L1 Triggering on High-Granularity Information at the HL-LHC

The CMS collaboration is building a high-granularity calorimeter (HGCAL) for the endcap regions as part of its planned upgrade for the High-Luminosity LHC. The calorimetric data will form part of the Level-1 trigger (hardware) of the CMS experiment, reducing the event rate from the nominal 40 MHz to 750 kHz with a decision time (latency) of 12.5 microseconds. In addition to basic tracking information, which will also be available in the Level-1 trigger system, the use of particle-flow techniques will be facilitated as part of the trigger system. Around 1-million “trigger channels” are read at 40 MHz from the HGCAL, presenting a significant challenge in terms of data manipulation and processing for the trigger system: the trigger data volumes will be an order of magnitude above those currently handled at CMS. In addition, the high luminosity will result in an average of 140 (or more) interactions per bunch crossing that produce a huge background rate in the forward region and these will need to be efficiently rejected by the trigger algorithms. Furthermore, the reconstruction of particle clusters used for particle flow in high hit-rate events presents a complex computational problem associated with the trigger. We present the status of the trigger architecture and design, as well as the algorithmic concepts needed in order to tackle these major issues.