A GEM Detector System for an Upgrade of the CMS Muon Endcaps

GEM data would be performed on a custom mezzanine placed on the GLIB board.
Figure 65: Off-detector Electronics option using the current RPC PAC trigger.
The number of optical links would depend on the GEM segmentation. However, it is important to note that the
standard TB has got 18 inputs for the optical links and covers an arc of 30 ◦ . Eight of these optical link inputs are
used to receive data from the RPC chambers in the RE3/1, RE3/2, R4/1 and RE4/2 regions[37]. The structure of
these optical links is an integral part of the RPC system; the GEM interface would need to adhere to this structure
to ensure compatibility. Thus 10 optical link inputs are available in each TB for receiving data from GEMs in
GE1/1 and GE2/1.
The custom AMC board would encode the data to be compatible with the existing RPC PAC trigger. This uses
effectively 21 bit/BX in one optical link. The data coding algorithm, used in the PAC trigger system[38], divides
channels into partitions and transmits channels hit within partitions plus a time stamp with the following code bit
allocation:
1. 8 bits of the partition data,
2. 9 bits of partition number,
3. 3 bits of partition delay,
4. 1 bit end-of-data
One further requirement for compatibility with the current RPC system is that the maximum latency of the GEM
data at the input of the TB cannot exceed the current latency of the RPC data. The maximum latency would
therefore be 42 BX, this includes: muons time of flight, detector and front-end electronics signal processing,
optical transmission from the UXC to the USC, processing in the GLIB and transmission from GLIBs to the TBs.
Option B: Design of integrated GEM Trigger Boards (GemTB) in the µTCA crate In this option, shown
in Fig. 65, the trigger algorithm is performed on a new GEM Trigger Board (GemTB) placed in the µTCA crate
together with the GLIB boards. The GEM data from the GLIBs are transmitted to the GemTBs through the crate
backplane. The advantage of this system is that full granularity data can be transmitted providing the trigger
algorithm can profit from it. It is likely that multiple µTCA crates would be needed to cover the full phi-range of
one detector side. Therefore the possibility of data exchange between adjacent crates would be provided in order
to assure the logical overlapping of sectors. Additionally, the GemTBs would receive data from the RPC chambers
RE3/1, RE3/2, R4/1. and RE4/2 via the current RPC optical links. The muon candidates found by each GemTB
would be collected by a GBSort board in which local ghost-busting and sorting of trigger candidates is performed.
Muon candidates would then be transmitted to the Trigger Crate (TC) and introduced into the TC GBSort chip for
additional ghost-busting and sorting. Interface Boards placed in the slots originally foreseen for the high-η TBs
could be used for this purpose.
Figure 66: Off-detector Electronics option using new AMC GEM Trigger Boards (GemTB).
The detector region covered by one GemTB in principle does not have to correspond to the PAC logical sector,
but can be smaller. The choice would be driven mainly by the granularity of the data used in the GemTB trigger
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