enclosed in aluminum Faraday cages which also shield the readout hedgehog
cards against the mezzanine cards.  
The modularity of the sMDT chambers in the NSW is given in Table~\ref{NSW_MDT_modularity}. 


%% =======================  section 5.3  ==============================

\newpage

% ==== 5.3 ============================================================
%%%
\subsection{Performance}
\label{Performance}
%%%
%%% ===================================================================

%% \newpage

%% ----- 5.3.1-----------------------------------------------------------
%%
\subsubsection{Performance of the TGC trigger chambers}
\label{TGC_performance}
%%
%% ----------------------------------------------------------------------

\if{0}

{\it
  Summary of chamber performance, details in appendix of technology.

  Spatial and angular resolution as functions of rate and angle of incidence\\
  Time resolution\\
  Efficiency (single measurement and segment)\\
  Double track resolution\\
  Rejection of fake and background tracks\\
}

\fi

The most important issue for a detector that needs to operate at high rate during a period of  more
than 10 years, is its ageing characteristics. For this reason, a series of small TGCs (10 $\times$
10\,cm$^2$) detectors were constructed  and exposed to $\gamma$ irradiation for various periods of
time. No deterioration was observed in any of these prototypes. The one with the longest exposure
was opened after a total irradiation of 6\,Cb/cm of wire (i.e. 33\,Cb/cm$^2$).  Fig.~\ref{TGC5} shows
the rate and current behavior of the detector during the irradiation. No noticeable degradation is
observed after the MIP dose equivalent to  2,000 days of operation at the hottest location in the
ATLAS Small Wheel.

%
% --- Fig. 12 ----------------------------------------------------------------
%

\begin{figure}[!htb]
   \centering
%%\vspace{-2 pt}
%%%   \includegraphics[width=8cm]{Figures/TGC5.pdf}
%%%   \includegraphics[width=8cm]{Figures/TGC5b.pdf}
\includegraphics[width=7cm]{Figures/TGC5}
%% \hspace{5mm}
\includegraphics[width=7cm]{Figures/TGC5b}
\caption{Counting rate and current as a function of the integrated charged in the detector.}

\label{TGC5}

\end{figure}


The irradiated detector was opened to see the effect of the irradiation. Fig.~\ref{TGC6} shows a
microscope picture of a wire in the irradiated region, as well as in the non-irradiated region. The
deposits in the irradiated region were analyzed, the deposits being mainly Carbon and Oxygen, as
expected from the gas composition.

%
% --- Fig. 13 ----------------------------------------------------------------
%

\begin{figure}[!htb]
   \centering
%%\vspace{-2 pt}
\includegraphics[width=0.4\linewidth,clip]{Figures/TGC6a}
\hspace{5mm}
%%\includegraphics[width=0.4\linewidth,clip]{Figures/TGC6b}
%\includegraphics[width=4cm]{Figures/TGC6c}
\caption{Deposits observed in the irradiated (left) and non-irradiated region of a wire.}
\label{TGC6}

\end{figure}


In order to perform the R\&D needed for the use of TGC's in the ATLAS Small Wheels, six large
(ranging from 120$\times$100\,cm$^2$ to 70$\times$40\,cm$^2$) packages of TGCs, each one containing
four gas-gaps, each gap with pads-strips and wire readout, have been constructed. These detectors
were irradiated in various facilities to evaluate their behavior and position resolution. The time,
position and angular resolutions were determined from measurements performed in the H8 facility at
CERN, while the efficiency for MIPs under a high full area irradiation with $\gamma$\'~s was
performed at the SOREQ Nuclear Center (Israel) using $^60$Co source, while triggering on cosmic
muons.

The sensitivity to neutrons was measured at the Demokritos Nuclear Center (Greece).  To simulate
the present proposal, the trigger used at the H8 test-beam was the expected 3-out-of-4 coincidence
of pads. The time resolution of this trigger with respect to a beam-defined scintillator is shown
in units of 0.8\,ns in Fig.~\ref{TGC7} (left). It can be seen that the time resolution of the system
is able to provide the bunch crossing identification with high efficiency.

%
% --- Fig. 14 ----------------------------------------------------------------
%

\begin{figure}[!htb]
   \centering
%%\vspace{-2 pt}
%%%   \includegraphics[width=12cm]{Figures/TGC7.pdf}
\includegraphics[width=0.4\linewidth,clip]{Figures/TGC7}
%%\hspace{5mm}
\includegraphics[width=0.4\linewidth,clip]{Figures/TGC9_BB}

\caption{Left: Time difference between 3-out-of-4 pad trigger with respect to the beam
scintillator in units of 0.8~ns. The FWHM is 11~ns. 99\% of the distribution is within 25~ns.
Right: Strip multiplicity for a MIP on a typical gas gap.}

\label{TGC7}

\end{figure}

Fig.~\ref{TGC8} shows the individual efficiencies of each of the layers for Pads,
Strips and wires of one such package, using the Time-over-Threshold method. It can be
see that they are all fully efficient at an operating voltage of 2.9\,kV, which was
used for the rest of the tests.

The position resolution of each individual gap was measured by using clean muon
tracks in the small tube MDT test detector that was part of the test beam set-up. The
strip multiplicity distribution that provides the charge centroid is shown in
Fig.~\ref{TGC7} (right).


%
% --- Fig. 15 ----------------------------------------------------------------
%

\begin{figure}[!htb]
   \centering

%%% \vspace{-2 pt}
%%%   \includegraphics[width=16cm]{Figures/TGC8.pdf}
\includegraphics[width=0.45\linewidth,clip]{Figures/TGC8_BB}
\caption{Plateau curve for each of the gaps used in the test, using the T-O-T method.}
\label{TGC8}

\end{figure}


The TGC position resolution was calculated from a fit to the difference between the measured
position in a given layer and the position extrapolated from a fit to the other three layers, as
shown in Table~\ref{tab:resolution} (before and after the correction for inter-gap misalignment). A
typical distribution for one layer is shown in Fig.~\ref{TGC10}. It can be concluded that the
position resolution achieved with the TGC layers fulfills the requirements for the NSW and provides
a valuable complement to the position measurements of the MDT.

The position resolution as a function of incident angle has been measured in the past using a
regular ADC to determine the charge in each individual strip. This measurement will be repeated in
the August 2011 test beam using Time-over-Threshold. The dependence of the single
layer resolution as a function of angle is shown in Fig.~\ref{TGC11}.


%
% --- Table  4 -------------------------------------------------------------------
%

 \begin{table}[!htb]
    \centering
    \begin{tabular}{crcc}
     Layer & $\sigma_{corr}(\sigma_{nocorr})\pm\Delta\sigma$[$\mu$m] \\
     \hline
     1  & 66.2 (120.9)  $\pm$ 1.2 \\
     2  & 66.7 (79.8)   $\pm$ 1.1 \\
     3  & 63.6 (76.0)   $\pm$ 1.0 \\
     4  & 63.8 (116.49) $\pm$ 1.0 \\

      \hline
    \end{tabular}

	\caption{Position resolution in the 4 layers after (before) correcting
             for individual layer position.}

    \label{tab:resolution}
  \end{table}

The angular resolution of a single sTGC package was obtained by comparing the angle
between one package of sTGC's (four gas gaps) with the one measured with the Small Tube MDT test
chamber. The difference has a resolution of 3~mrad. Having such a resolution with a
single sTGC package (5\,cm thick) implies that with two packages separated by 300\,mm, the
angular resolution of the system would be better than 0.3\,mrad at the Level-1 trigger.

The measured single layer efficiencies for MIP's in a $\gamma$ background has
been measured by triggering with cosmics under a high irradiation flux. The results
are shown in Fig.~\ref{TGC12}, left. Similarly, this was done with 5.5\,MeV neutrons. The
corresponding single layer efficiency is shown in Fig.~\ref{TGC12}, right.

%
% --- Fig. 16 ----------------------------------------------------------------
%

\begin{figure}[!htb]
   \centering
%% \vspace{-2 pt}
%%%   \includegraphics[width=12cm]{Figures/TGC10.pdf}
\includegraphics[width=0.4\linewidth,clip]{Figures/TGC10}
\caption{Typical residual for one of the layers with respect to the fitted line.}
\label{TGC10}

\end{figure}


%
% --- Fig. 17 ----------------------------------------------------------------
%

\begin{figure}[!htb]
   \centering
%%% \vspace{-2 pt}
%%%   \includegraphics[width=12cm]{Figures/TGC11.pdf}
\includegraphics[width=0.5\linewidth,clip]{Figures/TGC11_CC}
\caption{Resolution vs. incident angle as measured with muons in H8.}
\label{TGC11}
\end{figure}

%
% --- Fig. 18 ----------------------------------------------------------------
%

\begin{figure}[!htb]
   \centering
%%% \vspace{-2 pt}
%%%   \includegraphics[width=16cm]{Figures/TGC12.pdf}
\includegraphics[width=0.45\linewidth,clip]{Figures/TGC12}
\hspace{2mm}
\includegraphics[width=0.45\linewidth,clip]{Figures/TGC13}
\caption{Left: Single layer efficiency in kHz/cm$^2$ on a large
chamber under uniform $\gamma$-irradiation (strip length= 1.2~m).
         Right: Single layer efficiency for MIPs under 5.5~MeV neutron irradiation.}
\label{TGC12}

\end{figure}

%%%%%%%%%%%%%%%%%%%%%%%%

Finally, by using the Time-over-Threshold method, one can reject single gas-gaps measurements,
polluted by a $\delta$-ray, a $\gamma$ or a neutron-conversion, by demanding less than six
strips in a cluster and a cut on the ToT. This is illustrated in Fig.~\ref{TGC14}, where one can
see the passage of a muon through the four gas-gaps, (a) without contamination, (b) with a
$\delta$-ray in layer one and (c) with a neutron impinging in layer three.

%
% --- Fig. 19 ----------------------------------------------------------------
%

\begin{figure}[!htb]
   \centering
\includegraphics[width=4cm]{Figures/TGC14a}
\hspace{2mm}
\includegraphics[width=4cm]{Figures/TGC14b}
\hspace{2mm}
\includegraphics[width=4cm]{Figures/TGC14c}

\caption{Event display of a) a clean muon track, b) a muon track  with a $\delta$-ray in the first
gas-gaps and c) a neutron interacting in the third gas gap. The unit of the horizontal axis is
25\,ns/division.}

\label{TGC14}

\end{figure}

\newpage


%% ----- 5.3.2-----------------------------------------------------------
%%
\subsubsection{Performance of the sMDT tracking chambers}
\label{MDT_performance}
%%
%% ----------------------------------------------------------------------

In order to verify the performance of the 15\,mm diameter (''small'') drift tube chambers, extensive
tests of the   full-scale prototype chamber with 15\,mm diameter drift tubes have been
performed  in the H8 high-energy muon beam in 2010 and 2011 together with integrated TGC
chambers (see Fig.~\ref{fig_prototype_H8}, left) and,  using cosmic ray muon tracks, in the
CERN Gamma Irradiation Facility (GIF) with a 500\,GBq $^137$Cs source
(Fig.~\ref{fig_prototype_H8}, right). The standard ATLAS MDT readout electronics has been used
with the adjustable deadtime set to the minimum value corresponding to an overall effective
deadtime of 200\,ns.  The spatial resolution and efficiency of the individual drift tubes and of
the whole chamber have been measured for different $\gamma$ irradiation rates where the
individual drift tubes have been illuminated uniformly over the whole length.

As a measure for evaluating the tracking efficiency, we define the ''3$\sigma$-efficiency'' as the
probability of detecting a tube hit within a distance from the muon track of 3 times the drift tube resolution $\sigma$,
where the muon track is defined by the other tubes on the track and extrapolated to
the active volume of the tube. Similarly, the drift tube resolution as a function of the drift
radius is determined by an iterative method excluding the evaluated tube from the track
reconstruction in the chamber~\cite{ref_resolution_measurement}.

%
% --- Fig. 20 ----------------------------------------------------------------
%

\begin{figure}[!htb]
\begin{center}
\includegraphics[width=0.58\linewidth,clip]{Figures/efficiency_vs_flux_CC}

\caption{The $3\sigma$ efficiency of individual drift tubes of 0.5~m length (as at the inner bore of the Small Wheels) 
as measured at the GIF as a function of the background hit rate. Results for 15~mm and 30~mm diameter tubes are compared. The
corresponding calculated track segment reconstruction efficiencies in 2 x 4 layer MDT chambers and in 
sMDT chambers with 2 x 6 or, optionally, 2 x 8 tube layers are also shown.}

\label{fig_efficiency_GIF11_intro}
%
\end{center}
\end{figure}

Fig.~\ref{fig_efficiency_GIF11_intro} shows the average 3-$\sigma$ efficiency of 15\,mm diameter drift
tubes and the track segment reconstruction efficiency of the prototype chamber as a function of the
$\gamma$ background flux in comparison with the results for the 30\,mm diameter tubes from previous
tests of a BOS MDT chamber in GIF~\cite{ref_deile}, \cite{ref_horvat}.  At low background rate, the
$3\sigma$ tube  efficiency of 30 and 15\,mm diameter tubes is 94\%. The
efficiency loss is due to $\delta$-rays created by interactions of the muons in the tube walls
which produce hits earlier than the muons. The degradation of the efficiency with increasing rate
follows the predicted behaviour.  The redundancy of track measurements in the different drift tube
layers is sufficient  to achieve more than 99\% track segment reconstruction efficiency up to the
highest background rates.

F%
% --- Fig. 21 ----------------------------------------------------------------
%

\begin{figure}[!htb]
\centering
\includegraphics[width=0.58\textwidth]{Figures/dtresolrate_CC}

\caption{Dependence of the {\it average} drift tube resolution (with time-slewing corrections) 
on the background hit rate.
The measurements for 30~mm and 15~mm diameter drift tubes at the GIF are compared.}

\label{fig_dtresolrate_intro}
\end{figure}


ig.~\ref{fig_dtresolrate_intro} shows the dependence of the average 30~mm and 15~mm diameter
drift tube resolution (taking into account time slewing corrections) 
on the background hit rate measured at the GIF. 
The measurements confirm the expectation that
the degradation rate of the spatial resolution with the background flux is reduced by a factor of 10 for 15~mm
compared to 30~mm diameter tubes (see the discussion in the Appendix).

At low background rates, the average resolutions of 15 and 30\,mm diameter
drift tubes at low background rates have been measured to be $110\pm 2\,\mu$m and $83\pm 2\,\mu$m, respectively.
At the maximum background hit rate of 10~kHz/cm$^2$ at which the GIF measurements could be performed,
the average 15~mm diameter drift tube resolution is $143\pm 4~\mu$m. 
At 14 kHz/cm$^2$ rate it will still be better than $170~\mu$m.

\newpage

%% =======================  section 5.4  ==============================

% ==== 5.4 ============================================================
%%%
\subsection{sTGC Level-1 trigger and readout electronics}
\label{chapter:TGC}
%%%
%%% ===================================================================

\if{0}

{\it
  How the L1 signal are produced, starting from the detector signal
  till the formation of SL input.\\
  Latency (calculation, measurement with demonstrator)\\
  Compatibility with Phase II upgrade    RR to supply text !!! \\
}

{\it Comment: Here we will show a diagram showing the two packages with the MDT in
between. }

\fi


The strip signals in the sTGC of the NSW are used to extrapolate tracks to the Big Wheel where they are
corroborated by the Sector Logic with the existing 2-out-of-3 and 3-out-of-4 coincidences in the Big Wheel
chambers. The fine granularity of the extrapolation results in high efficiency for trigger rejection.
The electronics added to accomplish this must not cause the latency to the Sector Logic to exceed the existing 1.05\,$\mu$s.
A detailed breakdown of the sTGC trigger latency is given in the appendix (\ref{appendix:Latency}).

The sub-sector in the Big Wheel trigger station of the extrapolated muon is found as follows: In the two sTGC packages of the NSW, a
local trigger is made from two sets of 3-out-of-4 pad coincidences, corresponding to a tower of four
pads in each of the two quadruplets. Pads in alternating layers are shifted by half a pad in both dimensions to give a 4-times higher
granularity of logical towers. The triggering tower selects the strips in its region for transmission to
eight centroid finders, one for each layer. This selection leads to a 20-fold reduction in the number of
strips to be processed as well as in the number of centroid finders. The four centroids in each sTGC
package are averaged to give two points in 3-D space, defining a vector which points to a sub-sector in
$\eta$,\,$\phi$ of the Big Wheel. The $\sim$60\,$\mu m$ resolution together with their distance of 250\,mm provides an
angular accuracy of better than 0.4\,mrad.

A simple block diagram of the trigger processing chain is shown in Fig.~\ref{appendix:TGC1}.
Its elements are elaborated below.
The trigger functionality is split between on- and off-chamber locations.
Two architectural options are discussed.
A more detailed block diagram of the sTGC trigger logic for two quadruplets is shown in
appendix (\ref{appendix:Latency}) as part the description of a trigger demonstrator.
In order to measure the trigger latency for a realistic FPGA design,
a demonstrator has been implemented with a commercial FPGA evaluation board.


\textbf{ASD:}
The baseline is to use BNL's VMM Front End chip, now under development. The planned analog parameters of this chip,
0.11 to 2.0\,pC input charge range, 20-25\,ns peaking time, and time-over-threshold discriminator, match
the needs of the sTGC detectors. The first prototype chips will be
available early in 2012. The sTGC version of this ASD has LVDS discriminator outputs suitable
for the Time-over-Threshold trigger logic.

\textbf{Pad tower trigger:}
The Pad tower trigger, whose logic is described above, is on-chamber and implemented in radiation tolerant electronics.
The coincidence tags a bunch crossing and the group of strips passing through the triggered tower in each layer that must
be selected for processing by the Selector.

\textbf{Selector:} 
From the pad tower trigger, the Selector receives the Id of one of about 20
groups of 16~strips that must be routed off-chamber for further processing.

\textbf{Time-over-Threshold converter:}
This module measures the time between the rising and falling edges of the ASD Time-over-Threshold level
in steps of 1\,ns and with a 7-bit dynamic range.
This time is approximately in proportion to the charge on each strip.
Optimized parameters will depend on the final ASD characteristics.
The converter is fairly simple. The demonstrator implementation uses a 1-to-10 serial-to-parallel converter
and logic running at 1/10$^{th}$ the sampling clock speed to count the time between edges.

\textbf{Centroid finder and averager:}
The centroid finder currently calculates the centroid from the charge on up to five strips.
Centroids are included in the average only if the number of strips above threshold is below a
certain maximum value in order to exclude $\delta$-rays and conversions (see Fig.~\ref{TGC14}).
It is expected to implement this logic off-chamber in reprogrammable FPGAs.

\textbf{Extrapolator:}
The extrapolation of the track vector to the Big Wheel is done using Look-up-Tables (LUTs) for both
the R and $\phi$ coordinate. The LUT inputs for a given R are the centroid average, the radius and
the difference in the centroid averages of the two quadruplets. The LUT input for $\phi$ is the
triggering pad tower ID.

We seek to minimize the amount of on-chamber electronics, because access and power are very limited for the Small Wheel,
and on-chamber electronics must be radiation tolerant. Radiation tolerant ASIC's are likely required on-chamber
even though reprogrammable FPGAs would be more flexible.
%%Reprogrammable FPGAs rated to 700kRad are available, but are expensive.
Radiation tolerant FPGA's are possible for locations away from the Small Wheels.

Two architectural options for the Selector and Time-over-Threshold to digital converter are shown. One option requires
more complex radiation tolerant electronics, but can handle multiple triggers; the other option requires higher bandwidth links.
The two options, detailed below, are:

\textbf{Option A: Time-over-Threshold converter per channel, either in, or behind the ASD, but
BEFORE the Selector:} Here, on every beam crossing, the Selector selects 20~strip ToT's per layer
(7-bits each) indicated by the pad tower coincidence. The needed link bandwidth to the off-chamber
Centroid Finder (including ToT's, BCID, R and $\phi$ IDs, parity) is 140\,+\,20\,=\,160 bits at
40\,MHz = 6.4\,Gb/s.

\textbf{Option B: Time-over-Threshold AFTER the Selector:}
Here, the ToT converter can be off-chamber with the Centroid Finder; also fewer ToT converters are needed.
The needed link bandwidth from the Selector to the off-chamber TOT converter is: 20~channels at
1\,Gb/sec per channel = 20\,Gb/sec for the expected maximum ASD pulse width of 100\,ns.

The disadvantage of Option A is the increase in complexity and amount of on-chamber radiation tolerant electronics.
The ToT converters are not overly complex, resembling simple deserializers, but on-chamber radiation tolerant ASICs
are none-the-less higher risk than off-chamber FPGAs.

The disadvantage of Option B becomes clear when the background is high enough that another pad tower
trigger occurs in one of the 19~non-selected groups of strips during the 100\,nsec of the TOT conversion.
Such triggers will be lost since both the selector and ToT converter are busy until the end of the 100\,nsec maximum pulse duration.
One can increase the complexity of the selector and route subsequently triggered strips in a round-robin
fashion to additional TOT modules after the selector. This increased complexity of the radiation tolerant
selector probably eliminates any advantage of this option. Option A on the other hand can handle a pad
tower trigger anywhere every 25\,nsec.

%% !!! Comment RR: the following sentence sounds a bit open-ended. The BG estimates are now much
%%                 better than before the start-up and for the technical parameters, maybe one could
%%                 give an approximate time scale?
The choice between the options depends on reliable background estimates and details of radiation
tolerant ASICs and of serializer speeds and power, all of which are not sufficiently known at this
time.

Further details of the sTGC Level-1 trigger and readout electronics can be found in
appendix (\ref{appendix:Latency}).

\if{0}

  %%%%%%%%%%%%%%
\begin{figure}[!htb]
   \centering
\vspace{-2 pt}
%%%   \includegraphics[width=16cm]{Figures/TGC15.pdf}
\includegraphics[width=0.6\linewidth,clip]{Figures/TGC15}
\caption{The trigger demonstrator board based on a commercial evaluation board.}
\label{TGC15}
\end{figure}
%%%%%%%%%%%%%%%

\fi



\paragraph{{Latency (calculation, measurement with demonstrator)\\}\\}

A detailed discussion of the latency expected for the sTGC readout is presented in the appendix
(\ref{appendix:Latency}).


\paragraph{{Compatibility with Phase II upgrade\\}\\}

The pipelines and derandomizers will be designed to handle a 500\,kHz Level-1 trigger
rate and to accommodate up to 10\,$\mu$s Level-1 latency.


\paragraph{{Time schedule for the sTGC electronics development\\}\\}

The schedule for prototyping, development and production of the sTGC
trigger electronics is given in the appendix~\ref{appendix:TGC_project}, Table~\ref{tab:TGC2}.

\newpage

%% =======================  section 5.5  ==============================

% ==== 5.5 ============================================================
%%%
\subsection{Readout Electronics of the sMDT Chambers}
\label{chapter:MDT_electronics}
%%%
%%% ===================================================================

\if{0}

{\it
  Detailed description of electronics chain.-- Integration to DAQ (Mention the GBT?).--
  Readout related parameters, e.g. bandwidth requirements, number and granularity of
  read-out links.-- Compatibility with Phase II upgrade.
}
\fi

The readout of the Small MDT tubes will follow the proven architecture of the present MDT
system. A number of additions and modifications, however, will be necessary in order to
adapt the rate capability of the readout chain to the requirements of the SLHC. For the
mechanical integration of the readout electronics with the Small tube chambers, the
layout of the electronics will have to be adapted to the four times higher tube density at
the ends of the chambers. Finally, the radiation tolerance of all components
will have to comply with the high radiation levels, in particular at the inner boundary of
the Small Wheel.


%% ----- 5.5.1-----------------------------------------------------------
%%
\subsubsection{Architecture of the present MDT chamber readout}
\label{MDT_architecture}
%%
%% ----------------------------------------------------------------------

In the present readout scheme tube signals are routed via a PCB ("hedgehog card") to a
piggy-back card ("mezzanine card"), containing an Amplifier and Shaper, followed by a
Discriminator with adjustable threshold, all three functions being integrated in a
radiation tolerant ASIC (ASD). The discriminator outputs, in turn, are routed to a TDC,
where each leading and trailing edge signal receives a high-precision time stamp, which
is retained, together with the corresponding channel number, in a large internal buffer
of the TDC ("Level-1 buffer").

%
% --- Fig. 22 ----------------------------------------------------------------
%

\begin{figure}[!htb]
%
\begin{center}
%
\includegraphics[width=0.48\linewidth,clip]{Figures/MDT_electronics_scheme_old_BB}
%% \hspace{2mm}
\includegraphics[width=0.48\linewidth,clip]{Figures/NSW_ELX_diagram_w_GBT}

\caption{Schematic diagram of the present (left) and of the future MDT readout chain (right). Using
		 the GBT in the new readout scheme improves the readout bandwidth and simplifies logistics,
		 as TTC, Data readout and DCS are transferred over the same bi-directional link.
		 The omission of CANbus cabling due to GBT will reduce the risk of ground loops between
		 chambers and improve system reliability.}

\label{readout_chain}

\end{center}

\end{figure}

When the TDC receives a Level-1 trigger, a subset of the recorded hits, corresponding to
a pre-defined time window, are retained for readout and are forwarded to the data
concentrator of this chamber, the "Chamber Service Module" (CSM). From there, data are
sent to the off-chamber electronics in USA15, the "Readout Driver" (ROD). A CSM can
serve up to 18 mezzanine cards. The operation parameters of the analog frontend (ASD)
and the TDC are controlled by a JTAG string, which is distributed by the Detector
Control System (DCS) to the CSM, which sends it individually to each mezzanine card.
Fig.~\ref{readout_chain} gives the layout of the present system. A detailed presentation
of the MDT readout electronics is given in ~\cite{MDT_ELX_paper}. The ASD and TDC ASICs
are described in ~\cite{ASD_manual} and ~\cite{TDC_manual}.


%% ----- 5.5.2-----------------------------------------------------------
%%
\subsubsection{Architecture of the sMDT chamber readout}
\label{MDT_architecture_Small_tubes}
%%

For the evolution of this architecture into matching the requirements of the Small tube readout, a
number of problems limiting the performance of the present scheme has to be overcome. Given the
high rate capability of the Small MDT tubes, the bandwidth of the readout system has to be
increased. Due to the limitations of the internal buffering scheme and processing speed, the
present TDC can only handle average tube rates up to about 300~kHz per tube without losing data,
while tubes in the NSW would run up to 1200\,kHz (Table~\ref{NSW_efficiencies}). An improved TDC is
therefore an essential requirement for a new readout design.

Another limitation of the readout bandwidth comes from the optical link, connecting the CSM to the
ROD, which only provides a usable bandwidth of 1,4 Gbit/s. Fig.~\ref{occ_and_link}
shows occupancy and efficiency of Large and Small tubes as a function of tube hit rate.

%
% --- Fig. 23 ----------------------------------------------------------------
%

\begin{figure}[!htb]

\begin{center}

\includegraphics[width=0.45\linewidth,clip]{Figures/NSW_efficiency_occupancy_saturation_CC}

\caption
{Green: Occupancy as a function of tube hit rate, assuming a dead time of 200 ns (equal to the
maximum drift time) after each hit. Red: Resulting efficiency with 200\,ns dead time. Blue:
Saturation of the mezzanine-to-CSM link at 100 kHz L1 trigger rate. The load of 15\% at zero hit
rate is due to overheads in the transfer protocol.} 

\label{occ_and_link}

\end{center}

\end{figure}

The technologies used for the existing ASICs in the MDT readout are no more available from
industry, and the following new components have therefore to be introduced into the new readout
system.

\begin{enumerate}

  \item[(a)] The ASD is re-designed in the IBM 130\,nm technology. A 4-channel
  prototype, demonstrating the analog parameters, has already been produced and works
  correctly. Most analog parameters of the previous design are preserved. A full 4-channel
  prototype chip including the digital part (see Fig.~\ref{fig_ASD_new}) has been delivered 
  in September 2011 and is under test. The final step in the ASD chip design is the extension
  to 8 channels.  

  \item[(b)] The TDC will be replaced by the HPTDC, designed by the CERN-MIC group
  (~\cite{The_HPTDC}). This 32-channel device has an improved internal buffering
  scheme as well as higher transfer and processing speeds. If only 24 out of the available 32
  channels are used, the unused channels can be disabled to save power.

  \item[(c)] The CSM collects data from a MDT chamber, formats the event and sends data,
  trigger-by-trigger, via an optical link to the ROD in USA15. All logical operations are performed
  by an FPGA, which will need to be upgraded to higher radiation tolerance. While the present CSM
  serves up to 18 mezzanine cards, the new CSM will have to serve up to 24 cards.

  \item[(d)] The link connecting the CSM to the ROD will be replaced by a GigaBit
  Transmitter link (GBT), developed by CERN. This link provides a 3 times higher
  transfer rate (Gbit/s), compared to the presently used S-link~\cite{GBT}.


  \item[(d)] The readout of voltages and temperatures on the mezzanine cards will be done via the
  DCS channel of the GBT. To collect ADC readings from temperature sensors on the chamber and from
  B-field probes, a new version of the ELMB will be used. Aim would be to interface this new device
  (''ELMB++'') directly to the CSM, so these data could be read out through the DCS channel of
  the GBT. This way, the presently used CANbus cabling could be omitted, simplifying installation
  and reducing the risk of ground loops.

\end{enumerate}

%
% --- Fig. 24 ----------------------------------------------------------------
%

\begin{figure}[!htb]
\begin{center}

\includegraphics[width=0.6\linewidth,clip]{Figures/ASD_new}
%
\caption{The new 4-channel ASD chip with full analog and digital functionality on the test board.}

\label{fig_ASD_new}

\end{center}

\end{figure}


%
% --- Table  5 -------------------------------------------------------------------
%

\begin{table}[!htb]

 \begin{center}
 
    \begin{tabular}{|c|c|c|c|c|c|c|c|} \hline 
Chamb.& Tubes$\times$      & Tubes/    & Tubes & Mezz.cards/ & Mezz.cards & CSMs/    & CSMs  \\ 
type  & layers$\times$MLs  & chamber   & total & chamber     & total      & chamber  & total \\   
\hline 
\hline
  EIL0 &  $72\times6\times 2$ &  864   & 13824 &    36       &    576     &    2     & 32  \\
  EIL1 &  $72\times4\times 2$ &  576   &  9216 &    24       &    384     &    1     & 16  \\
  EIL2 &  $96\times4\times 2$ &  768   & 12288 &    32       &    512     &    2     & 32  \\
\hline
  EIS0 &  $80\times6\times 2$ &  960   & 15360 &    40       &    640     &    2     & 32  \\
  EIS1 &  $84\times4\times 2$ &  672   & 10752 &    28       &    448     &    2     & 32  \\
  EIS2 &  $72\times4\times 2$ &  576   &  9216 &    24       &    384     &    1     & 16  \\

\hline \hline

	  \multicolumn{3}{|l|}{Total side A + C:}
	 &\multicolumn{1}{c|}{70656}&\multicolumn{1}{l|}{}
	 &\multicolumn{1}{c|}{2944} &\multicolumn{1}{l|}{}
	 &\multicolumn{1}{c|}{160}\\
\hline

  \end{tabular}
 \end{center}
 
\caption{Electronics channels (drift tubes) and readout boards in the NSW. 
Each mezzanine card serves 24 drift tubes, matching the modularity of the chambers with 
6 and 4 tube layers per multilayer. Each CSM serves up to 24 mezzanine cards. Chambers with more
than 24 mezzanine cards are served by two CSMs.\label{NSW_MDT_modularity}} 

\end{table} 

Another stringent requirement for the realization of the new readout scheme is
the mechanical integration of the on-chamber readout electronics with the chamber
mechanics, as the density of channels at the tube ends is four times higher than in the
case of the 30\,mm tubes. The following design changes will therefore be implemented.

\begin{enumerate}

  \item[(a)]
  HV decoupling capacitors are located in 15\,mm diameter cylinders at the tube ends, see
  Fig.~\ref{fig_hedgehogs}. The density of signal routing on hedgehog cards
  is thus no longer limited by insulation distances between HV and ground or 
  signal traces on the PCB.

  \item[(b)] To match the higher tube density, the space on the mezzanine card has to be used
  more efficiently. For this purpose, the passive protection circuitry of the ASD inputs
  will be moved to the hedgehog card.

  \item[(c)] For the interconnection between mezzanine cards and new CSM, we foresee to use the
  same 40-wire cables as was used in the present system. Its moderate diameter and high
  flexibility may be important for fitting the new MDTs into the limited space in the Small Wheel.

\end{enumerate}

The modularity of the readout system of the Small tube sMDTs for the NSW is given in
Table~\ref{NSW_MDT_modularity}.


\paragraph{{Power consumption of the electronics\\}\\}

While the power consumption of the on-chamber frontend electronics of the present system is about
30--40~W/chamber (\cite{MDT_ELX_paper}, table 3), the consumption will now be about 2.5 times as
high, due to tube density and increased consumption of TDC, optical link and other component.


%
% --- Table  6 -------------------------------------------------------------------
%

\begin{table}[!ht]

 \begin{center}
    \begin{tabular}{|c||c|c|c|c|c|c|} \hline %{@{}llc@{}}

Chamber& mezz.s/&current/&current/&current/&power/  &power/  \\
type   & chamber& mezzan.& CSM    &chamber &chamber &chamber \\
       &        &        &        &        &at 3.3 V&at 4.5 V\\
\hline
       &        &\it{A}  &\it{A}  & \it{A} &\it{W}  &\it{W}  \\
\hline
  EIL0 &   36   &    18  &     3  &  21    &  69.3  &  94,5  \\
  EIL1 &   24   &    12  &   1.5  &  14    &  44.6  &  60.8  \\
  EIL2 &   32   &    16  &     3  &  19    &  62.7  &  85,5  \\
\hline
  EIS0 &   40   &    20  &     3  &  23    &  75.9  & 103.5  \\
  EIS1 &   28   &    14  &     3  &  17    &  56.1  &  76.5  \\
  EIS2 &   24   &    12  &   1.5  &  14    &  44.6  &  60.8  \\

\hline \hline

	    \multicolumn{5}{|l}{Power per Large + Small sector [W]}
	   &\multicolumn{1}{|c|}{353}&\multicolumn{1}{c|}{482}\\
\hline

        \multicolumn{5}{|l}{Power per side [W]}
	   &\multicolumn{1}{|c|}{2825}&\multicolumn{1}{c|}{3852}\\

\hline

  \end{tabular}
 \end{center}

 \caption{Power requirements for the readout electronics of the NSW. Each mezzanine card consumes
 about 0.5~A, each CSM about 1.5~A at 3.3~V. The voltage delivered to
 the CSM is assumed to be 4.5~V, the voltage difference being consumed in local voltage regulators.
		  \label{NSW_Small_tube_elx}}

\end{table}

A detailed breakdown of the expected power consumption of the MDT chambers in the NSW is presented
in Table~\ref{NSW_Small_tube_elx}. The total power consumption of 8.4 kW is less than the
consumption of the presently installed CSC (11.5 kW, see~\cite{Aad:2008zzm}, Table 46).  Yet, the
removal of the heat may require flushing with dry air, as significant temperature gradients may
influence the spatial resolution of the MDT and natural air convection is
obstructed by the tight space constraints in the high-$\eta$ region of the Small Wheel.


%% =======================  section 5.6  ==============================

% ==== 5.6 ============================================================
%%%
\subsection{Services, infrastructure and DCS}
\label{chapter:Services}
%%%
%%% ===================================================================


{\it
  \begin{enumerate}
  \item Description of service scheme (including power system,
    read-out, trigger, alignment), cooling needs and other special
    requirements
  \item Table with number of services (number of cables, outer
    diameter, cross section of leads)
  \item Table with power consumption (per channel, chamber, total)
  \item Required rack space
    \begin{enumerate}
    \item UX15 (include maximum allowed distance to detector if any)
    \item US15 (power system)
    \item USA15 (DAQ)
    \end{enumerate}
  \item Gas system and distribution\\
    Details on number of gas manifolds per sector (include drawings)
    and connections to chambers (serial, parallel?)). Size of pipes\\
    Required nominal, minimum, and maximum flow\\
    Required precision of gas mixture\\
    Safety measures in case of inflammable gas\\
    Required rack space for gas system in SGX1, USA15, UX15
  \item Integration in DCS system, requirements for DCS
  \end{enumerate}
}


\paragraph{Gas system and distribution}

The gas system is a natural extension of the present TGC gas system,  which will be extended from
the present eight manifolds  (four for EIL4 TGCs and four for the SW TGC chambers)  to one
containing 20 manifolds (four for the EIL4 chambers  and 16 for the new Small Wheel chambers
(one/layer for the  upper and similarly for the lower chambers). This will be less  than the normal
manifolds in the Big-Wheels and can be easily  be added to the present SW racks. The manifolds will
be running at a flow rate of 40l/hr of the regular TGC gas mixture, which corresponds to a volume
exchange every four hrs. The extra flow corresponds to a 25\% increase to the present TGC gas flow,
which can be implemented in the present gas system, without any additional modifications, except
the extra flowmeters in the present SW racks. The safety system for the new detectors will be the
same as for the present one, that were approved by TIS.

\paragraph{Integration in DCS system, requirements for DCS}

   \begin{itemize}
 \item Chamber charge monitor
 \item HV control and monitoring, trip recovery
 \item LV power control and monitoring
 \item Temperature monitoring
 \item Link health statistics
 \item ASD configuration parameters (threshold, peaking time, etc.)
 \item Front End trigger and readout configuration parameters (delays, channel masks,
       pipeline depth, etc.)

   \end{itemize}



% --- Table  xx -------------------------------------------------------------------
%
  \begin{table}
    \centering
    \begin{tabular}{lccc}
      Cable & Number of cables & Outer cable & Cross section\\
      & (granularity) & diam. (mm) & of leads (mm$^{2}$))\\
      \hline
      HV & & & \\
      LV & & & \\
      Monitoring and control & & & \\
      Front-end links & & & \\
      Calibration & & & \\
      Alignment & & & \\
      Miscellaneous & & & \\
      \hline
    \end{tabular}
    \caption{Example table number of services per chamber}
    \label{tab:services}
  \end{table}


  \begin{table}
    \centering
    \begin{tabular}{lccc}
      Chamber & Number of channels & Power consumption per channel &
      Total power consumption\\
      \hline
      & & & \\
      \hline
    \end{tabular}
    \caption{Example table: Number and types of chambers per sector}
    \label{tab:sector_type}
  \end{table}

%%% Sample tables commented out on 22.07.11 RR

\if{0}

Now comes a first example of a table

  \begin{table}
    \centering
    \begin{tabular}{lrr}
      Micromega/TGC/RPC/MDT & Type 1 & Type 2\\
      \hline
      Number of chambers & & \\
      Radial extension (mm)& & \\
      Minimum length in $\phi$ (mm)& & \\
      Maximum length in $\phi$ (mm)& & \\
      Thickness in z (mm)& & \\
      Mass (kg) & & \\
      Number of layers & & \\
      Number of channels & & \\
      \hline
    \end{tabular}
    \caption{Example table chamber types}
    \label{tab:chamber_type}
  \end{table}

Now comes a second example of a table

  \begin{table}
    \centering
    \begin{tabular}{lrr}
      Chamber & Small sector & Large sector\\
      \hline
              &  put in      &  a number! \\
      \hline
    \end{tabular}
    \caption{Example table: Number and types of chambers per sector}
    \label{tab:sector_type}
  \end{table}

\fi

