Global Muon Trigger (GMT)

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  • IN chips: LVDS Receiver: 512 bits / 40 MHz PHASE + BC synchronization, Spy mem, Ringbuffers + FIFOs
  • LFF/LFB: find matching muons between RPC and DT/CSC, cancel out logic order by rank, 1st-sorter stage
  • AUF/AUB: assign MIP+ISO bits
  • SRT: Final Sorter, Ringbuffers + FIFOs
  • ROP: VME decoding, Readout processor


The GMT Logic Board receives signals of the 16 input muons on 16 SCSI-2 cables connected to a special wide input board mounted in parallel to the front panel and connected to the GMT Logic Board by edge connectors. Parallel LVDS transfers (32 bits per muon including status bits) are used in order to keep latency low. The synchronized calorimeter bits are received via the custom-built backplane at 80 MHz in order to halve the number of connections. The output muon candidates (26 bits per muon) are sent to the Global Trigger directly via the backplane using 80 MHz GTL+ signals. VME, JTAG and control signals as well as a Channel Link for readout by Data Acquisition are also connected via the backplane. The GMT logic is implemented in several Xilinx FPGAs. In order to simplify routing of the board large FPGAs are placed on mezzanine boards.


The Global Muon Trigger receives in every bunch crossing up to four muon candidates each from the DT and RPC Triggers in the barrel region and up to four muon candidates each from the CSC and RPC Triggers in the forward region. Candidates consist of measurements of transverse momentum (5 bits), sign of charge (2 bits), azimuthal angle f (8 bits) and pseudorapidity ? (6 bits) at the muon system as well as a quality code (3 bits). From the Global Calorimeter Trigger the GMT receives MIP and Quiet bits for each of 252 calorimeter regions measuring ?? x ?f = 0.35 x 0.35 rad. MIP bits denote compatibility of the energy deposit in the calorimeters with the passage of a minimum ionising particle while Quiet bits indicate that the energy deposit in the region was below a certain threshold.


By combining candidates from the regional triggers the GMT naturally increases trigger efficiency, especially in detector regions of slightly different geometric acceptance of the regional triggers. The GMT matches candidates of complementary regional triggers based on their coordinates ? and f in order to confirm them. By applying selection criteria based on quality and pseudorapidity to the remaining unconfirmed candidates, the GMT can significantly lower the trigger rate caused by fake or improperly measured muons while keeping the efficiency close to the maximum possible. A further significant reduction of trigger rates can be achieved by careful merging of the parameters of matched candidates based on their detector region and qualities: proper combination of transverse momentum measurements can to a great extent reduce the effect of feed-through (assigning high transverse momenta to low-momentum muons). The suppression of certain types of low-quality unconfirmed muons in the di-muon trigger considerably reduces the rate contribution due to ghosts in some regions of pseudorapidity. A special case is the barrel/endcap overlap region, where ghosts can arise because the DT and CSC Triggers share several chambers. Special cancel-out units in the GMT can eliminate almost completely these ghosts. An additional function of the GMT is the correlation of muon candidates with calorimeter regions in order to check the candidates for confirmation and isolation.Starting from their positions (? and f in the muons system, the candidates are propagated to the calorimeter/vertex, based on their charge and transverse momentum measurements using tabulated parameterisations of muon trajectories. For confirmation, the calorimeter MIP bit at the propagated position of the muon in the calorimeter is attached to the muon candidate. In order to check a muon candidate for isolation, calorimeter Quiet bits are checked in calorimeter regions corresponding to the direction of the muon at the vertex (axis of a potential jet). An Isolation bit is attached to a muon candidate when the calorimeter region or multiple regions around the potential jet axis have their Quiet bit(s) set. Isolation and MIP confirmation can be used as additional criteria for trigger conditions in the Global Trigger.

  • A complete list of the functions performed by the GMT is given, below: Synchronizing the muon candidates and calorimeter bits to each other and to the LHC orbit.
  • Matching and Pairing DT with barrel-RPC muons in the barrel and CSC with forward-RPC muons in the endcaps based on their proximity in ? and f.
  • Merging the parameters of corresponding pairs of candidates from complementary systems.
  • Converting pseudorapidity measurements to a common scale.
  • Detecting possible ghosts or fake triggers by tagging unconfirmed low-quality candidates in certain regions of pseudorapidity in order to exclude them from certain trigger algorithms.
  • Cancelling-Out duplicate candidates in the barrel/endcap overlap region., especially candidates detected by both the DT and CSC Triggers.
  • Propagating muon candidates from the muon system to the calorimeter and vertex in order to check for calorimetric isolation or confirmation.
  • Ranking and Sorting the candidates in order to determine the four most important ones.

The algorithm of the GMT has been designed based on the above requirements. In order to process muon candidate data within a maximum latency of 10 slots of 25 ns, functions are performed in parallel wherever possible. Most of the GMT functions such as matching, merging, detection of low quality candidates, ranking and propagation depend on the detailed characteristics of the regional trigger systems or on the alignment of the trigger systems and calorimeters. In order to keep the GMT as flexible as possible, all the main functions are configurable via memory-based Look-Up Tables (LUTs). LUT contents have been optimised according to the simulated performance of the regional trigger systems based on the default detector geometry. In future the actual measured characteristics of the regional trigger systems and the alignment can easily be taken into account by updating the LUT contents. The algorithm has been modelled in C++ and simulated in the framework of the ORCA detector simulation software. Its performance has been demonstrated in many detailed simulation studies.


The GMT sends up to four muon candidates to the Global Trigger, each candidate consisting of measurements of transverse momentum (5 bits), sign of charge (2 bits), azimuthal angle f (8 bits) and pseudorapidity ? (6 bits), quality code (3 bits) as well as a MIP bit and an ISO bit denoting calorimetric confirmation and isolation.

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