%%% 
\setcounter{chapter}{6}
%
\setcounter{section}{1}
%
\setcounter{figure}{58}
%

This file is located in /home/iwsatlas1/richterr/ibericht/ATLAS-MDT.

%
\section{The Muon Spectrometer}
%
The muon spectrometer~\cite{muon_tdr,ATLAS_detector_paper} of the ATLAS experiment 
is equipped with three layers of muon detectors in a toroidal
magnetic field of $3-6\,$Tm bending power generated by a superconducting
air-core magnet system. 
The spectrometer is designed to provide muon momentum resolution of better
than $10\%$ for transverse momenta up to $1\,$TeV/c over a pseudo-rapidity
range of $|\eta|\leqslant 2.7$. This requires very accurate track
sagitta measurement with three layers of muon detectors which have to be aligned
relative to each other with an accuracy of up to $30\,$\micron\ in the bending direction
in the magnetic field. Drift chambers with very high
spatial resolution of $40\,$\micron\, the Monitored Drift Tube (MDT) chambers,
have been developed to cover the active area of the spectrometer of $5500\,$m$^2$
with only $5\%$ gaps mainly in the region of the detector feet.

%

The cylindrical central part of the spectrometer (barrel)
contains eight race-track shaped magnet coils of $25\,$m length and $5\,$m radial width.
The layout of the muon chambers follows the eight-fold symmetry of the magnet
around the proton beam axis in eight small and eight large azimuthal sectors.
%%   (see Fig.~\ref{fig:ATLAS_MDT_barrel_installation})
The barrel part of the spectrometer
is complemented by two endcaps each consisting of eight superconducting coils housed
in a common cryostat fitting into the inner bore of the barrel toroid magnet and three
wheel-shaped layers of muon detectors 
%%   (see Fig.~\ref{fig:ATLAS_MDT_ECT_installed}).

The MDT chambers built in Munich consist of two triple layers of $30\,$mm diameter aluminium drift
tubes of $3.8\,$m length equipped with a central gold-plated tungsten-rhenium sense wire which are
separated by an aluminum space frame. The drift tubes are operated with Ar:CO$_2$ (93:7) gas
mixture at a pressure of $3\,$bar and a gas amplification of 20000 (corresponding to a operating
voltage between tube wall and wire of $3080\,$V) and provide a position resolution of about
$80\,$\micron\ . The sense wires are positioned within a chambers with an accuracy of better than
$20\,$\micron\ in order to achieve the required spatial resolution of the
chambers~\cite{mdt_constr1}.

%In the years 2001 to 2006, 88 MDT chambers have been constructed at the MPI~\cite{mdt_constr2}
%containing about 36000 drift tubes of $3.8\,$m length.
They cover about $15\%$ of the active area of the spectrometer. The installation of these
chambers in the outermost layer of the barrel part of the spectrometer took place in 2005 and 2006.
The commissioning of the chambers in the ATLAS detector is expected to be complete
by summer 2008, just before the start of LHC data taking.

(The text of the first column is only a placeholder, 
so the upgrade part starts at the second column, similar to the version for the Fachbeirat.)
%
%   --------------------------------------------------------------------------------------
%
%    aaa bbb xxx zzz


%   =========================================================================================
\section{Upgrade activities}
%   =========================================================================================
%

The design luminosity of the LHC of
$10^{34}\,$cm$^{-2}$s$^{-1}$ is expected to be reached after a few years of data taking at lower
luminosity.
In an experimental run at the LHC the design luminosity will only be available at the beginning of
a machine fill, then it decays slowly due to proton losses and beam size increase. 
When the
luminosity has decreased to about half the initial value after typically 5 hours, the beams will be dumped and a new proton fill
will be dumped and a new proton fill will be accumulated. 
The {\it{integrated}} luminosity after 10 years of operation under these
conditions is expected to be about $\sim$ 300 fb$^{-1}$.

For the study of even lower cross section phenomena at the LHC, a substantial increase of the LHC
luminosity is needed.
With a series of upgrades of the CERN accelerator complex an order of magnitude could be achieved, 
which may yield 
an integrated luminosity of up to 3000 fb$^{-1}$ after 10 years of additional running 
(Super-LHC or SLHC). 
To help the LHC detectors to
cope with this luminosity increase, 
a new  operation mode of the machine is foreseen (''luminosity levelling''), 
reducing the decay rate of luminosity during a proton fill in such a way that the
peak luminosity at the beginning of the fill will be limited to  about 4 $\times$
$10^{34}\,$cm$^{-2}$s$^{-1}$, yet yielding a 10 times higher integrated luminosity for the fill. 

While this reduction of the {\it peak} relative to the {\it integrated} luminosity by a factor of
2.5 represents a substantial alleviation for the operation of the ATLAS detector, the increase of
luminosity by a factor of 4 relative to the original design value of $10^{34}\,$cm$^{-2}$s$^{-1}$
still calls for a major upgrade effort for most subsystems of the ATLAS detector. Tracking
detectors, for example, may need higher granularity to cope with the high particle rates while e.g.
the overall radiation dose corresponding to 3000 fb$^{-1}$ will require new radiation hard readout
electronics in all subsystems of the ATLAS detector. 

%   =========================================================================================
\subsection{Upgrade of the Muon system for High Luminosities}
%   =========================================================================================
%

The muon system will face two main challenges with luminosities beyond the design value:
%
\begin{enumerate}
%
\item[(a)] {maintenance of excellent tracking efficiency of the MDT drift tube chambers in the
presence of high background hit rates, due to converted neutrons and gammas and} 
%
\item[(b)] {limitation of trigger rates for high-p$_T$ muons at a level of about 20--30$\%$ of the
overall ATLAS trigger rate, which is planned to remain at the present level of 100 kHz.}
%
\end{enumerate}

In the following chapters we present (a) a R\&D project for the upgrade of drift tube chambers
to cope with high background rates at the SLHC and (b) a method to upgrade the muon trigger by
combining the high position accuracy of the MDT chambers with the high time resolution of
the existing muon trigger chambers, decisively improving the selectivity of the muon trigger for
high-p$_T$ tracks.


% --- Fig.  1 (59) --------------------------------------------------------------------------
%
\begin{figure}[h]
\figimage{0.85}{Large_small_tubes}
\labelcaption{fig:Large_small_tubes}
%
{
Illustration of the tracking quality in 30 mm and 15 mm drift tubes in a region of high n/$\gamma$
background, as expected at the SLHC. The occupancies from background hits (red dots) are 50 $\%$ in
the 30 mm tubes but only 7 $\%$ in the 15 mm tubes due to shorter drift time and smaller area. 
}
%
\end{figure}

% -------------------------------------------------------------------------------------------
%

%   =========================================================================================
%

\subsubsection{Drift tube chambers for tracking in a high--background environment}
%

The outer region of the ATLAS detector, where the muon chambers are located, receives high rates of
low-energy neutrons, 
mainly due to shower leakage from calorimeters and shielding structures in the
high-$\eta$ region. At the nominal luminosity gammas from neutron capture and related
conversion electrons are expected to generate hit rates in the range 50--300 kHz in each MDT tube.
A conversion electron may mask a muon hit if the signal arrives {\it before} the muon signal, 
which leads to a muon detection efficiency of 
exp(-$\tau\times$f) $\approx$ 1 - $\tau\times$f, 
where $\tau$ is the average drift time in the MDT tubes and f the rate of hits due to gamma
conversions.



% --- Fig.  2 (60) --------------------------------------------------------------------------

\begin{figure}[h] 
\figimage{0.9}{Cosmic_test_setup_BB} 
\labelcaption{fig:Cosmic_test_setup} 
% 
{In this test setup the coordinates of a cosmic track are measured in 12 layers of 30 mm tubes
(blue). The efficiency of 6 tubes of 15 mm diameter (red) is defined 
with respect to the fitted track, as explained in the text. 
Two scintillators below the setup are used for triggering.}
% 
\end{figure}

% -------------------------------------------------------------------------------------------

At high rates of n/$\gamma$ background, the efficiency may be further reduced by a decrease of the
gas amplification due to space charge from slowly drifting positive ions in the tubes,
while the {\it fluctuations} of the space charge tend to degrade the spatial resolution by up to 
about 20 $\%$ at the highest rates. 

% --- Fig.  3 (61) --------------------------------------------------------------------------

\begin{figure}[h] 
\figimage{0.85}{ATLAS_MDT_drifttime1} 
\labelcaption{fig:ATLAS_MDT_drifttime1} 
%
{
Comparison of the drift-time spectra of $30\,$mm and $15\,$mm diameter drift tubes at standard
gas mixture, gas pressure and gas amplification. The maximum drift times are 700 and 200 ns,
respectively, well reproduced by GARFIELD simulation.} 
\end{figure}

% -------------------------------------------------------------------------------------------

The effects of gamma conversions in the MDT tubes have been studied in detail using a muon beam in
the presence of intense $\gamma$-irradiation of up to $500\,$Hz/cm$^{2}$ (i.e. $\sim$300 kHz/tube),
as delivered by the Gamma Irradiation Facility at CERN (GIF)~\cite{mdt_highrates}. While the
$\gamma$-rates at the GIF correspond to only about 30$\%$ of the background levels expected for the
hottest regions at the SLHC, the results of these measurements already allow to define the baseline
of a chamber design with much improved tracking capability: MDT drift tube with only half the tube
diameter offer a reduction of the drift time by a factor 3,5, due to the non-linear relation
between track distance from the central wire and drift time (r-t relation) and in addition by a
factor 2 from the exposed area, thus yielding a factor of 7 in the reduction of the hit rate due to
n/$\gamma$ background. Moreover, up to two times more tube layers can be accommodated in the
available space, leading to improved track finding efficiency and position resolution (see
Fig.~\ref{fig:Large_small_tubes}).

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

\begin{figure}[h]
\figimage{0.9}{Large_small_efficiency}
\labelcaption{fig:Large_small_efficiency}
%
{
Efficiency vs. hit rate per tube for 30 mm and 15 mm drift tubes. 
For this measurement cosmic muon tracks were detected in the presence of 
gamma irradiation with adjustable intensity at the GIF facility at CERN.
}
%
\end{figure}


% -------------------------------------------------------------------------------------------

The reduction of the tube diameter of the MDT tubes allows to maintain the main advantages of the
drift tube concept:

\begin{itemize}

\item{independence of the position resolution from the angle of incidence onto the chamber plane
(contrary to drift chambers with rectangular drift geometry)}

\item{operational independance of each tube, where any malfunction of a tube can only generate a
negligible inefficiency} 

\item{modularity of chamber construction}

\end{itemize}    

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

\begin{figure}[h] 
\figimage{1.0}{Sm_tbs_design} 
\labelcaption{fig:Small_tube_structure} 
% 
{
Structure of a small drift tube with gas connection and decoupling capacitor in the longitudinal
direction (green zylinder). The plastic parts are injection moulded.}
% 
\end{figure}

% ----------------------------------------------------------------------------------------

Another advantage of maintaining the drift tube concept is that it allows to use the extensive
experience with design and operation of the present MDT chambers with 30 mm tube diameter. 


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

\begin{figure}[h] 
\figimage{0.85}{Sm_tbs_gas_and_elx} 
\labelcaption{fig:Sm_tbs_gas_and_elx} 
% 
{
Integration of gas distribution system and readout electronics with the small tubes.}
\end{figure}

% ----------------------------------------------------------------------------------------

To verify the performance of 15 mm (''small'') tubes a number of tests was executed, using cosmic
muon tracks. A pair of 30 mm (''large'') drift tube chambers was used as reference, 
defining the position of the muon track, 
while a layer of small tubes was the device under test (see
Fig.~\ref{fig:Cosmic_test_setup}). Tubes along the track are called 'efficient' when the hit is
detected inside a 3$\sigma$ road, as defined by the reference tubes. 

This measurement was done in the presence of various levels of gamma background due to a close-by,
adjustable source (GIF test area at CERN). First tests~\cite{mdt_slhc} confirmed the expectation
that by reducing the drift tube diameter by a factor of two whilst leaving all operational
parameters unchanged, 
the maximum drift time is reduced by a factor of 3,5 (Fig.~\ref{fig:ATLAS_MDT_drifttime1}). 
Fig.~\ref{fig:Large_small_efficiency} shows the efficiency
of small and large tubes vs. hit rate from conversions, 
small tubes providing a much better performance, as expected. 
The efficiency at rate zero deviates from 100 $\%$ due to tracks passing
across or close to the tube walls and due to $\delta$-electrons shifting the position of the hit
outside the 3$\sigma$ acceptance road.


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

\begin{figure}[h] 
\figimage{0.9}{Accuracy} 
\labelcaption{fig:Accuracy} 
% 
{
Distribution of the wire displacement from the nominal position, the decisive parameter 
for the measuring accuracy of the MDT chambers.}
% 
\end{figure}

% ----------------------------------------------------------------------------------------

Going from large to small tubes as construction elements for MDT chambers poses a number of
technical challenges, as the higher tube density requires more refined electrical and gas
connections on the same available service area. This is a particular problem for the supply of the
tubes with the operating voltage of 2750 V, requiring isolation distances which cannot be
realized on the area available for the readout boards. The integration of the HV decoupling
condensors into the end-plugs of the tubes was therefore a central requirement for the tube design.
In a similar way, gas supply of each MDT tube by individual tublets, 
as implemented for the 30 mm tube MDTs, did not seem feasible for the
production of chambers with four times higher tube density. 

% --- Fig.  8 -------------------------------------------------------------------------------

\begin{figure}[h] 
\figimage{0.8}{Small_wheel_module} 
\labelcaption{fig:Small_wheel_module} 
% 
{
Design of a full prototype for a chamber to be implemented into the forward region of the ATLAS
detector (''Small wheel'').}

% 
\end{figure}

% ----------------------------------------------------------------------------------------

Development work for small tube chambers started early in 2008 with an innovative tube design,
where HV decoupling and gas distribution are integrated into the design 
of the end-plug (Fig.~\ref{fig:Small_tube_structure}). 
Fig.~\ref{fig:Sm_tbs_gas_and_elx} shows the integrated
gas distribution scheme plus readout electronics for a 4-layer module of small tubes.

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

\begin{figure}[h] 
\figimage{1.0}{Sm_tbs_tube_placement} 
\labelcaption{fig:Sm_tbs_tube_placement} 
% 
{
Precision placement of the small MDT tubes in 
high accuracy 'combs' before glueing with epoxy.}
% 
\end{figure}

% ----------------------------------------------------------------------------------------

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

\begin{figure*}[ht] 
\figimage{1.0}{Sm_tbs_chamber_glueing} 
\labelcaption{fig:Sm_tbs_chamber_glueing} 
% 
{
The small tube prototype chamber after assembly, 
consisting of two packages of 8 tube layers, with 72 tubes per layer and
1152 tubes in total.}
% 
\end{figure*}

% ----------------------------------------------------------------------------------------

The integration of tubes into chambers is achieved by bonding tubes layer by layer with epoxy
glue. This requires a high level of positioning accuracy and fixation during the curing. 
In production tests the target accuracy of 20 $\mu$m (Fig.~\ref{fig:Accuracy}) was obtained
by placing the tubes into special supports (''combs''), see Fig.~\ref{fig:Sm_tbs_tube_placement}. 
All 8 tube layers
were glued in a time span of a few hours, curing was overnight, 
such that the assembly of a tube package took only one day.

Presently, a full prototype of a MDT chamber in small tube technology 
is under construction (Fig.~\ref{fig:Small_wheel_module}). 
It consists of 2$\times$8 tube layers and is designed to fit
into the inner part of the muon detector in the very forward direction, where rates are highest.
This prototype will be available for tests at the GIF facility by fall 2010. The readout will be
achieved with available electronics for the large tube chambers, 
specially adapted for use with the
new chamber geometry. 


\subsubsection{Development of new electronics for small drift tube readout}
%

The readout architecture of the present MDT system is described in ~\cite{MDT_ELX_paper}. 
While this proven concept is also applicable to the small tube system, the four times 
higher channel density is dictating a higher level of integration, 
more on-board data storage capacity
and higher band width for data transfer to the counting room. 

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

\begin{figure}[h] 
\figimage{0.9}{ASD_130_nm_output} 
\labelcaption{fig:ASD_130_nm_output} 
% 
{
Performance of two channels in an ASD chip 
to be used in the frontend electronics of the MDT drift tubes. 
Gain and pulse shape of both channels correspond
closely to the design parameters and to predictions from simulation.}
% 
\end{figure}

% ----------------------------------------------------------------------------------------
%

In addition, due to the ten times
higher target for the integrated luminosity of the SLHC, 
electronics components have to survive
correspondingly higher irradiation doses. 
Together with the Electronics Division of the MPI we are
currently developing a new 16-channel Amplifier-Shaper-Discriminator (ASD) 
chip in IBM 130 nm technology, 
known to be radiation hard beyond what is expected for the Muon detector at SLHC. 

To study the analog performance of the 130 nm technology in our application 
a prototype chip for
the ASD with four channels was produced in 2009, showing excellent 
matching with the design parameters as well as of the gains among the four
channels. Fig.~\ref{fig:ASD_130_nm_output} shows the response of two channels 
to the injection of a delta-charge.

For the TDC, measuring the drift times in the MDT tubes, a faster, 
radiation hard technology is introduced, 
based on developments of the CERN Micro Electronics Group. 

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

\begin{figure}[h] 
\figimage{0.8}{Large_Small_R_O_Architecture} 
\labelcaption{fig:Elx_diagram_sm_tubes} 
% 
{
Present readout scheme for large MDT tubes, 24 tubes being served 
by one TDC ('AMT') on a readout card (top). 
In the future architecture 96 small tubes will be served by a card of comparable size, 
requiring a higher level of integration (bottom).}
% 
\end{figure}

% ----------------------------------------------------------------------------------------

For the on-chamber FPGAs, responsible for data formatting and transmission to the counting room,  
a new firmware is developed by MPI's Electronics Division. 
The aim is to reduce the sensitivity of the configuration code
to single event upsets from ionizing particles by the implementation of a high level of 
code redundancy (Triple Modular Redundancy).  
Finally, optical data transmission to
the counting room will be based on the Gigabit Optical Link (GOL) chipset, developed by CERN. In
this transmission scheme 5 Gbit/s will be available, about three times more than in the
present SLINK scheme. 
A schematic diagram of the readout architecture of large and small tube chambers is
given in Fig.~\ref{fig:Elx_diagram_sm_tubes}.

\if{0}
%

In addition to a degradation of the spatial
resolution of the drift tubes from $80\,$\micron\ without background to about $100\,$\micron\ at
the highest rates caused by fluctuations of the increasing space charge in the tubes, the muon
detection efficiency of the tubes deteriorates from $94\%$ to about $70\%$. 

New muon chambers with faster response time and high track reconstruction efficiency are required 
for the four times higher background rates expected at a Super-LHC in the spectrometer endcap
regions, and are desirable for the highest-rate regions already at the LHC design luminosity.

The development of faster muon detectors for Super-LHC at MPI builds on the extensive experience
with the design and operation of the present ATLAS MDT chambers. First tests~\cite{mdt_slhc}
confirmed the expectation that by reducing the drift tube diameter  by a factor of two while
leaving the gas mixture, pressure and gain unchanged, the maximum drift time is reduced by a factor
of 3.5 (see Fig.~\ref{fig:ATLAS_MDT_mdt_tspectra}). At the same time, the count rates caused by
neutron and photon conversions in the drift tube walls will decrease proportional to the tube
circumference by a factor of two, reducing the drift tube occupancy (the product of maximum drift
time and count rate) and, consequently, the efficiency loss by a factor of 7. In addition, the
number of drift tube layers fitting into the same volume as the present muon chambers can be
increased by up to a factor of two (see Fig.~\ref{fig:Large_small_tubes}) improving the chamber
resolution and track reconstruction efficiency.
%
\fi

%
%   --------------------------------------------------------------------------------------
%

\subsubsection{Sharpening of the trigger threshold for high-p$_T\,$ tracks}
%

The capability to trigger on muon tracks with a transverse momentum (p$_T$) above a certain
threshold was one of the principal requirements for the design of the muon spectrometer. MDT
chambers with their long drift time of up to $750\,$ns, spanning 30 beam crossings, are not suited
for this task and had to be complemented by specialized trigger chambers, capable of identifying
tracks belonging to a given beam crossing.

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

\begin{figure}[h]  
%
\figimage{0.90}{L1_trigger_barrel}  
\labelcaption{fig:L1_trigger_barrel}  
%  
{Implementation of the Level-1 trigger in the muon barrel region.} 
%  
\end{figure}

% ----------------------------------------------------------------------------------------

In the barrel region of the muon detector, 
the Resistive Plate Chamber (RPC) technology was selected
for this purpose. 
This detector type uses pick-up strips  perpendicular to the z-direction to sense
the avalanches generated by traversing particles in the chamber gas, delivering the coordinates of
the tracks along the bending direction of the magnetic field~\cite{muon_tdr}. 
The time resolution of
about 20 ns of the RPC chambers is sufficient to tag the beam crossing 
with about 95$\%$ confidence.
Fig.~\ref{fig:L1_trigger_barrel} shows the schematics of the muon Level-1 
triggering system in the
barrel region. RPC trigger chambers (marked in green) are positioned 
at three radial positions of the barrel, 
one in the outer detector layer (BO) and two in the middle layer (BM), 
below and above the middle MDT. 
This way, three coordinate measurements are obtained along the track, defining the
sagitta and thus $p_T$.

The implementation of the Level-1 trigger in the muon barrel uses a system of fast coincidences
between the pick-up strips of the three RPC layers, 
estimating $p_T$ from the bending of the
track in the toroidal magnetic field. Due to the width of the RPC strips of 30 mm the p$_T$
resolution for tracks above 15--20 GeV is not very sharp, 
the sagitta of a 20 GeV track being only 24 mm. 
Fig.~\ref{fig:L1_resolution} depicts the situation in a quantitative way. 
With a threshold setting of 20 GeV (red curve) the trigger is still accepting about 60$\%$ of
the 15 GeV and 15$\%$ of the 10 GeV tracks. 
The corresponding, unwanted extra trigger rates from muons
below threshold are by far not negligible, 
as the cross sections for most muon channels are strongly
decreasing with p$_T$, 
while those of the more interesting physics channels (e.g. from W/Z decays) are roughly
constant or even increasing, see Fig.~\ref{fig:Muon_momenta}.

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

\begin{figure}[h] 
\figimage{0.9}{L1_resolution} 
\labelcaption{fig:L1_resolution} 
% 
{
Acceptance of the Level-1 trigger vs. p$_T$ for three typical trigger thresholds.
For the 20 and 40 GeV thresholds the transitions from 90--10$\%$ efficiency 
cover a wide p$_T$-range, leading to high rates of unwanted triggers.}
% 
\end{figure}

% ----------------------------------------------------------------------------------------

For practical and cost reasons the maximum Level-1 trigger rate 
for ATLAS at the SLHC is planned to
remain at the present value of 100 kHz, 
and therefore the efficient rejection of triggers from low-p$_T$ muon
tracks is a prime requirement for the upgrade towards SLHC.

Improving the p$_T$-selectivity of the muon trigger means improving the precision of the track
coordinates available for the Level-1 trigger decision.  In the present ATLAS trigger hierarchy
tracking information of the MDT is only used at the Level-2 trigger stage, 
where muon tracks are reconstructed using the precise MDT coordinates, 
leading to the rejection of more
than 90$\%$ of the Level-1 muon triggers. 
Due to considerable computing and data transfer overheads,
however, this result is only available after a latency of about 10 ms, 
three orders of magnitude
beyond what is acceptable for the latency of the Level-1 trigger. 
If a Level-1 trigger arrives later
than the maximum allowed latency, 
information in the front-end buffers of the ATLAS subdetectors may
be lost. 

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

\begin{figure}[h]  
\figimage{0.9}{Muon_momenta}  
\labelcaption{fig:Muon_momenta}  
%  
{ 
Transverse momentum distribution of muons in the ATLAS muon spectrometer for various production
channels.
} 
% 
\end{figure}

% ----------------------------------------------------------------------------------------

The challenge for an improvement of the Level-1 trigger is to design a MDT readout scheme,
able to deliver a refined p$_T$-value inside a latency of a few mycro seconds. 
The present latency budget of 2,5 $\mu$s was adapted to the situation 
at the original LHC and is insufficient for any refinement of the trigger decision.

For SLHC, however, an increase of the latency to
6,4 $\mu$s or even 10 $\mu$s will be implemented 
for the frontend data storage of all subdetectors, 
providing considerable design
freedom for Level-1 trigger improvements.

The implementation of an improved muon barrel trigger requires an additional readout path of
the MDT, in parallel and independent of the existing non-synchroneous, 
'standard' readout path (Fig.~\ref{fig:New_readout_diagram}).

For this fast MDT readout path the following design concept is pursued:

\begin{enumerate}

\item[(a)] {Readout is only activated when a high-$p_T$ candidate is flagged by the RPC trigger
logic, saving occupancy and readout bandwidth.}

\if{0}
\item[(b)] {The approximate location of the high-$p_T$ candidate track supplied by the RPC is used to
identify those tubes in a MDT chamber which are close to the muon trajectory; the other MDT tubes are
ignored saving readout and processing time.} 
\fi

\item[(b)] {All MDT tubes but those close to the trajectory of the high-$p_T$ muon candidate,
as supplied by the RPC, will be ignored in the fast readout, saving data transfer and processing time.} 

\item[(c)] {The resolution of the MDT drift time will be reduced from 12 to 7 bit.
The corresponding position
resolution of the MDT of about 1 mm (RMS) being
still about a factor 10 better than the one of the RPC, sufficient for a decisive sharpening
of the Level-1 trigger threshold. 
This way data volumina and transmission delays are reduced. 
For the same reason, 
data redundancy and format overheads will be reduced to the strict minimum.} 

\item[(d)] {Separate fast communication lines will be installed to reduce transmission delays, while
fast local processors will be used for the sagitta and p$_T$ estimates.} 

\end{enumerate}

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

\begin{figure}[h] 
\figimage{1.0}{New_readout_diagram} 
\labelcaption{fig:New_readout_diagram} 
% 
{
Readout architecture to combine the precision track coordinates determined 
in the MDT chambers with
the 'fast' trigger flag supplied by the RPC's. 
}
% 
\end{figure}

% ----------------------------------------------------------------------------------------

An analysis of the time
behaviour of such a readout model shows that a latency of 4,5--5,5 $\mu$s could be achieved and
thus would be a realistic option for the upgrade of the muon Level-1 trigger at the LHC.
%, see Fig.~\ref{fig:Latencies} 
A significant advantage of this scheme would be 
that the existing RPC trigger chambers would not need an
modifications (except electronics).   

A similar upgrade scheme could also be applied to the Level-1 trigger in the end-cap region where
trigger chambers of the TGC type are used~\cite{muon_tdr}. Because of the different geometry of the
toroidal magnetic field and the different location of trigger and MDT chambers, however, a modified
architecture and specialized algorithms will have to be used.

In the context of this research program we have submitted four proposals to the ATLAS upgrade
coordinator: 
(a) development of improved muon drift tube detectors~\cite{mdt_EoItubes} 
(b) development of radiation tolerant readout electronics~\cite{mdt_EoIelectronics} and 
(c) development of methods to reduce the data volume~\cite{mdt_EoIreadout} or, 
alternatively, increase the bandwidth of the data acquisition system 
of the MDT chambers~\cite{mdt_EoIelectronics} for operation at very high luminosities.

\clearpage

%%%%%%%%%%%%%%%%
% References
%%%%%%%%%%%%%%%%

\begin{Biblio}

\bibitem{muon_tdr}
ATLAS Collaboration,
\textsl{Technical Design Report for the ATLAS Muon Spectrometer},
CERN/LHCC/97-22, May 1997.


\bibitem{ATLAS_detector_paper}
The ATLAS collaboration,
\emph{The ATLAS Experiment at the CERN Large Hadron Collider},
JINST {\textbf 3}  S08003 (2008)
%
\bibitem{MDT_ELX_paper}
Y. Arai et al.,
\emph{ATLAS Muon Drift Tube Electronics},
JINST {\textbf 3}  P09001 (2008)
%
\bibitem{mdt_constr1}
J.~Dubbert, S.~Horvat, O.~Kortner, S.~Kotov, H.~Kroha, S.~Mohrdieck-M\"ock, R.Richter,
\textsl{Final Evaluation of the Mechanical Precision of the ATLAS Muon Drift Tube Chambers},
proceedings of the 2006 IEEE Nuclear Science Symposium, San Diego, USA, 29 October--4 November 2006,
MPI report, MPP-2006-167, November 2006.

\bibitem{mdt_constr2}
F.~Bauer et al.,
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Spectrometer},
Nucl.\ Instr.\ and Methods \textbf{A461} (2001) 17; \\
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ATLAS Muon Spectrometer},
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\textsl{The First Precision Drift Tube Chambers for the ATLAS Muon 
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Nucl.\ Instr.\ and Methods \textbf{A518} (2004) 69.

\bibitem{mdt_install1}
J.~Dubbert et al.,
\textsl{Integration, Commissioning and Installation of Monitored Drift Tube Chambers
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21.--27.5.2006,
Nucl.~Instr.\ and Methods \textbf{A572} (2007) 53;\\
\textsl{\sl Integration, Commissioning and Installation of Large Drift-Tube Chambers
for the ATLAS Barrel Muon Spectrometer},
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MPI report, MPP-2006-164, November 2006,
to be published in the IEEE Transactions on Nuclear Science.

\bibitem{mdt_install2}
M.~Groh,
\textsl{Test von Driftrohrkammern f\"ur das Myonspektrometer des ATLAS-Detektors
am Large Hadron Collider},
Diploma Thesis, \TUM, Januar 2006.

\bibitem{mdt_commissioning1}
J.~Dubbert (for the ATLAS collaboration),
\textsl{First Experience with the ATLAS Muon Spectrometer},
proceedings of the $11^{\mathrm th}$ Vienna Conference on Instrumentation, Vienna, Austria,
19.--24.2.2007,
Nucl.~Instr.\ and Methods \textbf{A581} (2007) 507.

\bibitem{mdt_commissioning2}
J.~von Loeben (for the ATLAS collaboration),
\textsl{First Cosmic Ray Results of the ATLAS Muon Spectrometer with Magnetic Field},
proceedings of the 2007 IEEE Nuclear Science Symposium, Honolulu, Hawaii, USA, 28 October--2 November 2007,
MPI report, MPP-2007-175, November 2007,
to be published in the IEEE Transactions on Nuclear Science.

\bibitem{mdt_calib1}
J.~von Loeben, M.~Deile, N.~Hessey, O.~Kortner, H.~Kroha, A.~Staude,
\textsl{An Efficient Method to Determine the Space-to-Drift Time Relationship of the ATLAS Monitored
Drift Tube Chambers},
proceedings of the 2007 IEEE Nuclear Science Symposium, Honolulu, Hawaii, USA, 28 October--2 November 2007,
MPI report, MPP-2007-176, November 2007,
to be published in the IEEE Transactions on Nuclear Science.

\bibitem{mdt_calib2}
J.~von Loeben, 
\textsl{Test und Kalibrierung der Pr\"a\-zi\-si\-ons\-drift\-rohr\-kam\-mern des
ATLAS-Myon\-spek\-tro\-me\-ters},
Diploma Thesis, \TUM, December 2006.

\bibitem{mdt_resol}
S.~Horvat, O.~Kortner, H.~Kroha,
\textsl{Determination of the Spatial Drift-Tube Resolution Using Muon Tracks},
ATLAS Note, ATL-MUON-PUB-2006-008, July 2006.

\bibitem{mdt_testbeam1}
C.~Adorisio et al.,
\textsl{System Test of the ATLAS Muon Spectrometer at the H8 Beam at the CERN SPS},
ATLAS Note, ATL-MUON-PUB-2007-005, September 2007.

\bibitem{mdt_testbeam2}
F.~Bauer et al.,
\textsl{Study of the ATLAS Muon Spectrometer Using High Energy Combined Test Beam Data},
ATLAS Note, ATL-COM-MUON-2007-013, December 2007.

\bibitem{mdt_highrates}
M.~Deile et al., 
\textsl{Performance of the ATLAS Precision Muon Chambers under LHC Operating Conditions},
proceedings of the $9^{\mathrm th}$ Pisa Meeting on Advanced Detectors, Isola d'Elba, Italy,
25--31 May 2003,
Nucl.~Instr.\ and Methods \textbf{A518} (2004) 65; \\
\textsl{Resolution and Efficiency of the ATLAS Muon Drift-Tube Chambers at High Background Rates},
proceedings of the $10^{\mathrm th}$ Vienna Conference on Instrumentation, Vienna, Austria,
16--21 February 2004,
Nucl.~Instr.\ and Methods \textbf{A535} (2004) 212; \\
S.~Horvat et al.,
\textsl{Operation of the ATLAS Muon Drift-Tube Chambers at High Background
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IEEE Transactions on Nuclear Science, Vol.~53, No.~2 (2006) 562.

\bibitem{mdt_calib3}
J.~Dubbert et al.,
\textsl{Modelling of the Space-to-Drift-Time Relationship of the
ATLAS Monitored Drift-Tube Chambers in the Presence of Magnetic Fields},
Nucl.~Instr.\ and Methods \textbf{A572} (2007) 50.

\bibitem{mdt_calib4}
P.~Bagnaia, O.~Biebel, C.~Guyot, H.~Kroha, D.~Orestano, B.~Zhou,
\textsl{A Proposal for the Calibration and Alignment of the ATLAS Muon Spectrometer},
ATLAS Note, ATL-MUON-INT-2006-006, October 2005; \\
P.~Bagnaia et al.,
\textsl{Calibration Model for the MDT Chambers of the ATLAS Muon Spectrometer},
ATLAS Note, ATL-COM-MUON-2008-006, February 2008.

\bibitem{mdt_calib5}
M.~Cirilli et al.,
\textsl{Conditions Database and Calibration Software Framework for ATLAS Monitored Drift Tube Chambers},
Nucl.~Instr.\ and Methods \textbf{A572} (2007) 38.

\bibitem{mdt_align1}
Ch.~Amelung et al.,
\textsl{The ATLAS Muon Alignment System},
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CERN report, CERN-2007-004, 2007; \\
J.C.~Barriere et al.,
\textsl{The Alignment System of the Barrel Part of the ATLAS Muon Spectrometer},
ATLAS Note, ATL-MUON-PUB-2008-007, April 2008.

\bibitem{mdt_align2}
I.~Potrap, O.~Kortner, S.~Kotov, H.~Kroha,
\textsl{Alignment of the ATLAS Muon Spectrometer with Straight Tracks Using MILLEPEDE Method},
ATLAS Note, ATL-COM-MUON-2007-017, December 2007.

\bibitem{mdt_align3}
 O.~Kortner, S.~Kotov, H.~Kroha, J.~Schmaler, Ch.~Valderanis,
\textsl{Alignment of the ATLAS Muon Spectrometer with Tracks and
Muon Identification at High Background Rates},
Nucl.~Instr.\ and Methods \textbf{A581} (2007) 545-548.

\bibitem{mdt_align4}
O.~Kortner, S.~Kotov, H.~Kroha, I.~Potrap, J.~Schmaler,
\textsl{New Methods for the Alignment of the ATLAS Muon Spectrometer with Tracks},
proceedings of the 2007 IEEE Nuclear Science Symposium, Honolulu, Hawaii, USA, 28 October--2 November 2007,
MPI report, MPP-2007-177, November 2007,
to be published in the IEEE Transactions on Nuclear Science.

\bibitem{mdt_align5}
J.~Schmaler,
\textsl{Test and Alignment of the ATLAS Precision Muon Chambers},
Diploma Thesis, \TUM, MPI report, MPP-2007-30, March 2007.

\bibitem{mdt_muondqa1}
M.~Schott, N.~Benekos, L.~Chevalier, J.-F.~Laporte, 
\textsl{Impacts of Misalignment on the Reconstructed Z-Boson Resonance at the ATLAS
Muon Spectrometer},
proceedings of the $10^{\mathrm th}$ Pisa Meeting on Advanced Detectors, Isola d'Elba, Italy,
21--27 May 2006, 
%MPI report, MPP-2006-176, June 2006,
Nucl.~Instr.\ and Methods \textbf{A572} (2007) 16;\\
\textsl{Z-Boson Resonance as a Calibration and Alignment Process for the ATLAS Muon Spectrometer},
proceedings of the 2006 IEEE Nuclear Science Symposium, San Diego, 10 October--4 November 2006,
MPI report, MPP-2006-173, November 2006.

\bibitem{mdt_muondqa2}
N.~Benekos, M.~Schott,
\textsl{Study of Z-Boson Measurements at the ATLAS Experiment},
proceedings of the "Physics at LHC" conference, Cracow, Poland, 3--8 July 2006,
MPI report, MPP-2006-174, July 2006.

\bibitem{mdt_identification}
D.~Primor, O.~Kortner, G.~Mikenberg, H.~Messer,
\textsl{A Novel Approach to Track Finding in a Drift Tube Detector},
Journal of INstrumentation, JINST \textbf{2} P01009, January 2007.

\bibitem{mdt_simulation1}
S.~Baranov et al.,
\textsl{Muon Detector Description as built and its Simulation for the ATLAS Experiment},
proceedings of the $10^{\mathrm th}$ Pisa Meeting on Advanced Detectors, Isola d'Elba, Italy,
21--27 May 2006,
Nucl.~Instr.\ and Methods \textbf{A572} (2007) 14;\\
D.~Rebuzzi et al.,
\textsl{Muon Detector Description as built and its Simulation for the ATLAS Experiment},
proceedings of the 2006 IEEE Nuclear Science Symposium, San Diego, 10 October--4 November 2006,
MPI report, MPP-2006-170, November 2006.

\bibitem{mdt_simulation2}
D.~Rebuzzi et al.,
\textsl{GEANT4 Muon Digitization in the ATHENA Framework},
proceedings of the 2006 Computing in High Energy Physics Conference, Mumbai, India, 13--17 February 2006,
MPI report, MPP-2006-178, February 2006.

\bibitem{mdt_EoItubes}
O.~Biebel, J.~Dubbert, S.~Horvat, O.~Kortner, H.~Kroha, R.~Richter, D.~Schaile,
\textsl{Expression of Interest: R\&D on Precision Drift Tube Detectors for Very High Background Rates at SLHC},
ATLAS document, ATL-M-MN-0006, March 2007.

\bibitem{mdt_EoIelectronics}
O.~Biebel, J.~Dubbert, S.~Horvat, O.~Kortner, H.~Kroha, R.~Richter, D.~Schaile,
\textsl{Upgrade of the MDT Readout Chain for the SLHC},
ATLAS document, ATL-M-MN-0003, March 2007.

\bibitem{mdt_EoIreadout}
O.~Biebel, J.~Dubbert, S.~Horvat, O.~Kortner, H.~Kroha, R.~Richter, D.~Schaile,
\textsl{Upgrade of the MDT Electronics for the SLHC Using Selective Readout},
ATLAS document, ATL-M-MN-0005, March 2007.

\bibitem{mdt_slhc}
J.~Dubbert, S.~Horvat, O.~Kortner, H.~Kroha, F.~Legger, R.~Richter, F.~Rauscher,
\textsl{Development of Precision Drift Tube Detectors for the Very High Background Rates at the Super-LHC},
proceedings of the 2007 IEEE Nuclear Science Symposium, Honolulu, Hawaii, USA, 28 October--2 November 2007,
MPI report, MPP-2007-172, November 2007,
to be published in the IEEE Transactions on Nuclear Science.
%
\end{Biblio}
