Links:

• Good general description: Trigger TDR Chapter 12
• Up-to-date detailed information: U. Florida Sector Processor information

The CSC Track-Finder is defined to be the collection of electronic boards which are on the receiving end of the optical links sent by the CSC local trigger and which transmit L1 muons to the Global Muon Trigger (GMT). The CSC Track-Finder is implemented as 12 Sector Processors (SP) that identify up to the three best muons in six 60° azimuthal sectors of each endcap. Information is not shared across sector boundaries. Each Processor is a 9U VME card housed in a crate in the counting room of CMS. Three Sector Receiver (SR) cards also in the crate collect the optical signals sent from the Muon Port Cards of one sector and transmit data to a Sector Processor via a custom point-to-point backplane. A maximum of six track segments are sent from the first muon station in one sector, and three each from the remaining three stations.

The Sector Processor reconstructs tracks from the track segments delivered by the Sector Receivers and the DT Track-Finder. The number of CSC track segments collected by one Sector Processor is 15 per bunch crossing, assuming that ME4 participates. Six track segments are delivered from ME1; three each are delivered from ME2–ME4. Additionally, four DT track segments are delivered from the MB 2/1 chambers.

The Sector Processor scheme is shown below and then described. (Refer to the Trigger TDR chapter 12 for more details.)

The reconstruction of complete tracks from individual track segments is partitioned into several steps to minimize the logic and memory size of the Track-Finder:

1. Bunch Crossing Analyzer: the track segments from the CSC and DT trigger systems must be synchronized and possibly held for more than one bunch crossing to accommodate bunch-crossing misidentification from the LCT and BTI processors.
2. Extrapolation Units: nearly all possible pairwise combinations of track segments are tested for consistency with a single track. That is, each track segment is extrapolated to another station and then compared to other track segments in that station. Successful extrapolations yield tracks composed of two segments, which is the minimum necessary to form a trigger. If an ambiguity is created when two muons enter the same CSC chamber, all possible eta, phi combinations are tried.
3. Track Assembler Units: a muon which traverses all four muon stations and registers four track segments would yield six track “doublets.” Thus, the next step is to assemble complete tracks from the extrapolation results and cancel redundant shorter tracks.
4. Final Selection Unit: the best three muons are selected, and the track parameters are measured.