
\section{Upgrade motivations  {\small [TK]}}

{\it 
Discussion of muon spectrometer performance at high luminosity - precision 
tracking and L1 trigger, and conclude that new detector and electronics are
needed.

Point out serious (?) performance degradation in the small
wheel region in both MDT and CSC, referring to the radiation background 
discussoin in Appendix.
Performance of the present detector should be evaluated for high lumi conditin,
either using high lumi Monte Carlo or overlay of real events.
CSC is 4 layers. 

In the L1 discussion, emphasise the importance of maintaining low $p_T$ 
threshold.
There are two issues.
the high rate of fake triggers
in the endcap region based mainly on the study using data.
Then introduce basic idea of how this can be 
mitigated by integrating the small wheel in the L1 trigger.
As the second point of trigger, discuss the pT resolution and possible 
improvement using the new small wheel.
Physics requirement asks low pT threshold (20-40 GeV), 
L1 rate should be maintained at 100 kHz.  
Need sharpening pT threshold. Here introduce 1 mrad requirement.

Finally conclude that NSW should be built and replace the present ones
as a phase-1 upgrade item in preparation to running with luminosity beyond 
the nominal luminosity.
}

The Small Wheel (SW) is the first station (''Inner station'') in the end-cap of the
Muon spectrometer. Together with the Middle and Outer station it delivers the spatial
coordinates for the momentum determination of charged tracks. Between the Small Wheel
and the Middle station (''Big Wheel'') a toroidal magnet is located with a bending
power ranging from 3--7 T $\times$ m, depending on $\eta$, allowing a determination
of the track momentum with high accuracy.

Designed to measure the momentum of a charged particle, the three stations of the
end-cap have two distinct functions. They provide a fast, raw determination of the
transverse momentum p$_T$, shortly after the passage of the particle, to be used by
the Level-1 (L1) trigger. The L1 trigger logic has the task to identify events of
physics interest, for which a high-p$_T$ muon is a crucial signature. 

When a L1 trigger decision is broadcast to the ATLAS subsystems, the DAQ has to retain all
relevant information for the corresponding beam crossing (BX). Because of technical
limitations on data storage of the electronics, however, the L1-trigger must arrive at the
subdetectors not later than 2,5 $\mu$s after particle passage (trigger ''latency''),
otherwise crucial information might be lost. This short L1 trigger latency of the present
ATLAS readout system strongly limits the selectivity of the L1 decision, as only a fraction
of the available tracking information can be used for a reduction of fake triggers, which
are mainly due to low-p$_T$ tracks. One of the design aims for the ATLAS upgrade in phase II
is an increase of the L1 trigger latency to 6,4 $\mu$s or even up to 10 $\mu$s. At this
point more selective trigger strategies can be implemented, partly at the level of the
frontend electronics, partly at the level of the Central Trigger Processor (CTP), where
trigger information from different subdetectors can be combined.

The second important function of the three muon stations is the momentum measurement of
the track with the highest available accuracy. This is achieved by combining high
resolution drift tube chambers with a precise optical alignment system. Due to the
alignment system the relative position of track hits in front and behind the bending
magnet are precisely known, resulting in the required momentum resolution.

In the present Small Wheel Thin Gap Chambers (TGC) are used for the selection of
high-$p_T$ tracks for the L1-trigger, while Monitored Drift Tube chambers (MDT) are
used for the precision determination of the track  coordinates.

The LHC upgrade in phase I and II will lead to an increase of the instantaneous
luminosity by a factor of 2 and 5, respectively. In the present SW, these high
luminosities and the resulting background rates will lead to an unacceptable reduction
in trigger and tracking efficiency.

The main shortcoming of the present L1-trigger in the end-cap is caused by the fact that
a majority of tracks crossing the SW 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. The trigger chambers of the New Small Wheel (NSW) will have to be able to
determine the slope of the track with an accuracy of $<$ 1 mrad in the r-z plane
(bending plane). The slope of the track must also be determined in the $\phi$ direction,
though with less accuracy, to further reduce the number of trigger candidates which do
not come from the interaction point.

\begin{table}[ht!]   %%% Table:  1
%
\centering
%	
\begin{tabular}{|l||c|c|c|c|c|c|} \hline 
%    \cline{2-5} 
										 
  Chamber& location& tube    & expected      & rate per  & occup-   & effi-  \\  
%\hline   
  Type   & in r    & length  & hit rate      & tube      & ancy     & ciency \\  
\hline 
         & {\it cm}& {\it cm}& {\it hits/cm$^{2}$}& {\it kHz}& {\it \%} & {\it \%} \\  
\hline
  EIL0 &  93  &   56 & 6644  &  558  & 11,2 & 89,4 \\
  EIL1 & 208  &  125 & 2971  &  556  & 11,1 & 89,5 \\
  EIL2 & 318  &  179 & 1375  &  369  &  7,4 & 92,9 \\
  EIL3 & 426  &  205 &  646  &  198  &  4,0 & 96,1 \\ \hline
  \end{tabular}
  \caption{Expected hit rates and efficiencies for 15 mm tubes in the NSW.
           Col. 2 gives the location of the innermost tube of each chamber. Rates in the Small
		   sectors are lower because of the shorter tubes.}
  \label{NSW_efficiencies}
\end{table}

The limitations of the MDT precision chambers at high luminosity are mainly caused by
isolated hits in the tubes (''fake hits''), caused by 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. While the length of the tubes in the trapezoidal geometry
of the SW decreases proportional to distance from the beam line (r), the background
rates increase considerably faster than with 1/r. The highest hit rates per tube thus
occur at the inner tip of the EIS and EIL chambers. The observed losses of efficiency
and spatial resolution in the drift tubes are partly due to positive ions, slowly
drifting in the tube gas, distorting electric field and drift velocity, partly due to a
reduction of the electric field close to the wire, reducing gas amplification and signal
height~\cite{mdt_highrates}. Bandwidth limitations of the present readout electronics
also contribute to the loss of efficiency (see section 5.4 below). Replacing the present
MDTs by a chamber type with higher rate capabilities and improving the readout
electronics correspondingly is therefore a central aim for the  Small Wheel upgrade
towards higher luminosities, as expected from the SLHC.

Table~\ref{NSW_efficiencies} gives background rates, occupancies and hit efficiencies
for MDT tubes at different radii in the SW, based on simulation work in
Ref.~\cite{RTF_rad_task_force}. Results form LHC running at luminosities up to $5
\times 10^{32}$ seem to indicate rates which are about a factor 3--4 above the
simulated ones for the innermost chambers EIS1 and EIL1 of the SW.

