The Muon Sorter in the CMS Drift Tubes Regional Trigger

Abstract

Drift Tubes chambers were designed to be used in CMS trigger to allow muon tagging in the barrel of the detector [1]. Local Trigger logic reconstructs chamber-level track segments that are assembled into full tracks by the DT Track Finder in parallel over smaller sections of the barrel. In the final stage of the regional trigger the Muon Sorter has to select the best four candidates in the barrel and to filter fake muons generated by system redundancy. The hardware implementation of Muon Sorter satisfies large I/O and fast timing requirements using latest FPGA technology. The hardware was tested with custom facilities. Constraints, design implementation and test setup will be reported. I. DRIFT TUBES REGIONAL TRIGGER A. Regional Trigger partitioning The arrangement of muon detectors determines the partition of trigger logic [2]. CMS Drift Tubes detectors [3] are arranged in stations. Each station is provided with Local Trigger logic that reconstructs track segments [4]. Dimensions and positioning of the stations are such that four stations along the radial direction form a sector that covers 30° in the r-φ projection. Thus, 12 sectors at the same η cover the full φ angle making a wheel, while 5 sectors at the same φ make a wedge. Figure 1: Block view of the DT Regional Trigger system. Inside blocks the number of boards is shown, while numbers close to data flow arrows show the maximum number of muon tracks delivered by each stage. The DT Regional Trigger structure is shown in Figure 1. φ Track Finder boards [5,6,7] perform local trigger segments correlation in the r-φ projection. In parallel, η Track Finder boards use a pattern recognition algorithm on r-z projection hits to assign η values to found tracks [8]. Up to two tracks are built from data from one sector. Thus, up to 144 muon tracks can be built in the barrel region. The Muon Sorter selects up to 4 muon tracks to be forwarded to the Global Muon Trigger in two stages: 12 Wedge Sorter (WS) boards select up to 2 muons out of the 12 tracks collected from a wedge of the barrel; 1 single Barrel Sorter (BS) board performs the final selection of 4 tracks out of 24 tracks collected from the WS boards. B. Location of Regional Trigger hardware Regional Trigger electronics is located in the underground counting room of the experiment (USC55). No radiation issues have to be considered. Thus, the trigger logic is designed using programmable logic (FPGA). Modules are VME 9U boards, with a depth of 40 cm. The crates host a single J1 VME backplane and a custom backplane that provides power supply (3.3 V and 5 V) and GTL+ busses to exchange data between the boards. The system is shown in Figure 2: two racks host six crates for the Track Finder system and Wedge Sorter boards. One more rack with one crate hosts the Barrel Sorter board. The WS to BS connection is built with twisted pairs cables for LVDS transmission. Figure 2: DT Regional Trigger hardware layout. Muon Sorter 72 xx φ Track Finder 12 xx η Track Finder 12 xx Wedge Sorter 1 xx Barrel Sorter Global Muon Trigger (2 WS/ crate) x 6 crates LVDS cables 1 BS /1 crate Figure 3: Partial longitudinal cross section of the muon detectors, showing two examples of muons crossing wheel boundaries. Each track is reconstructed twice, by two consecutive φ Track Finders. II. WEDGE SORTER BOARD