University of Wisconsin CMS Trigger Group

CMS Level 1 Regional Calorimeter Trigger (RCT)

The RCT is divided into 18 crates, covering the entire calorimeter eta-phi space (2 crates in eta x 9 crates in phi). Each crate's processing is handled by 7 receiver cards, 7 electron identification cards, and 1 jet/summary card. A crate is divided up into 14 "regions" (7 in eta x 2 in phi), which consist of 4x4 squares of trigger towers; for every ECAL tower there is an HCAL tower covering the same bit of eta-phi space. A compressed (non-linear) energy and a fine grain bit are received from each HCAL or ECAL trigger tower by a receiver card. The energies for each region are uncompressed (by the lookup tables on the receiver card) and processed to create two sets of information: the total energy sum for each region (done on the receiver card), and a pair of the highest-energy "electron candidates", one isolated and one non-isolated (done on the electron identification card). At this stage, each "candidate" consists of a trigger tower with an energy higher than that of its neighbors. Electron identification and isolation also depend on the fine grain bit received from the ECAL trigger tower. The region sums and electron candidates are passed on to the crate's jet/summary card, which sorts the electron candidates to find the 4 highest-energy isolated and the 4 highest-energy non-isolated candidates; the jet/summary card then passes the 8 sorted candidates along with the region sums to the Global Calorimeter Trigger. The RCT also receives energies from the HF trigger towers (8 per crate), but beyond energy lookup, does no further processing before passing them to the GCT.

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