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%%%           pdftitle={First Cosmic Ray Results of the ATLAS Barrel Muon
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\begin{document}

\begin{center}
\textbf{\Large Upgrade of the ATLAS Muon Trigger for the SLHC }\\

\vspace*{10mm}

J.~Dubbert$^{\text{1}}$,
S.~Horvat$^{\text{1}}$,
O.~Kortner$^{\text{1}}$,
H.~Kroha$^{\text{1}}$,
R.~Richter$^{\text{1}}$\\[3mm]

{\small $^{\text{1}}$Max-Planck-Institut f\"ur Physik, F\"ohringer Ring 6,
80805 M\"unchen, Germany\\[5mm]}

\end{center}

The SLHC project at CERN foresees an increase of the peak luminosity
of the LHC by a factor of 4-10 beyond the nominal value of
10$^{\text{34}}$~cm$^{\text{-2}}$~s$^{\text{-1}}$. As the total L1
trigger rate of the ATLAS experiment cannot be signifi­cantly
increased beyond 100~kHz, a higher selectivity of the L1 trigger for
high-pT event signatures is required. One of the main inputs to the
ATLAS L1 trigger are high-pT tracks identified in the muon
spectrometer. As the muon rate decreases strongly with pT, the trigger
rate is highly sensitive to the selected pT threshold. To keep the
trigger rate for single muons at an accep­table level of about 10 kHz,
a minimum pT threshold of 20--30 GeV/c is required. Due to the limited
spatial resolution of the trigger chambers and the corres­ponding pT
error, however, a large fraction of tracks with pT below the selected
threshold will be accepted, inflating the L1 rate.

We present a concept for using the much higher spatial resolution of
the Monitored Drift Tube (MDT) chambers for a precise track momentum
determination for the L1 trigger and a decisive improvement of the
rejection of muons below threshold. This scheme requires a
communication path between the trigger and MDT chambers to be
established within each trigger tower. The coordi­nates of the high-pT
track as determined by the trigger chambers are communicated to a
newly implemented local processor (\textit{TowerMaster}) which determines the
search path for MDT tubes to be read out (using a LUT) and sends this
information to the front-end boards of the MDT chambers. At the
front-end, a system of scalers (one scaler per tube) is added to the
existing readout logic, determining the drift time with the relaxed
time resolution of 25~ns. At this clock frequency the word size of the
drift time information is reduced to 6 bit, yet providing a spatial
resolution of about 0.5~mm. This is more than a factor 10 better than
the resolu­tion of the trigger chambers. The scalers are started by
the signals on the MDT wires and stopped by the L1 muon trigger
signal. This signal is timed to arrive a fixed number of beam
crossings following the passage of the muon. The scaler readings thus
correspond directly to the drift times of the primary electrons and to
the distances of the muons from the MDT wires. The drift-time readings
of 6 tubes along the track in a MDT chamber contain redundancy and
provide a robust measurement of the track coordinate. The track
coordinates measured by the three MDT chambers along the muon
trajectory yield the track sagitta and thus the pT of the muon. If
this is below the selected threshold, a L1 veto is issued to the
trigger logic.

For this triggering scheme an additional L1 latency of 1--3 $\mu$s is
required, depending on the details of implementation.

\end{document}
