D. Acosta (University of Florida)
S. M. Wang (University of Florida)
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The endcap muon system of the CMS experiment consists of four disks
of trapezodial shaped cathode strip chambers (CSC) on each end of
the experiment. The CSC chambers are fast (drift-time about 60 ns)
and participate in the Level-1 trigger of CMS.
The purpose of the Track-Finding processor is to link trigger
primitives (track segments) from individual muon stations into
complete tracks, calculate the transverse momentum Pt from the
sagitta induced by the magnetic bending, and report the highest
quality tracks to the Level-1 Global Muon Trigger.
Each Track-Finding processor operates on information from a
60-degree sector in azimuth only. Approximately 500 bits of
information from the track segments are sent into the processor
at the bunch crossing rate of 40 MHz.

The Track-Finding processor consists of Extrapolation units,
a Track Assembly unit, and an Assignment unit. An Extrapolation
unit takes the three-dimensional spatial information from two
track segments in different stations, and tests if these two segments
are compatible with a muon originating from the nominal collision
vertex with a curvature consistent with the magnetic bending in that
region. Several extrapolation catergories (extrapolations between
different pairs of stations) are performed in parallel.
The extrapolation units are implemented in
Field-Programmable-Gate-Arrays (FPGA).

The Track Assembly unit links successful extrapolations into
complete tracks. The three best track candidates are
then forwarded to the Assignment unit, where the Pt of a track
is determined from the azimuth and polar angles measured by
several stations. The information is sent into a memory
Look-Up-Table (LUT) for the calculation.

Each processor delivers up to three best muon candidates to the
Endcap Muon Sorter, which forwards the four best muon candidates
to the Level-1 Global Muon Trigger. The trigger algorithms of the
Track-Finding processor are fully programmable as they are implemented
in FPGA and RAM logic. Thus, the experiment is able to adapt to
different background conditions and colliding rates.
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In this paper we present the design of a fast three-dimensional
track-finding processor for the Level-1 trigger of the CMS Endcap
Muon System. Each track is assembled from track segments found in
the Endcap Muon chambers in both theta and phi planes. The processor
determines the transverse momenta of the best muon candidates from
the assembled track information. The trigger algorithms are
programmable since the processor is based on FPGA and RAM logic,
allowing the experiment to adapt to different background conditions
and colliding rates.
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